ArrayList集合

集合常用操作

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class list3 {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add(5);
        list.add(5);
        System.out.println(list);
        //添加元素
        list.add("50");
        list.add(.5);
        list.add(0,"java");
        list.addAll(1,List.of("aa","bb","cc"));
        System.out.println(list);
        //删除所有
        //list.clear();
        Integer n = 5;
        //删除下标为5的元素
//        list.remove(5);
//        System.out.println(list);
//        list.remove(n);
//        System.out.println(list);
        //删除集合中所有5
//        list.removeAll(List.of(5));
//        System.out.println(list);
        //删除集合中所有5
//        list.removeIf(e-> e==(Integer) 5);
//        System.out.println(list);
        //删除集合中的指定字符串
        list.removeIf(e->e instanceof String && e.toString().contains("aa"));
        System.out.println(list);
        //替换元素
        list.set(1,500);
        System.out.println(list);
//判断元素是否存在
        System.out.println(list.contains(5000));
        //查询元素位置下标
        System.out.println(list.indexOf("cc"));
        //遍历数组
        for(Object e: list){
            System.out.println(e);
        }
        System.out.println("-----------------");
        for(int i = 0;i<list.size();i++){
            System.out.println(list.get(i));
        }
        System.out.println("-----------------");
        for(int i = list.size()-1;i>=0;i--){
            System.out.println(list.get(i));
        }
        //只能读取不能修改
        List<Integer> nums = List.of(1,2,3);
        System.out.println(nums);

        List<Double> ds = new ArrayList<>(List.of( 1d,2d,3d,1d,2d,3d,4d));
        ds.add(4.5);
        ds.addAll(List.of(10d,20d,30d));
        System.out.println(ds);
        System.out.println(ds.contains(30d));//contains:包含
        //保留括号内的删除其他的
//        ds.retainAll(List.of(1d,2d,3d));
//        System.out.println(ds);
        //方法一
        //若集合中的数大于2，要乘以0.5,不大于输出原数
          ds.replaceAll(a ->a >= 2 ? a* .5 : a);
        System.out.println(ds);
////方法二
//        for(int i = 0 ;i<ds.size();i++){
//            if(ds.get(i) >=2){
//                ds.set(i,ds.get(i) * .5);
//            }
//        }
//        System.out.println(ds);

        List<Integer>list2 = new ArrayList<>();
        list2.add(111);
        list2.add(222);
        list2.add(333);
        list2.add(444);
        list2.addAll(List.of(555,666));
        System.out.println(list2);
//方法一
        Object[]  n2 = list2.toArray();
        System.out.println(n2);
        System.out.println(Arrays.toString(n2));
//方法二  推荐使用
        Integer[] n3 = new Integer[list2.size()];
        list2.toArray(n3);
        System.out.println(n3);
        System.out.println(Arrays.toString(n3));

        //将Integer数组，直接转换为List<Integer> 集合 再实例化，就是ArrayList
        List<Integer> list3 = new ArrayList<>(Arrays.asList(n3));
        list3.add(99);
        System.out.println(list3);


    }
}

排序(Collections工具类)

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

public class List4 {
    public static void main(String[] args) {
        //集合字符串的乱序 升序 降序

        List<String> list = new ArrayList<>(List.of("java","c","c++","go","ruby","javascrip","python"));
        System.out.println("排序前："+list);

        //乱序
        Collections.shuffle(list);
        System.out.println("排乱序："+list);
        //升序
        Collections.sort(list);
        System.out.println("排升序："+list);
        //降序
        Collections.sort(list,(a,b)->b.compareTo(a));
        //Collections.sort(list, Comparator.reverseOrder());
        System.out.println("排降序："+list);
//按字符个数
        Collections.sort(list,(a,b)->a.length()-b.length());
        System.out.println("排升序："+list);
        Collections.sort(list,(a,b)->b.length()-a.length());
        System.out.println("排降序："+list);

        //数字排序
        List<Integer> list2 = new ArrayList<>(List.of(10,3,5,23,0,26,9));
        System.out.println(list2);
        //乱序
        Collections.reverse(list2);
        System.out.println(list2);
        Collections.shuffle(list2);
        System.out.println(list2);
        //升序
        Collections.sort((list2));
        System.out.println(list2);
        //降序
        Collections.sort(list2,(a,b)->b-a);
        System.out.println(list2);
    }
}