Returning an unmodifiable map

Using Collections.unmodifiableMap(...), I'm trying to return an unmodifiable view of a map. Let's say I have the following method,

public final Map<Foo, Bar> getMap(){
    ...
    return Collections.unmodifiableMap(map);
}

Why is it legal elsewhere to do the following,

Map<Foo, Bar> map = getMap();
map.put(...);

This doesn't throw an UnsupportedOperationException like I thought it would. Ca开发者_开发技巧n someone please explain this, or suggest how I can successfully return a truly unmodifiable map?

Are you sure you're not masking your exceptions somehow? This works absolutely fine, in that it throws UnsupportedOperationException:

import java.util.*;

public class Test {

    public static void main(String[] args) {
        Map<String, String> map = getMap();
        map.put("a", "b");
    }

    public static final Map<String, String> getMap(){
        Map<String, String> map = new HashMap<String, String>();
        map.put("x", "y");
        return Collections.unmodifiableMap(map);
    }
}

I suggest you print out map.getClass() on the return value of the method - I would expect it to be an UnmodifiableMap.

I created a small test program and my program threw an 'UnsupportedOperationException' when I tried to put data in.

code:

import java.util.*;

public class TestUnmodifiableMap
{
    Map<Integer, String> myMap;

    public TestUnmodifiableMap()
    {
        myMap = new HashMap<Integer, String>();
    }

    public final Map<Integer, String> getMap()
    {
        return Collections.unmodifiableMap(myMap);
    }

    public static void main(String[] args)
    {
        TestUnmodifiableMap t = new TestUnmodifiableMap();
        Map<Integer, String> testMap = t.getMap();

        testMap.put(new Integer("1"), "Hello");
    }
}

What else are you doing in your class?

There must be something else wrong. There's no way you can put something in that map after you wrapped it as an unmodifiable map.

I would also suggest to return

return Collections.<Foo, Bar>unmodifiableMap(map);

otherwise you will get "unchecked" warnings when compiling your code with -Xlint:unchecked.