layout: post
title: "Java创建一个不可变的类"
category: Reading Notes

tags: ["读文章", "Java"]

{% include JB/setup %}

创建一个不可变的类

• 将类声明为final，所以它不能被继承
• 将所有的成员声明为私有的，这样就不允许直接访问这些成员
• 对变量不要提供setter方法
• 将所有可变的成员声明为final，这样只能对它们赋值一次
• 通过构造器初始化所有成员，进行深拷贝(deep copy)

• 在getter方法中，不要直接返回对象本身，而是克隆对象，并返回对象的拷贝

  package com.journaldev.java;
   
  import java.util.HashMap;
  import java.util.Iterator;
   
  public final class FinalClassExample {
   
      private final int id;
   
      private final String name;
   
      private final HashMap testMap;
   
      public int getId() {
          return id;
      }
   
      public String getName() {
          return name;
      }
   
      /**
       * 可变对象的访问方法
       */
      public HashMap getTestMap() {
          //return testMap;
          return (HashMap) testMap.clone();
      }
   
      /**
       * 实现深拷贝(deep copy)的构造器
       * @param i
       * @param n
       * @param hm
       */
   
      public FinalClassExample(int i, String n, HashMap hm){
          System.out.println("Performing Deep Copy for Object initialization");
          this.id=i;
          this.name=n;
          HashMap tempMap=new HashMap();
          String key;
          Iterator it = hm.keySet().iterator();
          while(it.hasNext()){
              key=it.next();
              tempMap.put(key, hm.get(key));
          }
          this.testMap=tempMap;
      }
   
      /**
       * 实现浅拷贝(shallow copy)的构造器
       * @param i
       * @param n
       * @param hm
       */
      /**
      public FinalClassExample(int i, String n, HashMap hm){
          System.out.println("Performing Shallow Copy for Object initialization");
          this.id=i;
          this.name=n;
          this.testMap=hm;
      }
      */
   
      /**
       * 测试浅拷贝的结果
       * 为了创建不可变类，要使用深拷贝
       * @param args
       */
      public static void main(String[] args) {
          HashMap h1 = new HashMap();
          h1.put("1", "first");
          h1.put("2", "second");
   
          String s = "original";
   
          int i=10;
   
          FinalClassExample ce = new FinalClassExample(i,s,h1);
   
          //Lets see whether its copy by field or reference
          System.out.println(s==ce.getName());
          System.out.println(h1 == ce.getTestMap());
          //print the ce values
          System.out.println("ce id:"+ce.getId());
          System.out.println("ce name:"+ce.getName());
          System.out.println("ce testMap:"+ce.getTestMap());
          //change the local variable values
          i=20;
          s="modified";
          h1.put("3", "third");
          //print the values again
          System.out.println("ce id after local variable change:"+ce.getId());
          System.out.println("ce name after local variable change:"+ce.getName());
          System.out.println("ce testMap after local variable change:"+ce.getTestMap());
   
          HashMap hmTest = ce.getTestMap();
          hmTest.put("4", "new");
   
          System.out.println("ce testMap after changing variable from accessor methods:"+ce.getTestMap());
   
      }
   
  }
  

输出：

Performing Deep Copy for Object initialization
true
false
ce id:10
ce name:original
ce testMap:{2=second, 1=first}
ce id after local variable change:10
ce name after local variable change:original
ce testMap after local variable change:{2=second, 1=first}
ce testMap after changing variable from accessor methods:{2=second, 1=first}