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!