java.util.Collections.unmodifiableMap 示例

1. 概述

public static <K,V> Map<K,V> unmodifiableMap(Map<? extends K,? extends V> m)

返回指定映射的不可修改视图。此方法允许模块为用户提供对内部映射的“只读”访问。在返回的映射上执行的查询操作将“读完”指定的映射。

试图修改返回的映射（不管是直接修改还是通过其 collection 视图进行修改）将导致抛出 UnsupportedOperationException。

如果指定映射是可序列化的，则返回的映射也将是可序列化的。

参数：

m - 将为其返回一个不可修改视图的映射。

返回：

指定映射的不可修改视图。

2. demo

2.1 code (摘自 http://stackoverflow.com/questions/3999086/when-is-the-unmodifiablemap-really-necessary)

package com.rocky.catest;

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;  

public class SeeminglyUnmodifiable {
   private Map<String, Point> startingLocations = new HashMap<String, Point>(3);  

   public SeeminglyUnmodifiable(){
      startingLocations.put("LeftRook", new Point(1, 1));
      startingLocations.put("LeftKnight", new Point(1, 2));
      startingLocations.put("LeftCamel", new Point(1, 3));
      //..more locations..
   }  

   public Map<String, Point> getStartingLocations(){
      return Collections.unmodifiableMap(startingLocations);
   }  

   public static void main(String [] args){
     SeeminglyUnmodifiable  pieceLocations = new SeeminglyUnmodifiable();
     Map<String, Point> locations = pieceLocations.getStartingLocations();  

     Point camelLoc = locations.get("LeftCamel");
     System.out.println("The LeftCamel's start is at [ " + camelLoc.getX() +  ", " + camelLoc.getY() + " ]");  

     //Try 1. update elicits Exception
     try{
        locations.put("LeftCamel", new Point(0,0));
     } catch (java.lang.UnsupportedOperationException e){
        System.out.println("Try 1 - Could not update the map!");
     }  

     //Try 2. Now let's try changing the contents of the object from the unmodifiable map!
     camelLoc.setLocation(0,0);  

     //Now see whether we were able to update the actual map
     Point newCamelLoc = pieceLocations.getStartingLocations().get("LeftCamel");
     System.out.println("Try 2 - Map updated! The LeftCamel's start is now at [ " + newCamelLoc.getX() +  ", " + newCamelLoc.getY() + " ]");       }
}  

class Point{
    public float x;
    public float y;
    public Point(float x, float y){
        setLocation(x, y);
    }
    public void setLocation(float x, float y){
        this.x = x;
        this.y = y;
    }  

    public float getX(){
        return x;
    }  

    public float getY(){
        return y;
    }
}  

2.2 控制台输出

The LeftCamel's start is at [ 1.0, 3.0 ]
Try 1 - Could not update the map!
Try 2 - Map updated! The LeftCamel's start is now at [ 0.0, 0.0 ]

3. 解释

unmodifiableMap方法返回的Map, 它本身不允许修改, 就是说其中每一个entry引用不允许修改，但是entry中的value如果是对象，value引用的对象的属性值是可以修改的