常用类.System

System系统类，主要用于获取系统的属性数据和其他方法，构造方法私有的

方法名	说明
static void arraycopy();	复制数组
static long currentTimeMillis();	获取当前系统时间，返回的是毫秒值
static void gc();	建议jvm赶快启动垃圾回收器回收垃圾
static void exit(int status);	退出jvm，如果参数是0表示正常退出，非0表示异常退出

package cylxt;

public class Demo01 {
    public static void main(String[] args) {
        //1.arraycopy:数组的复制
        int[] arr = {20,18,15,8,35,26,45,90};
        int[] dest = new int[8];
        //源数组arr，从哪个位置开始复制0，目标数组dest，目标数组的位置0，复制的长度arr.length
        System.arraycopy(arr,0,dest,0,arr.length);
        for (int i = 0; i < dest.length; i++) {
            System.out.println(dest[i]);
        }
        //2.获取毫秒数
        System.out.println(System.currentTimeMillis());

        long start = System.currentTimeMillis();
        for (int i1 = 0; i1 < 99999999; i1++) {
            for (int i2 = 0; i2 < -99999999; i2++) {
                int result = i1+i2;
            }
        }
        long end = System.currentTimeMillis();
        System.out.println("用时："+(end-start));

        //3.System.gc();告诉垃圾回收器回收
        Student s1 = new Student("aaa",19);
        new Student("bbb",19);
        new Student("ccc",19);

        System.gc();
        //4.退出jvm
        System.out.println("程序结束了");
        System.exit(0);//后面的代码不会执行
        System.out.println("程序结束了");
    }
}

package cylxt;

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

    public Student(String name, int age) {
        super();
        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;
    }

    public Student(){

    }
    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

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