首页 > 代码库 > java中不常见的keyword：strictfp，transient
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();
}
}
}
java中不常见的keyword：strictfp，transient