Java 的 Collections 类

Java 的 Collections 类

java.util.Collections 类提供了许多静态方法来操作集合（Collections）。以下是其中一些主要的方法，并附有相应的示例：

1. sort(List<T> list) / sort(List<T> list, Comparator<? super T> c)

   // 对列表进行排序（自然顺序）
   List<Integer> numbers = new ArrayList<>(Arrays.asList(3, 1, 4, 1, 5, 9, 2, 6, 5));
   Collections.sort(numbers);
   System.out.println(numbers); // 输出: [1, 1, 2, 3, 4, 5, 5, 6, 9]
   
   // 使用自定义Comparator对列表进行排序
   List<String> words = new ArrayList<>(Arrays.asList("apple", "orange", "banana", "grape"));
   Collections.sort(words, Comparator.comparing(String::length));
   System.out.println(words); // 输出: [grape, apple, orange, banana]
   

2. binarySearch(List<? extends Comparable<? super T>> list, T key) / binarySearch(List<? extends T> list, T key, Comparator<? super T> c)

   // 在已排序的列表中进行二分查找
   List<Integer> sortedNumbers = new ArrayList<>(Arrays.asList(1, 3, 5, 7, 9));
   int index = Collections.binarySearch(sortedNumbers, 5);
   System.out.println("Index of 5: " + index); // 输出: Index of 5: 2
   
   // 在自定义排序后的列表中进行二分查找
   List<String> sortedWords = new ArrayList<>(Arrays.asList("apple", "orange", "banana", "grape"));
   Collections.sort(sortedWords, Comparator.comparing(String::length));
   int indexCustom = Collections.binarySearch(sortedWords, "banana", Comparator.comparing(String::length));
   System.out.println("Index of 'banana': " + indexCustom); // 输出: Index of 'banana': 2
   

3. reverse(List<?> list)

   // 反转列表中的元素顺序
   List<Character> characters = new ArrayList<>(Arrays.asList('a', 'b', 'c', 'd', 'e'));
   Collections.reverse(characters);
   System.out.println(characters); // 输出: [e, d, c, b, a]
   

4. shuffle(List<?> list)

   // 打乱列表中的元素顺序
   List<Integer> shuffledNumbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));
   Collections.shuffle(shuffledNumbers);
   System.out.println(shuffledNumbers); // 输出: [3, 1, 5, 2, 4]（随机顺序）
   

5. fill(List<? super T> list, T obj)

   // 将列表中的所有元素替换为指定的对象
   List<String> strings = new ArrayList<>(Arrays.asList("apple", "orange", "banana"));
   Collections.fill(strings, "grape");
   System.out.println(strings); // 输出: [grape, grape, grape]
   

6. copy(List<? super T> dest, List<? extends T> src)

   // 将一个列表的元素复制到另一个列表
   List<Integer> source = new ArrayList<>(Arrays.asList(1, 2, 3));
   List<Integer> destination = new ArrayList<>(Arrays.asList(0, 0, 0, 0, 0));
   Collections.copy(destination, source);
   System.out.println(destination); // 输出: [1, 2, 3, 0, 0]
   

7. addAll(Collection<? super T> c, T... elements)

   // 将指定元素添加到集合
   List<String> fruits = new ArrayList<>(Arrays.asList("apple", "orange"));
   Collections.addAll(fruits, "banana", "grape");
   System.out.println(fruits); // 输出: [apple, orange, banana, grape]
   

8. frequency(Collection<?> c, Object o)

   // 获取集合中指定元素的出现次数
   List<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 1, 2, 1, 3, 4));
   int frequencyOfOne = Collections.frequency(numbers, 1);
   System.out.println("Frequency of 1: " + frequencyOfOne); // 输出: Frequency of 1: 3
   

这些方法使得对集合进行排序、搜索、反转、填充等常见操作变得更加简单和方便。