当前位置：首页 > news >正文

Java进阶教程（一）关键字

news 2026/3/26 23:54:10

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)