Java学习(尚硅谷)集合之Collections工具类

package Map;

import org.junit.Test;

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


public class CollectionsTest {

    

    

    


    @Test
    public void test1(){
        ArrayList list = new ArrayList();
        list.add(123);
        list.add(45);
        list.add(12);
        list.add(-75);
        list.add(89);
        //      错误写法，报异常IndexOutOfBoundsException: Source does not fit in dest
//        ArrayList dest = new ArrayList();

//        Collections.copy(dest,list);
//        System.out.printladest);
        //正确的：
        List dest = Arrays.asList(new Object[list.size()]);

        System.out.println(dest.size());//=list.size
        System.out.println(dest);
        Collections.copy(dest,list);
        System.out.println(dest);

        //返回的list1就是线程安全的
        List list1 = Collections.synchronizedList(list);

    }
    @Test
    public void test(){
        ArrayList list = new ArrayList();
        list.add(123);
        list.add(45);
        list.add(12);
        list.add(12);
        list.add(12);
        list.add(-75);
        list.add(89);

        System.out.println(list);


        // Collections.reverse(list);
        //Collections.shuffle(list);
//        Collections.sort(list);
//        Collections.swap(list,1,2);
        int frequency = Collections.frequency(list,12);
        System.out.println(list);
        System.out.println(frequency);


    }}