Collection Methods

Java Core Libraries Cheat Sheet

This cheat sheet lists commonly used methods from core Java libraries (java.util, java.lang, java.util.stream, etc.) for typical data structure and backend tasks. Each section covers a topic with a table of key functions, descriptions, and one-line example usage.

Collections

#	Method/Function	Description	Example
1	Collections.sort(List<T>)	Sorts the list into natural (ascending) order.	Collections.sort(list);
2	Collections.reverse(List<?>)	Reverses the order of elements in the list.	Collections.reverse(list);
3	Collections.shuffle(List<?>)	Randomly permutes (shuffles) the list.	Collections.shuffle(list);
4	Collections.binarySearch(List<?>, key)	Searches for key in a sorted list; returns index if found, or -(insertionPoint)-1 if not.	int idx = Collections.binarySearch(list, key);
5	Collections.max(Collection<? extends T>)	Returns the maximum element of the collection (natural order or by supplied comparator).	Collections.max(list);
6	Collections.min(Collection<? extends T>)	Returns the minimum element of the collection.	Collections.min(list);
7	Collections.frequency(Collection<?>, Object)	Counts how many times the specified element appears in the collection.	Collections.frequency(list, elem);
8	Collections.fill(List<? super T>, T)	Replaces all elements in the list with the given value.	Collections.fill(list, 0);
9	Collections.replaceAll(List<T>, T oldVal, T newVal)	Replaces all occurrences of oldVal with newVal.	Collections.replaceAll(list, 1, 100);

List

#	Method/Function	Description	Example
1	List.add(E)	Appends the element to the end of the list.	list.add("item");
2	List.add(int, E)	Inserts the element at the given index.	list.add(0, "first");
3	List.get(int)	Returns the element at the specified index.	String s = list.get(2);
4	List.set(int, E)	Replaces the element at index with a new value.	list.set(1, "new");
5	List.remove(int)	Removes the element at the specified index.	list.remove(0);
6	List.size()	Returns the number of elements.	int n = list.size();
7	List.isEmpty()	Checks if the list has no elements.	boolean empty = list.isEmpty();
8	List.contains(Object)	Checks if the list contains the given element.	boolean has = list.contains("item");
9	List.indexOf(Object)	Returns the index of the first occurrence or -1 if not found.	int i = list.indexOf("item");
10	List.subList(int, int)	Returns a view between indices.	List<String> part = list.subList(1, 3);
11	List.sort(Comparator<? super E>)	Sorts using comparator or natural order. (Java 8+)	list.sort(Comparator.naturalOrder());
12	List.removeIf(Predicate<? super E>)	Removes all elements that match predicate. (Java 8+)	list.removeIf(x -> x < 0);

Set

#	Method/Function	Description	Example
1	Set.add(E)	Adds the element (ignores duplicates).	set.add(42);
2	Set.remove(Object)	Removes element.	set.remove(42);
3	Set.contains(Object)	Checks if contains.	boolean ok = set.contains(5);
4	Set.size()	Returns number of elements.	int n = set.size();
5	Set.isEmpty()	Checks if empty.	boolean empty = set.isEmpty();
6	Set.clear()	Removes all elements.	set.clear();
7	Set.removeIf(Predicate<? super E>)	Removes elements matching predicate. (Java 8+)	set.removeIf(x -> x % 2 == 0);
8	Set.retainAll(Collection<?>)	Keeps only elements present in another collection (intersection).	set.retainAll(otherSet);

Map

#	Method/Function	Description	Example
1	Map.put(K, V)	Associates value with key.	map.put("key", 100);
2	Map.get(Object)	Returns value for key or null.	int v = map.get("key");
3	Map.containsKey(Object)	Checks if key exists.	map.containsKey("key");
4	Map.size()	Returns entry count.	int n = map.size();
5	Map.isEmpty()	Checks if map has no entries.	map.isEmpty();
6	Map.remove(Object)	Removes mapping for key.	map.remove("key");
7	Map.remove(Object, Object)	Removes entry if key mapped to value.	map.remove("key", 100);
8	Map.keySet()	Returns keys as Set.	Set<String> keys = map.keySet();
9	Map.values()	Returns values as Collection.	map.values();
10	Map.putIfAbsent(K,V)	Adds mapping if key missing. (Java 8+)	map.putIfAbsent("key", 50);
11	Map.getOrDefault(K, V)	Returns value or default. (Java 8+)	map.getOrDefault("key", 0);
12	Map.computeIfAbsent(K, Function)	Computes value if absent. (Java 8+)	map.computeIfAbsent("k", k -> new ArrayList<>());
13	Map.merge(K, V, BiFunction)	Merges values. (Java 8+)	map.merge("k", 1, Integer::sum);
14	Map.forEach(BiConsumer)	Iterates entries. (Java 8+)	map.forEach((k,v)->System.out.println(k+":"+v));

Queue

#	Method/Function	Description	Example
1	Queue.add(E)	Inserts element (throws if full).	queue.add(10);
2	Queue.offer(E)	Inserts element (false if full).	queue.offer(20);
3	Queue.poll()	Retrieves/removes head or null if empty.	queue.poll();
4	Queue.peek()	Retrieves head without removal.	queue.peek();
5	Queue.remove()	Removes head (throws if empty).	queue.remove();
6	Queue.element()	Returns head (throws if empty).	queue.element();
7	Queue.size()	Returns count.	queue.size();
8	Queue.isEmpty()	Checks if empty.	queue.isEmpty();

String

#	Method/Function	Description	Example
1	String.length()	Returns length.	"Hello".length();
2	String.charAt(int)	Returns char at index.	"Hello".charAt(1);
3	String.substring(int,int)	Returns substring.	"Hello".substring(1,4);
4	String.indexOf(String)	First occurrence index.	"abcabc".indexOf("bc");
5	String.lastIndexOf(String)	Last occurrence index.	"abcabc".lastIndexOf("bc");
6	String.contains(CharSequence)	Checks substring presence.	"Hello".contains("ell");
7	String.startsWith(String)	Checks prefix.	"Hello".startsWith("He");
8	String.endsWith(String)	Checks suffix.	"Hello".endsWith("lo");
9	String.equals(Object)	Compares for equality.	"a".equals("A");
10	String.equalsIgnoreCase(String)	Ignores case.	"a".equalsIgnoreCase("A");
11	String.toLowerCase()	Converts to lowercase.	"Hello".toLowerCase();
12	String.toUpperCase()	Converts to uppercase.	"Hello".toUpperCase();
13	String.trim()	Removes whitespace.	" Hi ".trim();
14	String.replace(CharSequence, CharSequence)	Replaces literal text.	"aabb".replace("a","c");
15	String.replaceAll(String, String)	Replaces using regex.	"abc123".replaceAll("\\d","");
16	String.split(String)	Splits using regex.	"a,b,c".split(",");
17	String.join(CharSequence, CharSequence...)	Joins with delimiter. (Java 8+)	String.join("-", "a","b","c");
18	String.format(String,Object...)	Formats string.	String.format("Hi %s", "Bob");

Arrays

#	Method/Function	Description	Example
1	Arrays.sort(int[])	Sorts ascending.	Arrays.sort(arr);
2	Arrays.sort(T[], Comparator)	Sorts with comparator.	Arrays.sort(arr, Collections.reverseOrder());
3	Arrays.binarySearch(int[], key)	Searches sorted array.	Arrays.binarySearch(arr, key);
4	Arrays.equals(int[], int[])	Compares arrays.	Arrays.equals(a,b);
5	Arrays.toString(Object[])	Returns string form.	Arrays.toString(arr);
6	Arrays.asList(T...)	Returns fixed-size List view.	Arrays.asList(1,2,3);
7	Arrays.copyOf(T[], int)	Copies to new length.	Arrays.copyOf(arr,5);
8	Arrays.copyOfRange(T[], int, int)	Copies range.	Arrays.copyOfRange(arr,0,3);
9	Arrays.fill(T[], T)	Fills array.	Arrays.fill(arr,0);
10	Arrays.parallelSort(int[])	Parallel sort (Java 8+)	Arrays.parallelSort(arr);
11	Arrays.stream(T[])	Creates Stream. (Java 8+)	Arrays.stream(arr);

Streams (Java 8+)

#	Method/Function	Description	Example
1	Collection.stream()	Creates stream.	list.stream()
2	Stream.filter(Predicate)	Filters elements.	list.stream().filter(n -> n>0)
3	Stream.map(Function)	Maps elements.	list.stream().map(String::length)
4	Stream.flatMap(Function)	Flattens nested streams.	listOfLists.stream().flatMap(List::stream)
5	Stream.distinct()	Removes duplicates.	list.stream().distinct()
6	Stream.sorted()	Sorts elements.	list.stream().sorted()
7	Stream.limit(long)	Limits count.	list.stream().limit(5)
8	Stream.skip(long)	Skips first n.	list.stream().skip(2)
9	Stream.forEach(Consumer)	Iterates elements.	list.stream().forEach(System.out::println)
10	Stream.collect(Collector)	Collects to container.	list.stream().collect(Collectors.toList())
11	Stream.reduce(T, BinaryOperator)	Reduces values.	list.stream().reduce(0, Integer::sum)
12	Stream.count()	Counts elements.	list.stream().count()
13	Stream.anyMatch(Predicate)	Any matches predicate.	list.stream().anyMatch(x->x>5)
14	Stream.allMatch(Predicate)	All match predicate.	list.stream().allMatch(x->x>0)
15	Stream.findFirst()	Returns first element Optional.	list.stream().findFirst()
16	Stream.min(Comparator)	Minimum element.	list.stream().min(Integer::compareTo)
17	Stream.max(Comparator)	Maximum element.	list.stream().max(Integer::compareTo)

Optional (Java 8+)

#	Method/Function	Description	Example
1	Optional.of(T)	Creates non-null Optional.	Optional.of("test")
2	Optional.ofNullable(T)	Creates Optional or empty.	Optional.ofNullable(maybeNull)
3	Optional.empty()	Empty Optional.	Optional.empty()
4	Optional.isPresent()	Checks if value present.	opt.isPresent()
5	Optional.ifPresent(Consumer)	Runs if present.	opt.ifPresent(x->System.out.println(x));
6	Optional.orElse(T)	Returns value or default.	opt.orElse("fallback");
7	Optional.orElseGet(Supplier)	Returns value or supplier result.	opt.orElseGet(() -> "fallback");
8	Optional.orElseThrow(Supplier)	Throws exception if empty.	opt.orElseThrow(() -> new RuntimeException("No value"));
9	Optional.filter(Predicate)	Filters value.	opt.filter(x -> x.startsWith("A"));
10	Optional.map(Function)	Transforms if present.	opt.map(String::toUpperCase);
11	Optional.flatMap(Function)	Flattens nested Optional.	opt.flatMap(x -> Optional.of(x.length()));
12	Optional.ifPresentOrElse(Consumer,Runnable)	Runs consumer or else. (Java 9+)	opt.ifPresentOrElse(x->System.out.println(x),()->System.out.println("Empty"));