Java_Intermediate: Collection Cheat Sheet

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]

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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]

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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

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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]

- **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]

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

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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]

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- **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]

Some methods to work with map

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- **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"));

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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

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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 
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- **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}