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<h2 title="Class HashMap" class="title">Class HashMap<K,​V></h2>
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<dt><span class="paramLabel">Type Parameters:</span></dt>
<dd><code>K</code> - the type of keys maintained by this map</dd>
<dd><code>V</code> - the type of mapped values</dd>
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<dt>All Implemented Interfaces:</dt>
<dd><code><a href="../io/Serializable.html" title="interface in java.io">Serializable</a></code>, <code><a href="../lang/Cloneable.html" title="interface in java.lang">Cloneable</a></code>, <code><a href="Map.html" title="interface in java.util">Map</a><K,​V></code></dd>
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<dd><code><a href="LinkedHashMap.html" title="class in java.util">LinkedHashMap</a></code>, <code><a href="../../../java.desktop/javax/print/attribute/standard/PrinterStateReasons.html" title="class in javax.print.attribute.standard">PrinterStateReasons</a></code></dd>
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<pre>public class <span class="typeNameLabel">HashMap<K,​V></span>
extends <a href="AbstractMap.html" title="class in java.util">AbstractMap</a><K,​V>
implements <a href="Map.html" title="interface in java.util">Map</a><K,​V>, <a href="../lang/Cloneable.html" title="interface in java.lang">Cloneable</a>, <a href="../io/Serializable.html" title="interface in java.io">Serializable</a></pre>
<div class="block">Hash table based implementation of the <code>Map</code> interface. This
implementation provides all of the optional map operations, and permits
<code>null</code> values and the <code>null</code> key. (The <code>HashMap</code>
class is roughly equivalent to <code>Hashtable</code>, except that it is
unsynchronized and permits nulls.) This class makes no guarantees as to
the order of the map; in particular, it does not guarantee that the order
will remain constant over time.
<p>This implementation provides constant-time performance for the basic
operations (<code>get</code> and <code>put</code>), assuming the hash function
disperses the elements properly among the buckets. Iteration over
collection views requires time proportional to the "capacity" of the
<code>HashMap</code> instance (the number of buckets) plus its size (the number
of key-value mappings). Thus, it's very important not to set the initial
capacity too high (or the load factor too low) if iteration performance is
important.
<p>An instance of <code>HashMap</code> has two parameters that affect its
performance: <i>initial capacity</i> and <i>load factor</i>. The
<i>capacity</i> is the number of buckets in the hash table, and the initial
capacity is simply the capacity at the time the hash table is created. The
<i>load factor</i> is a measure of how full the hash table is allowed to
get before its capacity is automatically increased. When the number of
entries in the hash table exceeds the product of the load factor and the
current capacity, the hash table is <i>rehashed</i> (that is, internal data
structures are rebuilt) so that the hash table has approximately twice the
number of buckets.
<p>As a general rule, the default load factor (.75) offers a good
tradeoff between time and space costs. Higher values decrease the
space overhead but increase the lookup cost (reflected in most of
the operations of the <code>HashMap</code> class, including
<code>get</code> and <code>put</code>). The expected number of entries in
the map and its load factor should be taken into account when
setting its initial capacity, so as to minimize the number of
rehash operations. If the initial capacity is greater than the
maximum number of entries divided by the load factor, no rehash
operations will ever occur.
<p>If many mappings are to be stored in a <code>HashMap</code>
instance, creating it with a sufficiently large capacity will allow
the mappings to be stored more efficiently than letting it perform
automatic rehashing as needed to grow the table. Note that using
many keys with the same <code>hashCode()</code> is a sure way to slow
down performance of any hash table. To ameliorate impact, when keys
are <a href="../lang/Comparable.html" title="interface in java.lang"><code>Comparable</code></a>, this class may use comparison order among
keys to help break ties.
<p><strong>Note that this implementation is not synchronized.</strong>
If multiple threads access a hash map concurrently, and at least one of
the threads modifies the map structurally, it <i>must</i> be
synchronized externally. (A structural modification is any operation
that adds or deletes one or more mappings; merely changing the value
associated with a key that an instance already contains is not a
structural modification.) This is typically accomplished by
synchronizing on some object that naturally encapsulates the map.
If no such object exists, the map should be "wrapped" using the
<a href="Collections.html#synchronizedMap(java.util.Map)"><code>Collections.synchronizedMap</code></a>
method. This is best done at creation time, to prevent accidental
unsynchronized access to the map:<pre>
Map m = Collections.synchronizedMap(new HashMap(...));</pre>
<p>The iterators returned by all of this class's "collection view methods"
are <i>fail-fast</i>: if the map is structurally modified at any time after
the iterator is created, in any way except through the iterator's own
<code>remove</code> method, the iterator will throw a
<a href="ConcurrentModificationException.html" title="class in java.util"><code>ConcurrentModificationException</code></a>. Thus, in the face of concurrent
modification, the iterator fails quickly and cleanly, rather than risking
arbitrary, non-deterministic behavior at an undetermined time in the
future.
<p>Note that the fail-fast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification. Fail-fast iterators
throw <code>ConcurrentModificationException</code> on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness: <i>the fail-fast behavior of iterators
should be used only to detect bugs.</i>
<p>This class is a member of the
<a href="../../../java.base/java/util/package-summary.html#CollectionsFramework">
Java Collections Framework</a>.</div>
<dl>
<dt><span class="simpleTagLabel">Since:</span></dt>
<dd>1.2</dd>
<dt><span class="seeLabel">See Also:</span></dt>
<dd><a href="../lang/Object.html#hashCode()"><code>Object.hashCode()</code></a>,
<a href="Collection.html" title="interface in java.util"><code>Collection</code></a>,
<a href="Map.html" title="interface in java.util"><code>Map</code></a>,
<a href="TreeMap.html" title="class in java.util"><code>TreeMap</code></a>,
<a href="Hashtable.html" title="class in java.util"><code>Hashtable</code></a>,
<a href="../../../serialized-form.html#java.util.HashMap">Serialized Form</a></dd>
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<h3>Nested classes/interfaces declared in class java.util.<a href="AbstractMap.html" title="class in java.util">AbstractMap</a></h3>
<code><a href="AbstractMap.SimpleEntry.html" title="class in java.util">AbstractMap.SimpleEntry</a><<a href="AbstractMap.SimpleEntry.html" title="type parameter in AbstractMap.SimpleEntry">K</a>,​<a href="AbstractMap.SimpleEntry.html" title="type parameter in AbstractMap.SimpleEntry">V</a>>, <a href="AbstractMap.SimpleImmutableEntry.html" title="class in java.util">AbstractMap.SimpleImmutableEntry</a><<a href="AbstractMap.SimpleImmutableEntry.html" title="type parameter in AbstractMap.SimpleImmutableEntry">K</a>,​<a href="AbstractMap.SimpleImmutableEntry.html" title="type parameter in AbstractMap.SimpleImmutableEntry">V</a>></code></li>
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<h3>Nested classes/interfaces declared in interface java.util.<a href="Map.html" title="interface in java.util">Map</a></h3>
<code><a href="Map.Entry.html" title="interface in java.util">Map.Entry</a><<a href="Map.Entry.html" title="type parameter in Map.Entry">K</a>,​<a href="Map.Entry.html" title="type parameter in Map.Entry">V</a>></code></li>
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<h3>Constructor Summary</h3>
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<caption><span>Constructors</span><span class="tabEnd"> </span></caption>
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<th class="colFirst" scope="col">Constructor</th>
<th class="colLast" scope="col">Description</th>
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<th class="colConstructorName" scope="row"><code><span class="memberNameLink"><a href="#%3Cinit%3E()">HashMap</a></span>()</code></th>
<td class="colLast">
<div class="block">Constructs an empty <code>HashMap</code> with the default initial capacity
(16) and the default load factor (0.75).</div>
</td>
</tr>
<tr class="rowColor">
<th class="colConstructorName" scope="row"><code><span class="memberNameLink"><a href="#%3Cinit%3E(int)">HashMap</a></span>​(int initialCapacity)</code></th>
<td class="colLast">
<div class="block">Constructs an empty <code>HashMap</code> with the specified initial
capacity and the default load factor (0.75).</div>
</td>
</tr>
<tr class="altColor">
<th class="colConstructorName" scope="row"><code><span class="memberNameLink"><a href="#%3Cinit%3E(int,float)">HashMap</a></span>​(int initialCapacity,
float loadFactor)</code></th>
<td class="colLast">
<div class="block">Constructs an empty <code>HashMap</code> with the specified initial
capacity and load factor.</div>
</td>
</tr>
<tr class="rowColor">
<th class="colConstructorName" scope="row"><code><span class="memberNameLink"><a href="#%3Cinit%3E(java.util.Map)">HashMap</a></span>​(<a href="Map.html" title="interface in java.util">Map</a><? extends <a href="HashMap.html" title="type parameter in HashMap">K</a>,​? extends <a href="HashMap.html" title="type parameter in HashMap">V</a>> m)</code></th>
<td class="colLast">
<div class="block">Constructs a new <code>HashMap</code> with the same mappings as the
specified <code>Map</code>.</div>
</td>
</tr>
</tbody>
</table>
</div>
</li>
</ul>
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<div role="tablist" aria-orientation="horizontal"><button role="tab" aria-selected="true" aria-controls="memberSummary_tabpanel" tabindex="0" onkeydown="switchTab(event)" id="t0" class="activeTableTab">All Methods</button><button role="tab" aria-selected="false" aria-controls="memberSummary_tabpanel" tabindex="-1" onkeydown="switchTab(event)" id="t2" class="tableTab" onclick="show(2);">Instance Methods</button><button role="tab" aria-selected="false" aria-controls="memberSummary_tabpanel" tabindex="-1" onkeydown="switchTab(event)" id="t4" class="tableTab" onclick="show(8);">Concrete Methods</button></div>
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<table aria-labelledby="t0">
<tr>
<th class="colFirst" scope="col">Modifier and Type</th>
<th class="colSecond" scope="col">Method</th>
<th class="colLast" scope="col">Description</th>
</tr>
<tbody>
<tr class="altColor" id="i0">
<td class="colFirst"><code>void</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#clear()">clear</a></span>()</code></th>
<td class="colLast">
<div class="block">Removes all of the mappings from this map.</div>
</td>
</tr>
<tr class="rowColor" id="i1">
<td class="colFirst"><code><a href="../lang/Object.html" title="class in java.lang">Object</a></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#clone()">clone</a></span>()</code></th>
<td class="colLast">
<div class="block">Returns a shallow copy of this <code>HashMap</code> instance: the keys and
values themselves are not cloned.</div>
</td>
</tr>
<tr class="altColor" id="i2">
<td class="colFirst"><code><a href="HashMap.html" title="type parameter in HashMap">V</a></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#compute(K,java.util.function.BiFunction)">compute</a></span>​(<a href="HashMap.html" title="type parameter in HashMap">K</a> key,
<a href="function/BiFunction.html" title="interface in java.util.function">BiFunction</a><? super <a href="HashMap.html" title="type parameter in HashMap">K</a>,​? super <a href="HashMap.html" title="type parameter in HashMap">V</a>,​? extends <a href="HashMap.html" title="type parameter in HashMap">V</a>> remappingFunction)</code></th>
<td class="colLast">
<div class="block">Attempts to compute a mapping for the specified key and its current
mapped value (or <code>null</code> if there is no current mapping).</div>
</td>
</tr>
<tr class="rowColor" id="i3">
<td class="colFirst"><code><a href="HashMap.html" title="type parameter in HashMap">V</a></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#computeIfAbsent(K,java.util.function.Function)">computeIfAbsent</a></span>​(<a href="HashMap.html" title="type parameter in HashMap">K</a> key,
<a href="function/Function.html" title="interface in java.util.function">Function</a><? super <a href="HashMap.html" title="type parameter in HashMap">K</a>,​? extends <a href="HashMap.html" title="type parameter in HashMap">V</a>> mappingFunction)</code></th>
<td class="colLast">
<div class="block">If the specified key is not already associated with a value (or is mapped
to <code>null</code>), attempts to compute its value using the given mapping
function and enters it into this map unless <code>null</code>.</div>
</td>
</tr>
<tr class="altColor" id="i4">
<td class="colFirst"><code><a href="HashMap.html" title="type parameter in HashMap">V</a></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#computeIfPresent(K,java.util.function.BiFunction)">computeIfPresent</a></span>​(<a href="HashMap.html" title="type parameter in HashMap">K</a> key,
<a href="function/BiFunction.html" title="interface in java.util.function">BiFunction</a><? super <a href="HashMap.html" title="type parameter in HashMap">K</a>,​? super <a href="HashMap.html" title="type parameter in HashMap">V</a>,​? extends <a href="HashMap.html" title="type parameter in HashMap">V</a>> remappingFunction)</code></th>
<td class="colLast">
<div class="block">If the value for the specified key is present and non-null, attempts to
compute a new mapping given the key and its current mapped value.</div>
</td>
</tr>
<tr class="rowColor" id="i5">
<td class="colFirst"><code>boolean</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#containsKey(java.lang.Object)">containsKey</a></span>​(<a href="../lang/Object.html" title="class in java.lang">Object</a> key)</code></th>
<td class="colLast">
<div class="block">Returns <code>true</code> if this map contains a mapping for the
specified key.</div>
</td>
</tr>
<tr class="altColor" id="i6">
<td class="colFirst"><code>boolean</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#containsValue(java.lang.Object)">containsValue</a></span>​(<a href="../lang/Object.html" title="class in java.lang">Object</a> value)</code></th>
<td class="colLast">
<div class="block">Returns <code>true</code> if this map maps one or more keys to the
specified value.</div>
</td>
</tr>
<tr class="rowColor" id="i7">
<td class="colFirst"><code><a href="Set.html" title="interface in java.util">Set</a><<a href="Map.Entry.html" title="interface in java.util">Map.Entry</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>>></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#entrySet()">entrySet</a></span>()</code></th>
<td class="colLast">
<div class="block">Returns a <a href="Set.html" title="interface in java.util"><code>Set</code></a> view of the mappings contained in this map.</div>
</td>
</tr>
<tr class="altColor" id="i8">
<td class="colFirst"><code><a href="HashMap.html" title="type parameter in HashMap">V</a></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#get(java.lang.Object)">get</a></span>​(<a href="../lang/Object.html" title="class in java.lang">Object</a> key)</code></th>
<td class="colLast">
<div class="block">Returns the value to which the specified key is mapped,
or <code>null</code> if this map contains no mapping for the key.</div>
</td>
</tr>
<tr class="rowColor" id="i9">
<td class="colFirst"><code>boolean</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#isEmpty()">isEmpty</a></span>()</code></th>
<td class="colLast">
<div class="block">Returns <code>true</code> if this map contains no key-value mappings.</div>
</td>
</tr>
<tr class="altColor" id="i10">
<td class="colFirst"><code><a href="Set.html" title="interface in java.util">Set</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#keySet()">keySet</a></span>()</code></th>
<td class="colLast">
<div class="block">Returns a <a href="Set.html" title="interface in java.util"><code>Set</code></a> view of the keys contained in this map.</div>
</td>
</tr>
<tr class="rowColor" id="i11">
<td class="colFirst"><code><a href="HashMap.html" title="type parameter in HashMap">V</a></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#merge(K,V,java.util.function.BiFunction)">merge</a></span>​(<a href="HashMap.html" title="type parameter in HashMap">K</a> key,
<a href="HashMap.html" title="type parameter in HashMap">V</a> value,
<a href="function/BiFunction.html" title="interface in java.util.function">BiFunction</a><? super <a href="HashMap.html" title="type parameter in HashMap">V</a>,​? super <a href="HashMap.html" title="type parameter in HashMap">V</a>,​? extends <a href="HashMap.html" title="type parameter in HashMap">V</a>> remappingFunction)</code></th>
<td class="colLast">
<div class="block">If the specified key is not already associated with a value or is
associated with null, associates it with the given non-null value.</div>
</td>
</tr>
<tr class="altColor" id="i12">
<td class="colFirst"><code><a href="HashMap.html" title="type parameter in HashMap">V</a></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#put(K,V)">put</a></span>​(<a href="HashMap.html" title="type parameter in HashMap">K</a> key,
<a href="HashMap.html" title="type parameter in HashMap">V</a> value)</code></th>
<td class="colLast">
<div class="block">Associates the specified value with the specified key in this map.</div>
</td>
</tr>
<tr class="rowColor" id="i13">
<td class="colFirst"><code>void</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#putAll(java.util.Map)">putAll</a></span>​(<a href="Map.html" title="interface in java.util">Map</a><? extends <a href="HashMap.html" title="type parameter in HashMap">K</a>,​? extends <a href="HashMap.html" title="type parameter in HashMap">V</a>> m)</code></th>
<td class="colLast">
<div class="block">Copies all of the mappings from the specified map to this map.</div>
</td>
</tr>
<tr class="altColor" id="i14">
<td class="colFirst"><code><a href="HashMap.html" title="type parameter in HashMap">V</a></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#remove(java.lang.Object)">remove</a></span>​(<a href="../lang/Object.html" title="class in java.lang">Object</a> key)</code></th>
<td class="colLast">
<div class="block">Removes the mapping for the specified key from this map if present.</div>
</td>
</tr>
<tr class="rowColor" id="i15">
<td class="colFirst"><code>int</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#size()">size</a></span>()</code></th>
<td class="colLast">
<div class="block">Returns the number of key-value mappings in this map.</div>
</td>
</tr>
<tr class="altColor" id="i16">
<td class="colFirst"><code><a href="Collection.html" title="interface in java.util">Collection</a><<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="#values()">values</a></span>()</code></th>
<td class="colLast">
<div class="block">Returns a <a href="Collection.html" title="interface in java.util"><code>Collection</code></a> view of the values contained in this map.</div>
</td>
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<h3>Methods declared in class java.util.<a href="AbstractMap.html" title="class in java.util">AbstractMap</a></h3>
<code><a href="AbstractMap.html#equals(java.lang.Object)">equals</a>, <a href="AbstractMap.html#hashCode()">hashCode</a>, <a href="AbstractMap.html#toString()">toString</a></code></li>
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<h3>Methods declared in class java.lang.<a href="../lang/Object.html" title="class in java.lang">Object</a></h3>
<code><a href="../lang/Object.html#finalize()">finalize</a>, <a href="../lang/Object.html#getClass()">getClass</a>, <a href="../lang/Object.html#notify()">notify</a>, <a href="../lang/Object.html#notifyAll()">notifyAll</a>, <a href="../lang/Object.html#wait()">wait</a>, <a href="../lang/Object.html#wait(long)">wait</a>, <a href="../lang/Object.html#wait(long,int)">wait</a></code></li>
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<h3>Methods declared in interface java.util.<a href="Map.html" title="interface in java.util">Map</a></h3>
<code><a href="Map.html#equals(java.lang.Object)">equals</a>, <a href="Map.html#forEach(java.util.function.BiConsumer)">forEach</a>, <a href="Map.html#getOrDefault(java.lang.Object,V)">getOrDefault</a>, <a href="Map.html#hashCode()">hashCode</a>, <a href="Map.html#putIfAbsent(K,V)">putIfAbsent</a>, <a href="Map.html#remove(java.lang.Object,java.lang.Object)">remove</a>, <a href="Map.html#replace(K,V)">replace</a>, <a href="Map.html#replace(K,V,V)">replace</a>, <a href="Map.html#replaceAll(java.util.function.BiFunction)">replaceAll</a></code></li>
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<h4>HashMap</h4>
<pre>public HashMap​(int initialCapacity,
float loadFactor)</pre>
<div class="block">Constructs an empty <code>HashMap</code> with the specified initial
capacity and load factor.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>initialCapacity</code> - the initial capacity</dd>
<dd><code>loadFactor</code> - the load factor</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="../lang/IllegalArgumentException.html" title="class in java.lang">IllegalArgumentException</a></code> - if the initial capacity is negative
or the load factor is nonpositive</dd>
</dl>
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<h4>HashMap</h4>
<pre>public HashMap​(int initialCapacity)</pre>
<div class="block">Constructs an empty <code>HashMap</code> with the specified initial
capacity and the default load factor (0.75).</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>initialCapacity</code> - the initial capacity.</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="../lang/IllegalArgumentException.html" title="class in java.lang">IllegalArgumentException</a></code> - if the initial capacity is negative.</dd>
</dl>
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<h4>HashMap</h4>
<pre>public HashMap()</pre>
<div class="block">Constructs an empty <code>HashMap</code> with the default initial capacity
(16) and the default load factor (0.75).</div>
</li>
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<h4>HashMap</h4>
<pre>public HashMap​(<a href="Map.html" title="interface in java.util">Map</a><? extends <a href="HashMap.html" title="type parameter in HashMap">K</a>,​? extends <a href="HashMap.html" title="type parameter in HashMap">V</a>> m)</pre>
<div class="block">Constructs a new <code>HashMap</code> with the same mappings as the
specified <code>Map</code>. The <code>HashMap</code> is created with
default load factor (0.75) and an initial capacity sufficient to
hold the mappings in the specified <code>Map</code>.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>m</code> - the map whose mappings are to be placed in this map</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="../lang/NullPointerException.html" title="class in java.lang">NullPointerException</a></code> - if the specified map is null</dd>
</dl>
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</ul>
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<h4>size</h4>
<pre class="methodSignature">public int size()</pre>
<div class="block">Returns the number of key-value mappings in this map.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#size()">size</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="AbstractMap.html#size()">size</a></code> in class <code><a href="AbstractMap.html" title="class in java.util">AbstractMap</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the number of key-value mappings in this map</dd>
</dl>
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<h4>isEmpty</h4>
<pre class="methodSignature">public boolean isEmpty()</pre>
<div class="block">Returns <code>true</code> if this map contains no key-value mappings.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#isEmpty()">isEmpty</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="AbstractMap.html#isEmpty()">isEmpty</a></code> in class <code><a href="AbstractMap.html" title="class in java.util">AbstractMap</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd><code>true</code> if this map contains no key-value mappings</dd>
</dl>
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<h4>get</h4>
<pre class="methodSignature">public <a href="HashMap.html" title="type parameter in HashMap">V</a> get​(<a href="../lang/Object.html" title="class in java.lang">Object</a> key)</pre>
<div class="block">Returns the value to which the specified key is mapped,
or <code>null</code> if this map contains no mapping for the key.
<p>More formally, if this map contains a mapping from a key
<code>k</code> to a value <code>v</code> such that <code>(key==null ? k==null :
key.equals(k))</code>, then this method returns <code>v</code>; otherwise
it returns <code>null</code>. (There can be at most one such mapping.)
<p>A return value of <code>null</code> does not <i>necessarily</i>
indicate that the map contains no mapping for the key; it's also
possible that the map explicitly maps the key to <code>null</code>.
The <a href="#containsKey(java.lang.Object)"><code>containsKey</code></a> operation may be used to
distinguish these two cases.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#get(java.lang.Object)">get</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="AbstractMap.html#get(java.lang.Object)">get</a></code> in class <code><a href="AbstractMap.html" title="class in java.util">AbstractMap</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>key</code> - the key whose associated value is to be returned</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the value to which the specified key is mapped, or
<code>null</code> if this map contains no mapping for the key</dd>
<dt><span class="seeLabel">See Also:</span></dt>
<dd><a href="#put(K,V)"><code>put(Object, Object)</code></a></dd>
</dl>
</li>
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<h4>containsKey</h4>
<pre class="methodSignature">public boolean containsKey​(<a href="../lang/Object.html" title="class in java.lang">Object</a> key)</pre>
<div class="block">Returns <code>true</code> if this map contains a mapping for the
specified key.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#containsKey(java.lang.Object)">containsKey</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="AbstractMap.html#containsKey(java.lang.Object)">containsKey</a></code> in class <code><a href="AbstractMap.html" title="class in java.util">AbstractMap</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>key</code> - The key whose presence in this map is to be tested</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd><code>true</code> if this map contains a mapping for the specified
key.</dd>
</dl>
</li>
</ul>
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<h4>put</h4>
<pre class="methodSignature">public <a href="HashMap.html" title="type parameter in HashMap">V</a> put​(<a href="HashMap.html" title="type parameter in HashMap">K</a> key,
<a href="HashMap.html" title="type parameter in HashMap">V</a> value)</pre>
<div class="block">Associates the specified value with the specified key in this map.
If the map previously contained a mapping for the key, the old
value is replaced.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#put(K,V)">put</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="AbstractMap.html#put(K,V)">put</a></code> in class <code><a href="AbstractMap.html" title="class in java.util">AbstractMap</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>key</code> - key with which the specified value is to be associated</dd>
<dd><code>value</code> - value to be associated with the specified key</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the previous value associated with <code>key</code>, or
<code>null</code> if there was no mapping for <code>key</code>.
(A <code>null</code> return can also indicate that the map
previously associated <code>null</code> with <code>key</code>.)</dd>
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<pre class="methodSignature">public void putAll​(<a href="Map.html" title="interface in java.util">Map</a><? extends <a href="HashMap.html" title="type parameter in HashMap">K</a>,​? extends <a href="HashMap.html" title="type parameter in HashMap">V</a>> m)</pre>
<div class="block">Copies all of the mappings from the specified map to this map.
These mappings will replace any mappings that this map had for
any of the keys currently in the specified map.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#putAll(java.util.Map)">putAll</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="AbstractMap.html#putAll(java.util.Map)">putAll</a></code> in class <code><a href="AbstractMap.html" title="class in java.util">AbstractMap</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>m</code> - mappings to be stored in this map</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="../lang/NullPointerException.html" title="class in java.lang">NullPointerException</a></code> - if the specified map is null</dd>
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<pre class="methodSignature">public <a href="HashMap.html" title="type parameter in HashMap">V</a> remove​(<a href="../lang/Object.html" title="class in java.lang">Object</a> key)</pre>
<div class="block">Removes the mapping for the specified key from this map if present.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#remove(java.lang.Object)">remove</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="AbstractMap.html#remove(java.lang.Object)">remove</a></code> in class <code><a href="AbstractMap.html" title="class in java.util">AbstractMap</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>key</code> - key whose mapping is to be removed from the map</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the previous value associated with <code>key</code>, or
<code>null</code> if there was no mapping for <code>key</code>.
(A <code>null</code> return can also indicate that the map
previously associated <code>null</code> with <code>key</code>.)</dd>
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<h4>clear</h4>
<pre class="methodSignature">public void clear()</pre>
<div class="block">Removes all of the mappings from this map.
The map will be empty after this call returns.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#clear()">clear</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="AbstractMap.html#clear()">clear</a></code> in class <code><a href="AbstractMap.html" title="class in java.util">AbstractMap</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
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<pre class="methodSignature">public boolean containsValue​(<a href="../lang/Object.html" title="class in java.lang">Object</a> value)</pre>
<div class="block">Returns <code>true</code> if this map maps one or more keys to the
specified value.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#containsValue(java.lang.Object)">containsValue</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="AbstractMap.html#containsValue(java.lang.Object)">containsValue</a></code> in class <code><a href="AbstractMap.html" title="class in java.util">AbstractMap</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>value</code> - value whose presence in this map is to be tested</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd><code>true</code> if this map maps one or more keys to the
specified value</dd>
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<pre class="methodSignature">public <a href="Set.html" title="interface in java.util">Set</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>> keySet()</pre>
<div class="block">Returns a <a href="Set.html" title="interface in java.util"><code>Set</code></a> view of the keys contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own <code>remove</code> operation), the results of
the iteration are undefined. The set supports element removal,
which removes the corresponding mapping from the map, via the
<code>Iterator.remove</code>, <code>Set.remove</code>,
<code>removeAll</code>, <code>retainAll</code>, and <code>clear</code>
operations. It does not support the <code>add</code> or <code>addAll</code>
operations.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#keySet()">keySet</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="AbstractMap.html#keySet()">keySet</a></code> in class <code><a href="AbstractMap.html" title="class in java.util">AbstractMap</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a set view of the keys contained in this map</dd>
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<pre class="methodSignature">public <a href="Collection.html" title="interface in java.util">Collection</a><<a href="HashMap.html" title="type parameter in HashMap">V</a>> values()</pre>
<div class="block">Returns a <a href="Collection.html" title="interface in java.util"><code>Collection</code></a> view of the values contained in this map.
The collection is backed by the map, so changes to the map are
reflected in the collection, and vice-versa. If the map is
modified while an iteration over the collection is in progress
(except through the iterator's own <code>remove</code> operation),
the results of the iteration are undefined. The collection
supports element removal, which removes the corresponding
mapping from the map, via the <code>Iterator.remove</code>,
<code>Collection.remove</code>, <code>removeAll</code>,
<code>retainAll</code> and <code>clear</code> operations. It does not
support the <code>add</code> or <code>addAll</code> operations.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#values()">values</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="AbstractMap.html#values()">values</a></code> in class <code><a href="AbstractMap.html" title="class in java.util">AbstractMap</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a view of the values contained in this map</dd>
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<pre class="methodSignature">public <a href="Set.html" title="interface in java.util">Set</a><<a href="Map.Entry.html" title="interface in java.util">Map.Entry</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>>> entrySet()</pre>
<div class="block">Returns a <a href="Set.html" title="interface in java.util"><code>Set</code></a> view of the mappings contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own <code>remove</code> operation, or through the
<code>setValue</code> operation on a map entry returned by the
iterator) the results of the iteration are undefined. The set
supports element removal, which removes the corresponding
mapping from the map, via the <code>Iterator.remove</code>,
<code>Set.remove</code>, <code>removeAll</code>, <code>retainAll</code> and
<code>clear</code> operations. It does not support the
<code>add</code> or <code>addAll</code> operations.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#entrySet()">entrySet</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a set view of the mappings contained in this map</dd>
</dl>
</li>
</ul>
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<pre class="methodSignature">public <a href="HashMap.html" title="type parameter in HashMap">V</a> computeIfAbsent​(<a href="HashMap.html" title="type parameter in HashMap">K</a> key,
<a href="function/Function.html" title="interface in java.util.function">Function</a><? super <a href="HashMap.html" title="type parameter in HashMap">K</a>,​? extends <a href="HashMap.html" title="type parameter in HashMap">V</a>> mappingFunction)</pre>
<div class="block">If the specified key is not already associated with a value (or is mapped
to <code>null</code>), attempts to compute its value using the given mapping
function and enters it into this map unless <code>null</code>.
<p>If the mapping function returns <code>null</code>, no mapping is recorded.
If the mapping function itself throws an (unchecked) exception, the
exception is rethrown, and no mapping is recorded. The most
common usage is to construct a new object serving as an initial
mapped value or memoized result, as in:
<pre> <code>
map.computeIfAbsent(key, k -> new Value(f(k)));
</code></pre>
<p>Or to implement a multi-value map, <code>Map<K,Collection<V>></code>,
supporting multiple values per key:
<pre> <code>
map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
</code></pre>
<p>The mapping function should not modify this map during computation.
<p>This method will, on a best-effort basis, throw a
<a href="ConcurrentModificationException.html" title="class in java.util"><code>ConcurrentModificationException</code></a> if it is detected that the
mapping function modifies this map during computation.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#computeIfAbsent(K,java.util.function.Function)">computeIfAbsent</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>key</code> - key with which the specified value is to be associated</dd>
<dd><code>mappingFunction</code> - the mapping function to compute a value</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the current (existing or computed) value associated with
the specified key, or null if the computed value is null</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="ConcurrentModificationException.html" title="class in java.util">ConcurrentModificationException</a></code> - if it is detected that the
mapping function modified this map</dd>
</dl>
</li>
</ul>
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<pre class="methodSignature">public <a href="HashMap.html" title="type parameter in HashMap">V</a> computeIfPresent​(<a href="HashMap.html" title="type parameter in HashMap">K</a> key,
<a href="function/BiFunction.html" title="interface in java.util.function">BiFunction</a><? super <a href="HashMap.html" title="type parameter in HashMap">K</a>,​? super <a href="HashMap.html" title="type parameter in HashMap">V</a>,​? extends <a href="HashMap.html" title="type parameter in HashMap">V</a>> remappingFunction)</pre>
<div class="block">If the value for the specified key is present and non-null, attempts to
compute a new mapping given the key and its current mapped value.
<p>If the remapping function returns <code>null</code>, the mapping is removed.
If the remapping function itself throws an (unchecked) exception, the
exception is rethrown, and the current mapping is left unchanged.
<p>The remapping function should not modify this map during computation.
<p>This method will, on a best-effort basis, throw a
<a href="ConcurrentModificationException.html" title="class in java.util"><code>ConcurrentModificationException</code></a> if it is detected that the
remapping function modifies this map during computation.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="Map.html#computeIfPresent(K,java.util.function.BiFunction)">computeIfPresent</a></code> in interface <code><a href="Map.html" title="interface in java.util">Map</a><<a href="HashMap.html" title="type parameter in HashMap">K</a>,​<a href="HashMap.html" title="type parameter in HashMap">V</a>></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>key</code> - key with which the specified value is to be associated</dd>
<dd><code>remappingFunction</code> - the remapping function to compute a value</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the new value associated with the specified key, or null if none</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="ConcurrentModificationException.html" title="class in java.util">ConcurrentModificationException</a></code> - if it is detected that the
remapping function modified this map</dd>
</dl>
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<pre class="methodSignature">public <a href="HashMap.html" title="type parameter in HashMap">V</a> compute​(<a href="HashMap.html" title="type parameter in HashMap">K</a> key,
<a href="function/BiFunction.html" title="interface in java.util.function">BiFunction</a><? super <a href="HashMap.html" title="type parameter in HashMap">K</a>,​? super <a href="HashMap.html" title="type parameter in HashMap">V</a>,​? extends <a href="HashMap.html" title="type parameter in HashMap">V</a>> remappingFunction)</pre>
<div class="block">Attempts to compute a mapping for the specified key and its current
mapped value (or <code>null</code> if there is no current mapping). For
example, to either create or append a <code>String</code> msg to a value
mapping:
<pre> <code>
map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))</code></pre>