JAVA Collection Cheat Sheet

Important methods:

ArrayList<String> list = new ArrayList<>();

ArrayList(class) — implements →List(Interface) — extends → Collection

ArrayList is not Synchronized, Its equivalent synchronized class is Vector.


An ArrayList can also be created using an existing Collection. The newly created ArrayList will contain all the elements in the same order in the original collection.


e = element (Object), i = index , c = Collection

  • list.size();
  • list.contains(e); //returns bool
  • list.indexOf(e);
  • list.lastIndexOf(e);
  • list.get(i);
  • list.set(i,e);
  • list.add(e); // Add element
  • list.add(i,e);
  • list.addAll(c) // Add Collections
  • list.addAll(i, c)
  • list.remove(i);//returns bool : return at index

eg : arr.remove(3) -> this will remove element at index 3

  • list.remove(e) or list.remove(object);

//returns bool : This is tricky in case of integer list,pass Integer object

NOTE: It will only remove first occurence of the object

eg : arr.remove(new Integer(3)) -> this will remove first occurence of object with value 3

We saw that remove(int index) removes a method at the given index and remove(Object o) removes the given object from the ArrayList. Suppose we have an ArrayList that contains five elements i.e [13, 21, 43, 2, 9]. Now, if we do list.remove(2), then which overloaded method will be called. Will remove(int index) be called or remove(Object o) be called? remove(int index) will be called because we are passing a primitive to remove method. If we want to delete element 2, we should call remove(new Integer(2)) because elements are stored in an ArrayList as objects and not primitives.

  • list.removeRange(int fromIndex, int toIndex)
  • list.removeAll(Collection<?> c)

We can use the removeAll(Collection<?> c) method to remove, from the given list, all of the elements that are contained in the specified collection.

  • list.toArray(); // convert list to array
  • Collections.sort(list); //sorts in Ascending Order
  • Collections.sort(list, Collections.reverseOrder()); // Descending order

The Collections class contains a sort(List<T> list) method, which is used to sort an ArrayList. This method takes an ArrayList as input and sorts it in ascending order.

In the sort(List<T> list) method, T represents the type of object that is stored in the ArrayList. The Collections.sort(List<T> t) method takes an ArrayList of type T objects as the input. It is a must that T should implement the Comparable interface; otherwise, the code will not compile.

  • List<Integer> sortedList =; // USING STREAM
  • List<Integer> sortedList =


The ArrayList can be sorted in reverse order using streams bypassing Comparator.reverseOrder() to the sorted() method.

  • list.clear() //remove all element from the list
  • list.replaceAll(UnaryOperator<E> operator)


  • List indexes start from ‘0’, just like array index.
  • List allows “null”
  • List supports Generics and we should use it whenever possible. Using Generics with List will avoid ClassCastException at runtime.

copyOnWriteArrayList — thread-safe version of list

HashSet<Integer> set= new HashSet<>();

  • set.add(e);

If the element is not already present, then this method puts the element and returns true. If the element is already present, then it returns false.

  • set.contains(e);
  • set.remove(e);
  • set.clear(); // remove all elements in set
  • set.isEmpty();


Since a HashSet stores the elements in random order, it is not possible to store the elements in a HashSet in sorted order. If we want to sort the elements of a HashSet, then we should convert it into some other Collection such as a List,TreeSet, or LinkedHashSet.

HashSet does not allow duplicate elements.

HashSet allows only one null element.

The elements are inserted in random order in a HashSet.

A HashSet is internally backed by a HashMap.

initial capacity of 16 and a load factor of 0.75

TreeSet<Integer> set= new TreeSet<>();

Since all the elements are stored in sorted order in a TreeSet, storing elements should either implement the Comparable interface or a custom Comparator while creating the TreeSet.

  • set.add(e)
  • set.addAll(Collection c);
  • set.first() //Fetching the first element in TreeSet
  • set.last(); // Fetching the last element in TreeSet
  • set.headSet(40) // Fetching all the elements less than 40 | 40 is not inclusive.
  • set.tailSet(40) // Fetching all the elements greater than 40 | 40 is not inclusive.
  • set.remove(e);
  • set.isEmpty();
  • set.size();
  • set.conatins(e);

TreeSet does not allow duplicate elements.

TreeSet class doesn’t allow null elements.

Since elements are stored in a tree, the access and retrieval times are quite fast in a TreeSet.

The elements are stored in ascending order in a TreeSet.

Difference between a HashSet and TreeSet

  1. The HashSet allows one null element, whereas a TreeSet does not allow a null element.
  2. The elements are stored in random order in a HashSet, whereas it is stored in sorted order in TreeSet.
  3. HashSet is faster than Treeset for the operations like add, remove, contains, size, etc.

HashMap<String, Integer> map= new HashMap<>();

k = key, v = value

  • map.put(k,v);
  • map.putIfAbsent(k,v);
  • map.putAll(anotherMap);
  • map.get(k) //returns v mapped to k
  • map.getOrDefault(key , defaultValue);
  • map.remove(k) //returns v and removes it.
  • map.size();
  • map.containsKey(k);
  • map.containsValue(v);

Looping through a HashMap:

  • map.entrySet()
  • map.keySet() // This method returns a Set containing all the keys present in the Map.
  • map.values() // This method returns a Collection containing all the values present in the Map.

Replace :

  • map.replace(K key, V oldValue, V newValue);

// replace value of the key k if value of the key is same as old value

  • map.replace(K key,V newValue); // replace value of the key with new value
  • map.replaceAll( bifunction);

Using the replace(K key, V oldValue, V newValue) method#

The replace(K key, V oldValue, V newValue) method takes three parameters: the key, the old value, and a new value. It checks if the current value of the key is equal to the oldValue provided in the parameter. If yes then it replaces the value with newValue and returns true; otherwise, it returns false.

Using the replace(K key, V value) method#

This method takes only two parameters: a key and a value. It replaces the value of the key with the new value provided as a parameter and returns the old value. If the key is not present, then it returns null.

Using the replaceAll(BiFunction<? super K, ? super V, ? extends V> function) method#

This method takes a BiFunction as input and replaces the values of all the keys with the result of the given function. Suppose we need to add ten to the stock price of each company. Instead of updating the value for each stock one by one, we can use this method. The lambda expression to do this task will look like this:

(key, value) -> value + 10

The keys should be unique.

HashMap allows only one null key.

The values can be null or duplicate.

The keys are stored in random order.

an initial capacity of 16 and load factor of 0.75

TreeMap: TreeMap is a class in the java.utils package that stores the keys in sorted order. Some of the features of TreeMap are:

TreeMap<String, Integer> treeMap = new TreeMap<>();

TreeMap<String, Integer> reverseMap = new TreeMap<>(Comparator.reverseOrder());

  • put(k,v);
  • putAll(map);
  • get(k);
  • remove(k)
  • firstKey() // gives smallest key ( since TreeMap stores elements in sorted order)
  • firstEntry() // give smallest Entry
  • lastKey() // give largest key
  • lastEntry() // giva largest entry
  • replace(key,value);
  • replace(key,oldValue,newValue);

The entries in TreeMap are sorted in the natural ordering of its keys.

It does not allow null keys, however there can be null values.

The TreeMap is not thread-safe, although it can be made thread-safe using the synchronizedMap() method of the Collections class.

Since a TreeMap stores the keys in sorted order, the objects that we are storing in the TreeMap should either implement the Comparable interface or we should pass a Comparator while creating the TreeMap object.

PriorityQueue<Integer> pq = new PriorityQueue<>()

PriorityQueue<Integer> minHeap = new PriorityQueue<Integer>();

PriorityQueue<Integer> maxHeap = new PriorityQueue<Integer>((a,b) -> b-a); //(max elements on top/descending order)

Let’s say you want your own comparator, example, the pq has keys of a hashmap as it’s elements, and they need to be arranged according to their values in a hashmap.

pq = new PriorityQueue<>((a,b) -> map.get(a)-map.get(b));

You can define your own comparator separately( named MyComparator for example):

pq = new PriorityQueue<>(new MyComparator());

String s = “hello”;

  • s.toCharArray();
  • s.indexOf(substring);
  • s.chatAt(i);
  • s.toUpperCase();
  • s1.equalsIgnoreCase(s2);
  • s1.compareTo(s2); //returns s1-s2, in dictionary order so a-b returns -1
  • s1.contains(s2);
  • s.length();
  • s.startsWith(“hell”); //s.endsWith(“ello”), returns boolean
  • s.substring(incl,excl); // incl: inclusive index, included, excl: exclusive index, excluded
  • str_array = s.split(“ “);//return string array separated by spaces. “Hello world” returns [“Hello”,”world”]

Iterator itr = array_name.iterator(); //or list.iterator()

  • itr.hasNext(); //returns bool
  • itr.remove(); //removes curr element

Random r = new Random();

  • r.nextInt(n); //return a random int from 0 to n


  • Math.random(); //returns a random double between 0.0 and 1.0

Stack<Integer> s = new Stack<>();

  • s.push();
  • s.peek();
  • s.pop();
  • s.empty(); //returns bool
  • s.size();

Queue<Integer> q = new ArrayList<>();

  • q.add(e);
  • q.remove(); //throws exception if empty
  • q.poll(); //same as remove but returns null if empty
  • q.peek();
  • q.size();


Other Useful Methods:

  • Character.isDigit(c); //returns bool
  • Character.isAlphabetic(c);
  • Integer.parseInt(str); //if str = “1234”, it returns integer 1234.
  • Float.parseFloat(str); //same as above for floats
  • Integer.toBinaryString(num); //returns binary representation of num, input = 10 returns “1010”
  • Arrays.asList(arr); //converts array to list
  • Integer.MIN_VALUE //returns least possible int in Java
  • object.hashCode(); //returns hashcode value for object
  • IntegerList.get(i).intValue(); // to convert Integer to int;


  • Arrays.sort(arr, (a.b)->b-a); //sorts in descending order
  • Defining MySort to use in other sorting methods (example sort arrays based on their first element)

And then using it like:

  • Arrays.sort(arr, MySort)
  • For ArrayLists, Collections.sort(arraylist, MyArrListSort());

Comparable VS Comparator

the Collections.sort() method sorts the given List in ascending order. But the question is, how does the sort() method decide which element is smaller and which one is larger?

Each wrapper class(Integer, Double, or Long), String class, and Date class implements an interface called Comparable. This interface contains a compareTo(T o) method which is used by sorting methods to sort the Collection. This method returns a negative integer, zero, or a positive integer if this object is less than, equal to, or greater than the object passed as an argument.

If we use the Collections.sort(List<T> list) method to sort an ArrayList, then the class whose objects are stored in the ArrayList must implement the Comparable interface. If the ArrayList stores an Integer, a Long, or a String, then we don’t need to worry as these classes already implement the Comparable interface. But if the ArrayList stores a custom class object, then that class must implement the Comparable interface.

If we need some flexibility in sorting, we should use the Comparator interface instead of the Comparable interface. The Comparator interface has a method, compare(T o1, T o2), which takes two objects, o1 and o2 as parameters. It returns -1 if o1 << o2, 1 if o1 >> o2 and 0 if o1 is equal to o2.

If we need to use the Comparator interface, then we can’t use the Collections.sort(List<T> t) method as T should implement the Comparable interface. There is another overloaded method, sort(List<T> list, Comparator<? super T> c), that takes the list as well as a Comparator object as input. It then sorts the List on the basis of logic, which is provided in the Comparator implementation.

Associate Software Engineer | JP Morgan & Chase