关于Collections.unmodifiableMap

Collections.unmodifiableMap返回的是一个不可修改的Map，这个不可修改的Map指的是Map中的对象地址不可修改，里面的对象若支持修改的话，其实也还是可以修改的。下面用一个例子说明：

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;
    }
}

程序运行结果：