java中不常见的keyword：strictfp，transient

1.strictfp, 即 strict float point (精确浮点)。

　　strictfp keyword可应用于类、接口或方法。使用 strictfp keyword声明一个方法时，该方法中全部的float和double表达式都严格遵守FP-strict的限制,符合IEEE-754规范。当对一个类或接口使用 strictfp keyword时，该类中的全部代码，包含嵌套类型中的初始设定值和代码，都将严格地进行计算。严格约束意味着全部表达式的结果都必须是 IEEE 754 算法对操作数预期的结果，以单精度和双精度格式表示。

　　假设你想让你的浮点运算更加精确，并且不会由于不同的硬件平台所运行的结果不一致的话，能够用keywordstrictfp.

演示样例 1

　　以下的演示样例演示了一个使用 strictfp 修饰符声明的类。

Java代码

package com.magical;

// Example of precision control with strictfp

public strictfp class MyClass {

public static void main(String[] args)

{

float aFloat = 0.6710339f;

double aDouble = 0.04150553411984792d;

double sum = aFloat + aDouble;

float quotient = (float)(aFloat / aDouble);

System.out.println("float: " + aFloat);

System.out.println("double: " + aDouble);

System.out.println("sum: " + sum);

System.out.println("quotient: " + quotient);

}

}

package com.magical;

// Example of precision control with strictfp

public strictfp class MyClass {

public static void main(String[] args)

{

float aFloat = 0.6710339f;

double aDouble = 0.04150553411984792d;

double sum = aFloat + aDouble;

float quotient = (float)(aFloat / aDouble);

System.out.println("float: " + aFloat);

System.out.println("double: " + aDouble);

System.out.println("sum: " + sum);

System.out.println("quotient: " + quotient);

}

}

执行结果：

float: 0.6710339

double: 0.04150553411984792

sum: 0.7125394529774224

quotient: 16.167336

2.transient

当串行化某个对象时，如果该对象的某个变量是transient，那么这个变量不会被串行化进去。也就是说，如果某个类的成员变量是transient，那么当通过

ObjectOutputStream把这个类的某个实例

保存到磁盘上时，实际上transient变量的值是不会保存的。由于当从磁盘中读出这个对象的时候，对象的该变量会没有被赋值。

另外这篇文章还提到，当从磁盘中读出某个类的实例时，实际上并不会运行这个类的构造函数，而是读取这个类的实例的状态，而且把这个状态付给这个类的对象。

import java.util.*;

public class LoggingInfo implements java.io.Serializable

{

private Date loggingDate = new Date();

private String uid;

private transient String pwd;

LoggingInfo(String user, String password)

{

uid = user;

pwd = password;

}

public String toString()

{

String password=null;

if(pwd == null)

{

password = "NOT SET";

}

else

{

password = pwd;

}

return "logon info: \n " + "user: " + uid +

"\n logging date : " + loggingDate.toString() +

"\n password: " + password;

}

}

import java.io.*;

public class Serializable{

public static void main(String args[]){

LoggingInfo logInfo = new LoggingInfo("小徐", "不知道");

System.out.println(logInfo.toString());

try

{

ObjectOutputStream o = new ObjectOutputStream(

new FileOutputStream("logInfo.out"));

o.writeObject(logInfo);

o.close();

}

catch(Exception e) {//deal with exception

e.printStackTrace();

}

// To read the object back, we can write

try

{

ObjectInputStream in =new ObjectInputStream(

new FileInputStream("logInfo.out"));

LoggingInfo logInfo1 = (LoggingInfo)in.readObject();

System.out.println(logInfo1.toString());

}

catch(Exception e)

{//deal with exception

e.printStackTrace();

}

}

}

import java.util.*;

public class LoggingInfo_ implements java.io.Serializable

{

private Date loggingDate = new Date();

private String uid;

private transient String pwd;

public LoggingInfo_()

{

this.uid = "小徐";

this.pwd = "不知道";

}

public String toString()

{

String password=null;

if(pwd == null)

{

password = "NOT SET";

}

else

{

password = pwd;

}

return "logon info: \n " + "user: " + uid +

"\n logging date : " + loggingDate.toString() +

"\n password: " + password;

}

}

import java.io.*;

public class Serializable_{

public static void main(String args[]){

LoggingInfo_ logInfo_ = new LoggingInfo_();

System.out.println(logInfo_.toString());

try

{

ObjectOutputStream o = new ObjectOutputStream(

new FileOutputStream("logInfo_.out"));

o.writeObject(logInfo_);

o.close();

}

catch(Exception e) {//deal with exception

e.printStackTrace();

}

// To read the object back, we can write

try

{

ObjectInputStream in =new ObjectInputStream(

new FileInputStream("logInfo_.out"));

LoggingInfo_ logInfo_1 = (LoggingInfo_)in.readObject();

System.out.println(logInfo_1.toString());

}

catch(Exception e)

{//deal with exception

e.printStackTrace();

}

}

}