Java中不常见的关键字：strictfp，transient

原文：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 strictfppublic 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。

mport 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();   }   } }