Collections工具类

引言：Collections工具类

Collections是Java提供的操作集合的工具类，提供了大量静态方法用于操作集合，如排序、查找、填充、反转等。

排序操作

sort() - 排序

import java.util.*;

List list = new ArrayList<>();
list.add(5);
list.add(2);
list.add(8);
list.add(1);

System.out.println("排序前: " + list);
Collections.sort(list);
System.out.println("排序后: " + list);

reverseOrder() - 反向排序

List list = Arrays.asList(5, 2, 8, 1);

// 反向排序
Collections.sort(list, Collections.reverseOrder());
System.out.println("反向排序: " + list);

shuffle() - 随机打乱

List list = Arrays.asList(1, 2, 3, 4, 5);
System.out.println("打乱前: " + list);
Collections.shuffle(list);
System.out.println("打乱后: " + list);

查找操作

binarySearch() - 二分查找

List list = Arrays.asList("Apple", "Banana", "Orange");
Collections.sort(list);

int index = Collections.binarySearch(list, "Banana");
System.out.println("Banana的索引: " + index);

max() / min() - 最大值最小值

List list = Arrays.asList(5, 2, 8, 1, 9);

int max = Collections.max(list);
int min = Collections.min(list);

System.out.println("最大值: " + max);
System.out.println("最小值: " + min);

批量操作

fill() - 填充

List list = new ArrayList<>(Arrays.asList("a", "b", "c", "d"));
System.out.println("填充前: " + list);

Collections.fill(list, "X");
System.out.println("填充后: " + list);  // [X, X, X, X]

addAll() - 批量添加

List list = new ArrayList<>();
Collections.addAll(list, "A", "B", "C", "D");
System.out.println("批量添加后: " + list);

copy() - 复制

List source = Arrays.asList("A", "B", "C");
List dest = new ArrayList<>(Arrays.asList("X", "Y", "Z", "W"));

Collections.copy(dest, source);
System.out.println("复制后: " + dest);

集合替换和反转

replaceAll() - 替换所有

List list = new ArrayList<>(Arrays.asList("A", "B", "A", "C", "A"));
Collections.replaceAll(list, "A", "X");
System.out.println("替换后: " + list);

reverse() - 反转

List list = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));
Collections.reverse(list);
System.out.println("反转后: " + list);

rotate() - 旋转

List list = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));
Collections.rotate(list, 2);
System.out.println("旋转2位后: " + list);

集合同步控制

Collections提供了将普通集合转换为线程安全集合的方法：

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

Map map = new HashMap<>();
Map synchronizedMap = Collections.synchronizedMap(map);

Set set = new HashSet<>();
Set synchronizedSet = Collections.synchronizedSet(set);

不可变集合

Collections提供了创建不可变（只读）集合的方法：

// 创建不可变列表
List unmodifiableList = Collections.unmodifiableList(
    Arrays.asList("A", "B", "C")
);

// 尝试修改会抛出UnsupportedOperationException
// unmodifiableList.add("D");  // 运行时异常！

小结