System类

1. exit(0)退出当前程序

2. arrarycopy：复制数组元素，比较适合底层调用，一般使用Arrary.copyOf完成复制数组

   int[] src={1,2,3};
   int[] dest = new int[3];
   System.arraycopy(src,0,dest,0,3);

3. currentTimeMillens：返回当前时间距离的毫秒数
4. gc：运行垃圾回收机制 System.gc()

垃圾回收实例——学生类

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

    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("回收了"+name+" "+age);
    }
}

垃圾回收——测试类

public class Test01 {
    public static void main(String[] args) {
        new Student("小明",20);
        new Student("小红",28);
        new Student("小刚",22);
        System.gc();
    }
}