Thinking in java-19 final、finally、finalize关键字

这里有关于该问题的stackoverflow问答。

1.final

final可以用来修饰一个变量，则表明该变量是不可改变的；

private final String  name = "foo";
//the reference name can never change

final可以用来修饰一个方法，则表明该方法是不可重写override的；

//The method below can not be overridden by its derived class
public final String toString(){
}

final可以用来修饰一个类，则表明该类是不可继承的。

public final class finalClass{
//Some code here.
}
//The class below is not allowed.
public class classNotAllowed extends finalClass{
//some code
}

2.finally

finally关键字是用在异常处理时，执行一些总是要完成的工作。在异常发生时，相匹配的catch子句会被执行，然后就进入了finally语句块（如果有的话）。

Lock lock = new ReentrantLock();
try{
lock.tryLock();
}catch(Exception ex){
}finally{
lock.unlock();
}

3.finalize

finalize关键字是方法名，当垃圾回收操作发生时，该方法被垃圾回收器调用。一般该方法很少被重写。

protected void finalize(){
//Usually used to free memory
super.finalize();
}

package com.fqyuan.thinking;

public class Termination {

public static void main(String[] args) {
Book novel = new Book(true);
novel.checkIn();

new Book(true);
/*
* Runs the garbage collector.
*
* Calling the gc method suggests that the Java Virtual Machine expend
* effort toward recycling unused objects in order to make the memory
* they currently occupy available for quick reuse.
*/
System.gc();
}
}

class Book {
boolean checkedOut = false;

Book(boolean checkOut) {
checkedOut = checkOut;
}

void checkIn() {
checkedOut = false;
}

/*
* (非 Javadoc) Called by the garbage collector on an object when garbage
* collection determines that there are no more references to the object. A
* subclass overrides the finalize method to dispose of system resources or
* to perform other cleanup.
*
* @see java.lang.Object#finalize()
*/
protected void finalize() {
if (checkedOut)
System.out.println("Error: checked out!");
}
}

运行结果：

Error: checked out!