Java collection cheat sheet

Collection trong java
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For java_intermediate course. https://giangtester.com/khoa-hoc-java-intermediate/

I. Collection

  • is a “bag” = unordered lists
  • No direct implem­ent­ation
Collection<String> names = new ArrayList<>();

- Add one element
names.add("john");
System.out.println(names); //[john]

- Add multiple elements
names.addAll(List.of("james, harry"));
System.out.println(names); //[james, harry]

-----------------------------------------

Collection<String> names = new ArrayList<>(List.of("john", "james", "harry"));

- Remove one element
names.remove("john");
System.out.println(names); //[james, harry]

- Remove multiple elements
names.removeAll(List.of("james", "harry"));
System.out.println(names); //[john]

- Remove based on a condition
names.removeIf(s -> s.startsWith("j"));
System.out.println(names); //[harry]

- Remove all elements
names.clear();
System.out.println(names); //[]

- Remove the differences
Collection<String> names2 = new ArrayList<>(List.of("john", "james", "Steve"));
names.retainAll(names2);
System.out.println(names); //[john, james]

-----------------------------------------

Collection<String> names = new ArrayList<>(List.of("john", "james", "harry"));

- Print each element
names.forEach(System.out::println); //john james harry

- Get size
System.out.println(names.size()); //3

- Check empty
System.out.println(names.isEmpty()); //false

- Check contain one element
System.out.println(names.contains("harry")); //true

- Check contain multiple elements
System.out.println(names.containsAll(List.of("harry", "john"))); //true

-----------------------------------------

Collection<String> names = new ArrayList<>(List.of("john", "james", "harry"));

- Convert to array with array type
String[] strings = names.toArray(new String[]{});
System.out.println(Arrays.toString(strings)); //[john, james, harry]

II. List

  • A collection in which order is signif­icant
  • Allow duplicate elements
  • Insert and get element fast → ArrayList
  • Insert and remove fast → LinkedList
  • When need multi-­thread → CopyOn­Wri­teA­rra­yList

Methods

List contains all methods that defines in collection interface, and some methods work with index.

List<String> chars = new ArrayList<>(List.of("a", "b", "c"));

- Add an element at position
chars.add(2, "new");
System.out.println(chars); //[a, b, new, c]

- Add multiple elements at position
chars.addAll(2, List.of("newA", "newB"));
System.out.println(chars); //[a, b, newA, newB, c]

- Edit an element at position
chars.set(2, "newA");
System.out.println(chars); //[a, b, newA]

- Get an element at position
System.out.println(chars.get(0)); //a

- Check index of an element
System.out.println(chars.indexOf("b")); //1

- Remove an element at position
chars.remove(1);
System.out.println(chars); //[a, c]

-------------------------------------------------

- Create an unmodified list
List<String> chars = List.of("a", "b", "c");

- Create list from an unmodified list
List<String> chars = new ArrayList<>(List.of("a", "b", "c"));

- Sort a list a --> z
List<String> chars = new ArrayList<>(List.of("b", "a", "c"));
chars.sort(Comparator.naturalOrder());
System.out.println(chars); //[a, b, c]

- Sort a list z --> a
chars.sort(Comparator.reverseOrder());
System.out.println(chars); //[c, b, a]

- Replace/Update all elements
List<String> chars = new ArrayList<>(List.of("a", "b", "c"));
chars.replaceAll(s -> s + "_1");
System.out.println(chars); //[a_1, b_1, c_1]

III. Set

  • No duplicates
  • Order is not signif­icant
  • No more methods than Collection interface
  • Don’t care order → HashSet
  • Keep insertion order → Linked­HashSet
  • Store Enum in a Set → EnumSet
  • When need multi-­thread, size small, read more – less write → CopyOn­Wri­teA­rraySet
  • Auto sort element → TreeSet
  • When need multi-­thread + Auto sort → Concur­ren­tSk­ipL­istSet

Methods

- Create a set
Set<String> uniqueNames = new HashSet<>();

- Create an unmodified set
Set<String> uniqueNames = Set.of("b", "a", "c");
System.out.println(uniqueNames); //[a, b, c]

- Create a set from a list
Set<String> uniqueNames = new HashSet<>(List.of("a", "b", "a", "c"));
System.out.println(uniqueNames); //[a, b, c]

IV. Map

  • key-value associ­ations to store and retrieve elements
  • Normal map → HashMap
  • Keep insertion order → Linked­HashMap
  • Key is Enum → EnumMap
  • Auto sort → TreeMap
  • When multi-­thread → Concur­ren­tHa­shMap

Methods

- Create an normal map
Map<Integer, String> names = new HashMap<>();
Map<String, Person> names = new HashMap<>();

- Create an unmodified map
Map<Integer, String> names = Map.of(1, "john", 2, "james");

- Create a modified map from an unmodified map
Map<Integer, String> names = new HashMap<>(Map.of(1, "john", 2, "james"));

---------------------------------------

Map<Integer, String> names = new HashMap<>();

- Add one element
names.put(1, "john");
System.out.println(names); //{1=john}

- Add multiple elements
names.putAll(Map.of(1, "john", 2, "james"));
System.out.println(names); //{1=john, 2=james}

- Get value by key
String name = names.get(1);
System.out.println(name); //john

- Get value by key, if don't exist return default value
String name = names.getOrDefault(3, "NOT_FOUND");
System.out.println(name); //NOT_FOUND

---------------------------------------

Map<Integer, String> names = new HashMap<>(Map.of(1, "john", 2, "james"));

- Get all keys
Set<Integer> keys = names.keySet();
System.out.println(keys); //[1, 2]

- Get all values
Collection<String> values = names.values();
System.out.println(values); //[john, james]

- Print key-value
names.forEach((k, v) -> System.out.printf("k=%s, v=%s %n", k, v));
//k=1, v=john 
//k=2, v=james 
---------------------------------------

- Update multiple values
Map<Integer, String> names = new HashMap<>();
for (int i = 0; i < 4; i++) {
    names.compute(i, (k, v) -> "john_" + k);
}
System.out.println(names); // {0=john_0, 1=john_1, 2=john_2, 3=john_3}
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