java中不常见的关键字：strictfp，transient

 

1.strictfp, 即 strict float point (精确浮点)。
　　strictfp 关键字可应用于类、接口或方法。使用 strictfp 关键字声明一个方法时，该方法中所有的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 关键字时，该类中的所有代码，包括嵌套类型中的初始设定值和代码，都将严格地进行计算。严格约束意味着所有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。
　　如果你想让你的浮点运算更加精确，而且不会因为不同的硬件平台所执行的结果不一致的话，可以用关键字strictfp.
示例 1
　　下面的示例演示了一个使用 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);
 }
}

运行结果：
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，假设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。因为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，实际上并不会执行这个类的构造函数，而是读取这个类的实例的状态，并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();
   }  
 }
 
}