java Collections 工具类

Posted 2020-10-13

1.reverse反转
2.shuffle随机排序
3.sort自然排序
4.sort指定比较器排序
5.swap将下标位置为x和y的元素进行交换
6.max 最大值
7.min 最小值
8.frequency 计算元素个数
9.copy复制List
10.replaceAll 替换所有元素

import java.util.*;

/**
 * Created by meicai on 2017/11/6.
 */
public class TestCollections {
    public static  void  main(String[] args){
        test10();
    }

    /*reverse反转
    44
    85
    22
    123
     */
    private static void test1() {
        List<Integer> numList= Arrays.asList(123,22,85,44);
        Collections.reverse(numList);
        numList.forEach(System.out::println);
    }

    /**
     * shuffle随机排序
     *22
     85
     123
     44
     */
    private static void test2() {
        List<Integer> numList= Arrays.asList(123,22,85,44);
        Collections.shuffle(numList);
        numList.forEach(System.out::println);
    }

    /**3.sort自然排序
     * 22
     44
     85
     123
     */
    private static void test3() {
        List<Integer> numList= Arrays.asList(123,22,85,44);
        Collections.sort(numList);
        numList.forEach(System.out::println);
    }

    /**
     * sort指定比较器排序，使用匿名内部类
     * 123
     85
     44
     22
     */
    private static void test4_1() {
        List<Integer> numList= Arrays.asList(123,22,85,44);
        Collections.sort(numList, new Comparator<Integer>() {
                    @Override
                    public int compare(Integer o1, Integer o2) {
                        return -o1.compareTo(o2);
                    }
                }
                );
                numList.forEach(System.out::println);
    }

    /**
     * sort指定比较器排序，使用java8的lambda表达式
     * 123
     85
     44
     22
     */
    private static void test4_2() {
        List<Integer> numList= Arrays.asList(123,22,85,44);
        Collections.sort(numList,(t1,t2)->{return -t1.compareTo(t2);});
        numList.forEach(System.out::println);
    }

    /**
     * swap将下标位置为x和y的元素进行交换
     * 123
     44
     85
     22
     */
    private static void test5() {
        List<Integer> numList= Arrays.asList(123,22,85,44);
        Collections.swap(numList,1,3);
        numList.forEach(System.out::println);
    }

    /**
     * max
     * max:123
     */
    private static void test6() {
        List<Integer> numList= Arrays.asList(123,22,85,44);
        Integer max=  Collections.max(numList);
        System.out.println("max:"+max);
    }

    /**
     * frequency返回每个元素在集合中的个数
     *count:2
     */
    private static void test7() {
        List<Integer> numList= Arrays.asList(123,22,85,44,22);
        int count=  Collections.frequency(numList,22);
        System.out.println("count:"+count);
    }

    /**
     * copy 异常，错误使用方式
     *Exception in thread "main" java.lang.IndexOutOfBoundsException: Source does not fit in dest
     */
    private static void test8() {
        List<Integer> numList= Arrays.asList(123,22,85,44,22);
        List<Integer> numListCopy=new ArrayList<>();
        Collections.copy(numListCopy,numList);
        numListCopy.forEach(System.out::println);
    }

    /**copy 正确使用方式
     * 123
     22
     85
     44
     22
     */
    private static void test9() {
        List<Integer> numList= Arrays.asList(123,22,85,44,22);
        List<Integer> numListCopy=Arrays.asList(new Integer[numList.size()]);
        Collections.copy(numListCopy,numList);
        numListCopy.forEach(System.out::println);
    }

    /**
     * replaceAll 替换所有元素
     * 123
     222
     85
     44
     222
     */
    private static void test10() {
        List<Integer> numList= Arrays.asList(123,22,85,44,22);
        Collections.replaceAll(numList,22,222);
        numList.forEach(System.out::println);
    }
}

基本方式使用demo

 

11.从线程不安全的list转成成线程安全的list

private static void test11() {
        List<Integer> numList= Arrays.asList(123,22,85,44);
        List<Integer> synchronizedList= Collections.synchronizedList(numList);
        synchronizedList.forEach(System.out::println);
    }

从线程不安全的list转成成线程安全的list

12.Enumeration的使用

  /**
     * Enumeration的使用
     * a1
     a2
     a3
     */
    private static void test12() {
        Enumeration e=new StringTokenizer("a1-a2-a3","-");
        while (e.hasMoreElements()){
            System.out.println(e.nextElement());
        }
    }

Enumeration的使用