Java Collections工具类 #

List<Integer> list = new ArrayList<>(Arrays.asList(3, 1, 4, 1, 5, 9));

// 自然排序
Collections.sort(list);
System.out.println(list);  // [1, 1, 3, 4, 5, 9]

// 自定义排序
Collections.sort(list, (a, b) -> b - a);  // 降序
System.out.println(list);  // [9, 5, 4, 3, 1, 1]

List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));

Collections.reverse(list);
System.out.println(list);  // [5, 4, 3, 2, 1]

List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));

Collections.shuffle(list);
System.out.println(list);  // 随机顺序

List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));

Collections.swap(list, 0, 4);  // 交换索引0和4的元素
System.out.println(list);  // [5, 2, 3, 4, 1]

List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));

Collections.rotate(list, 2);  // 向右旋转2位
System.out.println(list);  // [4, 5, 1, 2, 3]

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);

// 必须先排序
int index = Collections.binarySearch(list, 3);
System.out.println(index);  // 2

// 未找到返回负数
int index2 = Collections.binarySearch(list, 6);
System.out.println(index2);  // -6

List<Integer> list = Arrays.asList(3, 1, 4, 1, 5, 9);

System.out.println(Collections.max(list));  // 9
System.out.println(Collections.min(list));  // 1

// 自定义比较
System.out.println(Collections.max(list, Comparator.reverseOrder()));  // 1

List<Integer> list = Arrays.asList(1, 2, 2, 3, 3, 3);

int count = Collections.frequency(list, 3);
System.out.println(count);  // 3

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
List<Integer> subList = Arrays.asList(3, 4);

int index = Collections.indexOfSubList(list, subList);
System.out.println(index);  // 2

List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));

Collections.fill(list, 0);
System.out.println(list);  // [0, 0, 0, 0, 0]

List<Integer> src = Arrays.asList(1, 2, 3);
List<Integer> dest = Arrays.asList(4, 5, 6);

Collections.copy(dest, src);
System.out.println(dest);  // [1, 2, 3]

List<Integer> list = new ArrayList<>();

Collections.addAll(list, 1, 2, 3, 4, 5);
System.out.println(list);  // [1, 2, 3, 4, 5]

List<Integer> list = Collections.nCopies(5, 1);
System.out.println(list);  // [1, 1, 1, 1, 1]

List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3));
List<Integer> unmodifiable = Collections.unmodifiableList(list);

// unmodifiable.add(4);  // UnsupportedOperationException

Set<Integer> set = Collections.unmodifiableSet(new HashSet<>(Arrays.asList(1, 2, 3)));
Map<String, Integer> map = Collections.unmodifiableMap(new HashMap<>());

List<String> emptyList = Collections.emptyList();
Set<String> emptySet = Collections.emptySet();
Map<String, String> emptyMap = Collections.emptyMap();

Set<String> set = Collections.singleton("A");
List<String> list = Collections.singletonList("A");
Map<String, String> map = Collections.singletonMap("A", "1");

List<String> list = new ArrayList<>();
List<String> syncList = Collections.synchronizedList(list);

// 线程安全的List
syncList.add("A");

Set<String> syncSet = Collections.synchronizedSet(new HashSet<>());
Map<String, String> syncMap = Collections.synchronizedMap(new HashMap<>());

List<Integer> list1 = Arrays.asList(1, 2, 3);
List<Integer> list2 = Arrays.asList(4, 5, 6);

boolean noCommon = Collections.disjoint(list1, list2);
System.out.println(noCommon);  // true

List<String> list = new ArrayList<>(Arrays.asList("A", "B", "A"));

Collections.replaceAll(list, "A", "C");
System.out.println(list);  // [C, B, C]

List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));

Collections.sort(list, Collections.reverseOrder());
System.out.println(list);  // [5, 4, 3, 2, 1]