【转】JAVA的可变类与不可变类

jdk的可变类和不可变类 

primitive变量: boolean,byte, char, double ,float, integer, long, short 
jdk的不可变类：jdk的java.lang包中 Boolean, Byte, Character, Double, Float, Integer, Long, Short, String. 

  StringBuffer 可变类 
  java.util.Date 可变类 

1.  可变类和不可变类(Mutable and Immutable Objects)的初步定义： 
      可变类：当你获得这个类的一个实例引用时，你可以改变这个实例的内容。 
      不可变类：当你获得这个类的一个实例引用时，你不可以改变这个实例的内容。不可变类的实例一但创建，其内在成员变量的值就不能被修改。 
   2. 如何创建一个自己的不可变类： 
      .所有成员都是private 
      .不提供对成员的改变方法，例如：setXXXX 
      .确保所有的方法不会被重载。手段有两种：使用final Class(强不可变类)，或者将所有类方法加上final(弱不可变类)。 
      .如果某一个类成员不是原始变量(primitive)或者不可变类，必须通过在成员初始化(in)或者get方法(out)时通过深度clone方法，来确保类的不可变。 
   3. 一个示例 

import java.util.Date;                                          
public final class BrokenPerson                                
{                                                              
  private String firstName;                                    
  private String lastName;                                     
  private Date dob;                                            
                                         
  public BrokenPerson( String firstName,                   public BetterPerson( String firstName, 
    String lastName, Date dob)                                             String lastName, Date dob)          
  {                                                                                      {                                     
   this.firstName = firstName;                                             this.firstName = firstName;         
   this.lastName = lastName;                                             this.lastName = lastName;           
   this.dob = dob;     //error               this.dob = new Date( dob.getTime() ); //correct 
  }                                                                                       }                                      
                                                             
  public String getFirstName() 
  { 
   return this.firstName; 
  } 
  public String getLastName() 
  { 
   return this.lastName; 
  } 
  public Date getDOB()                                               public Date getDOB()                     
  {                                                                                    {                                      
   return this.dob;    //error                  return new Date( this.dob.getTime() );//correct 
  }                                                                                     }                                      
}