Collections.unmodifiableMap

最新推荐文章于 2022-12-16 10:28:43 发布

原创最新推荐文章于 2022-12-16 10:28:43 发布 · 429 阅读

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探讨了Java中Collections.unmodifiableMap的特性，指出虽然该方法创建的Map不能通过put等方法修改，但可以通过修改已存在的值对象来间接变更Map的内容。

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对于不可变的Collections.unmodifiableMap 是否真的不可以变？

博客：http://blog.youkuaiyun.com/l2tp1012/article/details/39338209

 Java Code 

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package com.example.demo;

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;
    }
}

从这个代码可以看出：Collections.unmodifiableMap不是真正的不可以改变，其原理可以从Collections.unmodifiableMap的源代码看出来：

源代码如下：

 Java Code 

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public V put(K key, V value)
{
    throw new UnsupportedOperationException();
}
public V remove(Object key)
{
    throw new UnsupportedOperationException();
}
public void putAll(Map<? extends K, ? extends V> m)
{
    throw new UnsupportedOperationException();
}
public void clear()
{
    throw new UnsupportedOperationException();
}

结论：Collections.unmodifiableMap是可以对map的值进行修改的，不可以采用put方式进行，正确的姿势是对Value 采用Set方法进行修改

但是真实的使用场景Collections.unmodifiableMap是不需要对值进行修改的