Java進階教程（一）關鍵字__Java

Java進階教程（一） 關鍵字

synchronized：當它用來修飾一個方法或者一個代碼塊的時候，能夠保證在同一時刻最多隻有一個線程執行該段代碼。

instanceof：在運行時指出對象是否是特定類的一個執行個體。
result = object instanceof class

strictfp：strict float point (精確浮點)，保證平台移植之後，浮點運算結果是一致的。

transient：如果用transient聲明一個執行個體變數，當Object Storage Service時，它的值不需要維持。即用transient關鍵字標記的成員變數不參與序列化過程。

assert：斷言可以證明程式的正確性。
assert expression1
assert expression1: expression2
expression1表示一個boolean運算式，expression2表示一個基本類型、運算式或者是一個Object，用於在失敗時輸出錯誤資訊。
在eclipse中，必須開啟相關設定。具體在Window->Preferences->Java->Installed JREs，選擇相關版本，點擊edit，在Default VM arguments裡面輸入-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();            //還原序列化            ObjectInputStream ois = new ObjectInputStream(new FileInputStream("people.dat"));            people = (People) ois.readObject();            ois.close();        } catch (Exception e) {            e.printStackTrace();        }        //線程        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();        //斷言        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();        }    }}

運行結果如下：
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) 請讀者思考如下問題，歡迎在評論中互動：
如果People的set方法去掉synchronized關鍵字，結果會怎樣。
為什麼school是null。