Java进阶教程（一）关键字

Java进阶教程（一）

关键字

synchronized：当它用来修饰一个方法或者一个代码块的时候，能够保证在同一时刻最多只有一个线程执行该段代码。

instanceof：在运行时指出对象是否是特定类的一个实例。
result = object instanceof class

strictfp：strict float point (精确浮点)，保证平台移植之后，浮点运算结果是一致的。

transient：如果用transient声明一个实例变量，当对象存储时，它的值不需要维持。即用transient关键字标记的成员变量不参与序列化过程。

assert：断言可以证明程序的正确性。
assert expression1
assert expression1: expression2
expression1表示一个boolean表达式，expression2表示一个基本类型、表达式或者是一个Object，用于在失败时输出错误信息。
在eclipse中，必须开启相关设置。具体在Window->Preferences->Java->Installed JREs，选择相关版本，点击edit，在Default VM arguments里面输入-ea。

import java.io.FileInputStream;import java.io.FileOutputStream;import java.io.ObjectInputStream;import java.io.ObjectOutputStream;import java.io.Serializable;public class Demo {    public static void main(String[] args) {        People people = new People();        try {            //序列化            ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("people.dat"));            oos.writeObject(people);            oos.close();            //反序列化            ObjectInputStream ois = new ObjectInputStream(new FileInputStream("people.dat"));            people = (People) ois.readObject();            ois.close();        } catch (Exception e) {            e.printStackTrace();        }        //线程        MyRunner boy = new MyRunner(people, "GG", "boy");        MyRunner girl = new MyRunner(people, "MM", "girl");        Thread thread1 = new Thread(boy,"boy");        Thread thread2 = new Thread(girl,"girl");        thread1.start();        thread2.start();        //断言        boolean isPeople = people instanceof People;        assert !isPeople:"it's wrong";    }}class People implements Serializable{    private String name;    private String sex;    private transient String school = "FJNU";    public synchronized void set(String name,String sex) {        this.name = name;        this.sex = sex;    }    public void print(){        System.out.println("name="+name+",sex="+sex+",school="+school);    }}class MyRunner implements Runnable{    private People people;    private String name;    private String sex;    public MyRunner(People people,String name,String sex) {        this.people = people;        this.name = name;        this.sex = sex;    }    @Override    public void run() {        for(int i=0;i<10;i++){            people.set(name, sex);            people.print();        }    }}

运行结果如下：
Exception in thread “main” name=MM,sex=girl,school=null
name=MM,sex=girl,school=null
name=MM,sex=girl,school=null
name=GG,sex=boy,school=null
name=MM,sex=girl,school=null
name=MM,sex=girl,school=null
name=MM,sex=girl,school=null
name=MM,sex=girl,school=null
name=MM,sex=girl,school=null
name=MM,sex=girl,school=null
name=MM,sex=girl,school=null
name=MM,sex=girl,school=null
name=GG,sex=boy,school=null
name=GG,sex=boy,school=null
name=GG,sex=boy,school=null
name=GG,sex=boy,school=null
name=GG,sex=boy,school=null
name=GG,sex=boy,school=null
name=GG,sex=boy,school=null
name=GG,sex=boy,school=null
java.lang.AssertionError: it’s wrong at Demo.main(Demo.java:32)

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请读者思考如下问题，欢迎在评论中互动：
如果People的set方法去掉synchronized关键字，结果会怎样？
为什么school是null？