关于collections.unmodifiablemap 的一点理解

这是collections.unmodifiablemap的API文档

* Returns an unmodifiable view of the specified map.  This method
* allows modules to provide users with "read-only" access to internal
* maps.  Query operations on the returned map "read through"
* to the specified map, and attempts to modify the returned
* map, whether direct or via its collection views, result in an
* <tt>UnsupportedOperationException</tt>.<p>

简单来说就是返回一份map的只读视图，任何企图修改map的操作都会直接返回UnsupportedOperationException。

其实这个不可修改的Map指的是Map本身不可修改，里面的对象若支持修改的话，其实也还是可以修改的。

 unmodifiableMap是Map接口的一个实现类，里面包装了一个map对于可能修改map的方法

通过下面的两段代码说明一下（代码和collections.unmodifiablemap 关系不大，只是增加一点理解）

/**
 * Created by Andrew  on 2017/5/30.
 */
public class SafePoint {
    private int x,y;

    private SafePoint(int [] a){ this(a[0],a[1]); }

    public SafePoint(SafePoint p){ this(p.get()); }
    public SafePoint(int x, int y){
        this.x = x;
        this.y = y;
    }

    public synchronized int[] get(){
        return new int[]{x,y};
    }

    public synchronized void set( int x, int y){
        this.x = x;
        this.y = y;
    }
}

import java.util.Collections;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

/**
 * Created by Andrew  on 2017/5/30.
 */

public class PublishingVehicleTracker {
    private final Map<String,SafePoint> location;
    private final Map<String,SafePoint> unmodifiableMap;

    public PublishingVehicleTracker(Map<String,SafePoint> location){
        this.location = new ConcurrentHashMap<>(location);
        this.unmodifiableMap = Collections.unmodifiableMap(this.location);
    }

    public Map<String,SafePoint>getLocations(){
        return unmodifiableMap;
    }

    public SafePoint getLocation(String id){
        return location.get(id);
    }

    public void  setLocation(String id, int x, int y){
        if (!location.containsKey(id)){
            throw  new IllegalArgumentException("invalid vehicle name: "+id);
        }
            location.get(id).set(x,y);
    }
}

上例中location 和 unmodifiableMap都指向相同的数据， unmodifiableMap不支持修改。但在本例中可以通过unmodifiableMap.get(id).set(x,y)修改map中的元素；可以将SafePoint的x, y设置为final来防止修改。