Java 基础类库（System类）

public class Demo {
public static void main(String[] args) throws Exception {
long start = System.currentTimeMillis();
String str = "";
for (int i = 0; i < 2000; i++) {
str += i;
}
long end = System.currentTimeMillis();
System.out.println("本次操作所用时间:" + (end - start));
}

}

/*
* Object类提供的finalize方法protect void finalize() throws Throwble
*/
class Message {
public Message() {
System.out.println("绝地求生！");
}

@Override
protected void finalize() throws Throwable {
System.out.println("落地成盒！");
throw new Exception("大吉大利！今晚吃鸡！");
}
}

public class Demo {
public static void main(String[] args) throws Exception {
Message msg = new Message();
msg = null;// 形成垃圾
System.gc();//System的gc()方法间接调用Runtime.getRuntime.gc()的方法

}
}