System类

import javax.swing.plaf.synth.SynthLookAndFeel;

public  class Demo01 {
    public static void main(String[] args){
        //1.arraycopy:数组复制
        //src:源数组
        //srcPos:从源数组哪个位置开始复制
        //dest:目标数组
        //destPos:从目标数组哪个位置开始放
        //length:复制的长度
        int[] array={1,2,3,4,5,6,7,8};
        int[] dest = new int[8];
        System.arraycopy(array,0,dest,0,4);
        for (int i = 0; i <dest.length ; i++) {
            System.out.print(dest[i]+"\t");//1 2  3  4  0  0  0  0
        }

        //2.System.currentTimeMillis();获取当前时间的毫秒数  作用：可计时
        long start = System.currentTimeMillis();
        for (int i = -999999999; i <999999999 ; i++) {
            for (int j = -999999999; j <999999999 ; j++) {
                int result = i + j;
            }
        }
        long end =System.currentTimeMillis();
        System.out.println("用时："+(end-start));//用时：11(毫秒)

        //3.System.gc();通知jvm回收垃圾
        new Student("a",18);
        new Student("b",19);
        new Student("c",20);

        System.gc();
        /*
        a回收了
        c回收了
        b回收了
         */

        //4.System.exit(0);退出jvm
        System.exit(0);
        System.out.println("程序结束");//已经退出jvm，后面的不再执行
    }

}

public class Student {
    private String name;
    private int age;

    public Student() {

    }

    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    protected void finalize() throws Throwable {
        System.out.println(this.name+"回收了");
    }
}