Java SimpleDateFormat与System类使用示例详解

package com.classes.SimpleDateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
public class Demo01 {
    public static void main(String[] args) throws Exception{
        SimpleDateFormat sdf = new SimpleDateFormat("yyy年MM月dd日HH:mm:ss");
        Date date = new Date();
//        把日期转成字符串
        String str = sdf.format(date);
        System.out.println(str);
//        把字符串转换成日期
        Date date1= sdf.parse("2019年12月12日19:19:19");
        System.out.println(date1.toLocaleString());
    }
}

package com.classes.system;
public class Demo01 {
    public static void main(String[] args) {
//        System类是私有的，不用new
//        1.arraycopy：数组复制
//        5个参数：src：源数组 srcPos:从哪个位置开始复制 dest:目标数组 destPos:目标数组的位置 length:目标长度
        int [] arr = {22,18,15,8,35,26,45,90};
        int [] dest = new int[8];
        System.arraycopy(arr,0,dest,0,arr.length);
//        遍历数组
        for (int i = 0; i <dest.length ; i++) {
            System.out.println(dest[i]);
        }
//    2.计算当前时间的毫秒数
        System.out.println(System.currentTimeMillis());
//        一般用于计时
        long start = System.currentTimeMillis();
        for (int i = 0; i <99999999 ; i++) {
            for (int j = 0; j <99999999 ; j++) {
                int result = i+j;
            }
        }
        long end = System.currentTimeMillis();
        System.out.println(end-start);
//        3.System.gc()告诉垃圾回收器进行垃圾回收
        new Student("aaa",18);
        new Student("bbb",17);
        new Student("ccc",16);
        System.gc();
//        4.退出jvm
        System.exit(0);
        System.out.println("程序结束了");
    }
}

package com.classes.system;
public class Student {
    private String name;
    private int age;
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public Student() {
    }
    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
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
    @Override
    protected void finalize() throws Throwable {
        System.out.println(name+"被回收了"+"年龄为"+age);
    }
}