可变参数以及不可变集合

可变参数：

格式：

public class ArgsDemo {
    public static void main(String[] args) {

        System.out.println(getSum(1,2,3,4,5));
    }

    //可变参数
    public static int getSum(int...args){
        int sum = 0;
        for (int arg : args) {
            sum += arg;
        }
        return sum;
    }
}

可变参数的形参中只能写一个可变参数

Collections集合类

java.util.Collections：是集合工具类

作用：Collections不是集合，而是集合工具类

常用API：

图来自黑马程序员网课

package com.lazyGirl.collectionsdemo;

import java.util.ArrayList;
import java.util.Collections;

public class Demo1 {
    public static void main(String[] args) {

        ArrayList<String> list = new ArrayList<>();
        Collections.addAll(list,"abc","bcd","quen");
        System.out.println(list);

        Collections.shuffle(list);
        System.out.println(list);

        
    }
}

输出：

package com.lazyGirl.collectionsdemo;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Random;

public class Demo2 {
    public static void main(String[] args) {

        ArrayList<String> list = new ArrayList<>();

        Collections.addAll(list, "a", "b", "c");


        ArrayList<String> list1 = new ArrayList<>();
        Random rand = new Random();

        for (int i = 0; i < 10; i++) {
            int count = list.size();
            for (int j = 0; j < count; j++) {
                int index = rand.nextInt(list.size());
                String name = list.remove(index);
                list1.add(name);
                System.out.println(name);
            }

            list.addAll(list1);
            list1.clear();
        }


    }
}

不可变集合

应用场景：

不想让别人修改集合中的内容

书写格式：

图来自黑马程序员网课

package com.lazyGirl.immytable;

import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

public class Demo1 {
    public static void main(String[] args) {
        List<String> list = List.of("zs","ls","ww","zl");
//        System.out.println(list.get(0));

        for (String s : list) {
            System.out.println(s);
        }

//        System.out.println(list.remove("zs"));
//        System.out.println(list.add("aaa"));

        Set<String> set = Set.of("zs","ls","ww","zl");//保证元素唯一
        for (String s : set) {
            System.out.println(s);
        }
        System.out.println("----");
//        System.out.println(set.add("aaa"));

        //键值对数量小于10
        Map<String,String> map1 = Map.of("a","b","c","d","e","f");
        for (String s : map1.keySet()) {
            System.out.println(s + " " + map1.get(s));
        }
        System.out.println("----");

        Map<String,String> map2 = Map.copyOf(map1);
//        System.out.println(map2);
//        map2.put("e","u");


    }
}