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find the jar of the className

java -verbose:class -classpath $(echo *.jar | sed ‘s/ /:/g’)  com.anything.yourclass | grep “yourclass”

What classes are implementing Stack interface?

  • There is no stack interface in Java
  • push + pop + peek - are the methods generally used by classes that supports stack functionality (Stack and ArrayDeque)

What are the methods that can return special value or timeout in BlockingQueue

  1. offer(e) and offer(e, time, unit) (boolean)
  2. poll() and poll(time, unit) (returns E or null when timeout)

What are the methods that Blocks in BlockingQueue

  1. put(E e)
  2. take()

What was the HashMap change in JDK8

  1. TREEIFY_THRESHOLD - In Java 8, HashMap replaces the linked list with another useful data structure i.e. binary tree on breaching a certain threshold
  2. Which is known as TREEIFY_THRESHOLD. Once this threshold is reached the linked list of Entries is converted to the TreeNodes
  3. which reduces the time complexity from O(n) to O(log(n)).

How Hashmap works in Java

How LinkedBlockingQueue works?

What is Queue API? What are exception throwing API? What are boolean return API?

  • Queue
  • Exception throwing API
    • add(e)
    • remove()
    • element()
  • Special value returning API
    • offer(e)
    • poll()
    • peek()

How locking works inside concurrent hashmap

  • Lock-strip segment (before 8)
  • First element in each bucket is locked

Java Deque API

1. Deque behaves differently based on its interface
2. Stack (Interface)
   1. push - addFirst
   2. pop - removeFirst
   3. peek - getFirst
3. Queue (Interface) - Equivalent Deque Method
   1. add(e) - addLast(e)
   2. offer - offerLast
   3. remove() - removeFirst()
   4. poll - pollFirst()
   5. element - getFirst()
   6. peek - peekFirst

Java Comparators

  1. Comparator.comparing(Function.identity())
  2. Comparator c = String::compareTo
  3. Comparator that compares Person objects by their last name,
    1. Comparator byLastName = Comparator.comparing(Person::getLastName);
    2. Java doesn't support field reference similar to method reference, hence above style comparator may not work for List.val or Node.val
  4. Java creating composite comparator
    1. Comparator cmp = Comparator.comparingInt(p -> p.x).thenComparingInt(p -> p.y);
  5. Comparator.reverseOrder
    1. Comparator.comparingInt(String::length).reversed()
      1. // stream is now [test, foo, a], sorted by descending length
  6. Sort the indices of two dimensional array named intevals: int[][] 
         Integer[] indices = IntStream.range(0, intervals.length).boxed().toArray(Integer[]::new);
     Arrays.sort(indices, Comparator.comparingInt( (Integer i) -> intervals[indices[i]][0] ).thenComparingInt((Integer i) -> intervals[indices[i]][1]));

How String::compareTo method is type-casted as Comparator

  1. If type A has method, that accepts type B as parameter, java can convert that method into interface that can accepts type A and B.
    1. "The type to which the method belongs precedes the delimiter, and the invocation's receiver is the first parameter of the functional interface method"
    2. State of lambda

What is the type of String Comparator

  • Comparator<? super E>
  • Comparator<? super String>
  • why?
    • Comparator accepts String

Java generics - Type covariance

  • The container types have the same relationship to each other as the payload types do. This is expressed using the extends keyword.
  • If Cat extends Pet, then List is a subtype of List<? extends Pet>
  • It is read-only

Java generics - Type contravariance

  • The container types have the inverse relationship to each other as the payload types do. This is expressed using the super keyword.
  • Container type that is acting purely as a consumer of instances of a type
  • It can be used to write types into collection

What is PECS?

  • "PECS" is from the collection's point of view.
  • If you are only pulling items from a generic collection, it is a producer, and you should use extends;
  • if you are only stuffing items in, it is a consumer and you should use super.
  • If you do both with the same collection, you shouldn't use neither extends nor super. You should use simple class/interface (without wildcards)
  • What is a producer?
    • A producer is allowed to produce something more specific, hence extends, a consumer is allowed to accept something more general, hence super.
    • Producer refers to the return type of method.
  • What is a consumer?
    • Consumer refers to the parameter type of method.
  • A nice mnemonic you can use is to imagine returns for extends and accepts for super.
    • Tree<? extends T> reads Tree<? returns T>

What are Integer related Comparators

Integer.compare(int x, int y)   //static method
Integer.max(int x, int y)   //static method
Integer.min(int x, int y)   //static method
Integer.sum(int x, int y)   //static method
compareTo(Integer anotherInteger) //instance method

How to sum the Stack using lambda?

Optional<Integer> result = stack.stream().reduce(Integer::sum);

In frequencyCountMap, find the key that has maximum frequency

Collections.max(count.entrySet(), Map.Entry.comparingByValue()).getKey();

How to convert String into Stream?

"String".chars().mapToObj(c -> (char)c)"

How to add only non-null using stream

Arrays.stream(new Integer[]{1,2,3,null}).filter(Objects::nonNull).forEach(System.out::println)
Arrays.stream(lists).filter(Objects::nonNull).forEach(pq::offer);

How to check if there are any null

stream.anyMatch(Objects::isNull)
stream.anyMatch(x -> x == null)

How to convert List containing Integers to primitive int array i.e, int[]?

listOfIntegers.stream().mapToInt(Integer::intValue).toArray()

How to String-Stream as Array?

String[] stringArray = stringStream.toArray(String[]::new);

Quick analysis for xception handling in java code

find . -name \*java | grep -v “test.*est” | xargs grep -A 4 “catch.*xception” > exceptionHandling.txt
grep -A 4 catch.*xception `find . -type f -name \*java | grep -v test` > xception.log

Initialize two dimensional array

new int[][]{{1, 2}, {3}, {3}, {}}

How to create K rows with 2 columns each

int[][] closestPoints = new int[k][2];
jshell> var t = new int[5][2]
t ==> int[5][] { int[2] { 0, 0 }, int[2] { 0, 0 }, int[ ...  0, 0 }, int[2] { 0, 0 } }

Sort List in reverse order

Collections.sort(list, Collections.reverseOrder());

How many ways are there to find min/max?

  1. 3 Ways (reduce-lambda, min, reduce+method-reference)
int min = Stream.of(14, 35, -7, 46, 98).reduce(Integer::min).get();
min = Stream.of(14, 35, -7, 46, 98).min(Integer::compare).get();
min = Arrays.stream(nums).reduce(nums[0], (x,y) -> x<y ? x : y );

How to iterate List in reverser order

        Iterator lit = obj.descendingIterator();
        System.out.println("Backward Iterations");
        while(lit.hasNext()){
            System.out.println(lit.next());
        }

How to check if String is already sorted?

public boolean isSorted(String[] words) {
    return IntStream.range(0, words.length-1).noneMatch( i -> words[i+1].compareTo(words[i]) < 0 );
}

Comparator - Sorting Strings - sortedByNaturalOrder()

List<String> bonds = Arrays.asList("Connery","Lazenby","Moore", "Dalton", "Brosnan","Craig");
List<String> sortedByNaturalOrder = bonds.stream().sorted(Comparator.naturalOrder()).collect(Collectors.toList());
  • output: sortedByNaturalOrder ==> [Brosnan, Connery, Craig, Dalton, Lazenby, Moore]

Comparator.reverseOrder() - Static method

List<String> sortedByReverseOrder = bonds.stream().sorted(Comparator.reverseOrder()).collect(Collectors.toList());
  • output: sortedByReverseOrder ==> [Moore, Lazenby, Dalton, Craig, Connery, Brosnan]

How to compare strings using lowercase

List<String> sortedByLowerCase = bonds.stream().sorted(Comparator.comparing(String::toLowerCase)).collect(Collectors.toList());
  • output: sortedByLowerCase ==> [Brosnan, Connery, Craig, Dalton, Lazenby, Moore]
  • Note
    • The data is not changed to lowercase
    • Only while comparing, the Comparator uses toLowerCase of the all the input values

How to compare strings using string length

List<String> sortedByLength = bonds.stream().sorted(Comparator.comparingInt(String::length)).collect(Collectors.toList());
  • output: sortedByLength ==> [Moore, Craig, Dalton, Lazenby, Connery, Brosnan]
  • Note
    • The data is not changed to lowercase
    • Only while comparing, the Comparator uses length of the input string values

Composite comparator - string.length and naturalOrder

List<String> sortedByLengthThenByNaturalOrder = bonds.stream().sorted(Comparator.comparingInt(String::length).thenComparing(Comparator.naturalOrder())).
    collect(Collectors.toList());
  • output: sortedByLengthThenByNaturalOrder ==> [Craig, Moore, Dalton, Brosnan, Connery, Lazenby]

Where to find Comparator API

Static methods in java.util.Comparator

  1. naturalOrder()
  2. reverseOrder()
  3. comparing(Function<? super T,? extends U> keyExtractor)
  4. comparing(Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)
  5. comparingDouble(ToDoubleFunction<? super T> keyExtractor)
  6. comparingInt(ToIntFunction<? super T> keyExtractor)
  7. comparingLong(ToLongFunction<? super T> keyExtractor)
  8. nullsFirst(Comparator<? super T> comparator)
  9. nullsLast(Comparator<? super T> comparator)

Instance Methods in java.util.Comparator

  1. thenComparing(Comparator<? super T> other)
  2. thenComparing(Function<? super T,? extends U> keyExtractor)
  3. thenComparing(Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)
  4. thenComparingDouble(ToDoubleFunction<? super T> keyExtractor)
  5. thenComparingInt(ToIntFunction<? super T> keyExtractor)
  6. thenComparingLong(ToLongFunction<? super T> keyExtractor)

Examples from JDK - CASE_INSENSITIVE_ORDER

  • To sort a collection of String based on the length and then case-insensitive natural ordering, the comparator can be composed using following code,
Comparator<String> cmp = Comparator.comparingInt(String::length).thenComparing(String.CASE_INSENSITIVE_ORDER);

To obtain a Comparator that compares Person objects by their last name ignoring case differences,

static <T,U> Comparator<T> comparing(Function<? super T,? extends U> keyExtractor, Comparator<? super U> keyComparator)
  • Type Parameters:
  • T - input type of element to be compared
  • U - the type of the sort key
  • Parameters:
  • keyExtractor - the function used to extract the sort key
  • keyComparator - the Comparator used to compare the sort key
  • Returns:
  • a comparator that compares by an extracted key using the specified Comparator
Comparator<Person> cmp = Comparator.comparing(
    Person::getLastName,
    String.CASE_INSENSITIVE_ORDER);

to obtain a Comparator that compares Person objects by their last name,

Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);

How to Map<Integer, List<>> in one line

    Map<Integer, List<Integer>> adj = new HashMap<>();
    for (int[] edge : edges) {
        int a = edge[0], b = edge[1];
        adj.computeIfAbsent(a, value -> new ArrayList<Integer>()).add(b);
        adj.computeIfAbsent(b, value -> new ArrayList<Integer>()).add(a);
    }

Java format arguments while printing (or String manipulation)

String.format("%1$10s-%2$-10s-%3$10s-%4$10d","1000",2500,"123",123);
//        "      1000-2500      -       123-       123"
String.format("%1$-10s-%2$10s-%3$-10s-%4$10d","1000",2500,"123",123);
//        "1000      -      2500-123       -       123"

What are all important method in StringBuilder

strBuilder.deleteCharAt(strBuilder.length() - 1)
public StringBuilder replace(int start,int end, String str); //end-1 would be the last character affected

How to remove last character in StringBuilder

StringBuilder.deleteCharAt(sb.length()-1);

Java String methods, Initialize, Join and Strip

String[] strings = {"abc", "dbca"};
jshell> String.join(", ", new String[]{"abc", "dbca"})
$16 ==> "abc, dbca"


jshell> " abc ".strip() //stripLeading and stripTrailing - also available
$18 ==> "abc"
jshell> "abc".repeat(3)
$19 ==> "abcabcabc"

Java regular expression - tricks

jshell>"\\mohan\\root".split("\\\\")
$11 ==> String[3] { "", "mohan", "root" }
jshell> System.out.print("\nmohan\nroot")

mohan
root
jshell> "/a/b".split("/")
$15 ==> String[3] { "", "a", "b" }

How to find a digit inside array using binary search after index

java.util.Arrays.binarySearch(sortedArr, index + 1, sortedArr.length, key);
//If return value >=0, we found the key

Java primitive int[] sort reverse order

    Arrays.sort(int[], Collections.reverseOrder()) // will not work for primitive
    Arrays.stream(candidates).boxed().sorted (Collections.reverseOrder()).mapToInt(Integer::intValue).toArray();

How to convert List<int[]> into int[][]?

   List<int[]> common = new ArrayList<int[]>();
   return (int[][])common.toArray(new int[common.size()][]);

How to sort only elements after 1'st element (leave the 0th element in its place)

Collections.sort(mergedAccount.subList(1, mergedAccount.size()));

How to reverse elements between two indices of java array?

private void reverse(int[] nums, int first, int last) {
    while (first < last) {
        swap(nums, first++, last--);
    }
}

How to reverse Java int[] Array?

IntStream.rangeClosed(1, a.length).map(i -> a[a.length-i]).toArray();
Collections.reverse(Arrays.asList(yourArray));

Find sum/count in int stream

Integer sum = integers.stream().reduce(0, Integer::sum);
return (int)freq.values().stream().filter( v -> v>=2).count();

How to generate List with 11212312341234512345612345671234567812345678912345678910 in java?

    IntStream.rangeClosed(1, 10).flatMap(i -> IntStream.rangeClosed(1, i)).boxed().collect(Collectors.toList());

How to ensure two arrays/lists are equal?

word1FrequencyList.equals(word2FrequencyList);
Arrays.equals(array1, array2);

How to iterate ArrayDeque as stack

Deque<String> deque = new ArrayDeque<String>();deque.push("1");deque.push("2");
for(String data: deque) { System.out.println(deque.remove());}  //2 and 1
for(String data: deque) { System.out.println(data);}  //2 and 1

How to use ArrayDeque as stack while populating, and consume as queue

Deque<String> deque = new ArrayDeque<String>();deque.push("1");deque.push("2");
while(!deque.isEmpty()) { System.out.println(deque.removeLast());}  //1,2

What are new String methods added to JDK

What does toMap API return type?

  • Collector<T,?,Map<K,U>>

What are all toMap API?

toMap(Function<? super T,? extends K> keyMapper,  Function<? super T, ? extends U> valueMapper)
toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier)

What are the Map related comparators

  1. Map.Entry.comparingByValue()
  2. Map.Entry.comparingByValue(Comparator.reverseOrder())
  3. Map.Entry.comparingByKey()
  4. Map.Entry.comparingByKey(Comparator.reverseOrder())

Predicate till condition matches

  • Can be used only when containers are sorted
List<Dish> slicedMenuDishes = specialMenu.stream()
     .takeWhile(dish -> dish.getCalories() < 320)
     .collect(toList());
List<Dish> slicedMenuDishes = specialMenu.stream()
    .dropWhile(dish -> dish.getCalories() < 320)
    .collect(toList());

How to invoke script from url into JShell

jshell https://kishida.github.io/misc/jframe.jshell
jshell https://gist.githubusercontent.com/mohanmca/88de9d6115587f9b8c6e8ac73b80f46e/raw/a6f272479026f8bb5d79f01f9cbab631e04cb78c/jshell.jshell

Sample equals and hashcode for order

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Order order = (Order) o;
            return quantity == order.quantity 
                    && Float.compare(order.price, price) == 0 
                    && orderedTime == order.orderedTime 
                    && arrivedTime == order.arrivedTime 
                    && Objects.equals(orderId, order.orderId) 
                    && Objects.equals(parentOrderId, order.parentOrderId) 
                    && side == order.side && Objects.equals(instrument, order.instrument);
        }

        @Override
        public int hashCode() {
            return Objects.hash(orderId, parentOrderId, side, instrument, quantity, price, orderedTime, arrivedTime);
        }

Reference

How to create anki from this markdown file

  • mdanki java_oneliner.md Java_OneLiner.apkg --deck "Mohan::Core::Java::OneLiner"
  • node /Users/alpha/.nvm/versions/node/v16.5.0/lib/node_modules/mdanki/src/index.js java_oneliner.md Java_OneLiner.apkg --deck "Mohan::Core::Java::OneLiner" -s TOTAL_MEMORY=26777216