How to declare that method is returning Collections.unmodifiableMap

Question

I'm returning an unmodifiable map from a method. How do I make sure that any code that tries to modify the returned map will get a compile time error, as opposed to a run time error?

My class:

public class Foo
{
    private final Map<String, Bar> aMap;

    public Foo()
    {
    ...
    }

    public Map<String, Bar> getMap(){
        return Collections.unmodifiableMap(aMap);
    }
}

I want this to create a compile time error:

Foo foo = new Foo();
Map<String, Bar> m = foo.getMap();
m.put('key',null); // user should be notified that the map is unmodifiable here, not at run time

Can I change the return type? Can I add an appropriate annotation?

There is no way to achieve this if the return type of the Method is `java.util.Map`. You can only change the return type to a custom class that wraps a `Map`. — jlordo, Sep 25 '13 at 15:27
As a side note, the problem relaxes when you properly unit test your application - this should catch attempts to modify your unmodifiable collections. — Gyro Gearless, Sep 25 '13 at 15:48
If you can use third-party libraries, Guava's `ImmutableMap` implements the `Map` interface, but marks the mutation methods as `@Deprecated`, so it'll be a compile-time warning. — Louis Wasserman, Sep 25 '13 at 16:51

jlordo · Accepted Answer · 2013-09-25T15:44:37.070

5

You could create a class like

class ReadOnlyMap<K, V> {

    private final Map<K, V> map;

    public ReadOnlyMap(Map<K, V> map) {
       this.map = map;
    }

    public V get(Object key) {
        return map.get(key);
    }

    // also implement size() method
    // and everything else you want to expose
}

and use it as the return type of your method. Then your method would be

public ReadOnlyMap<String, Bar> getMap() {
    return new ReadOnlyMap(aMap);
}

Beware that this won't hinder the caller to mutate the value objects, if they're not instances of immutable classes.

edited Sep 25 '13 at 15:44

answered Sep 25 '13 at 15:36

jlordo

37,490
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2

As pointed out in another answer, be sure to pay attention to this: "Beware that the values() and entrySet() methods returns modifiable collections, so you would need to wrap their returned value too." – StormeHawke Sep 25 '13 at 16:04

score 2 · Answer 2 · answered Sep 25 '13 at 15:33

A default Map is modifiable, period.

If you want it to be unmodifiable, you'll have to write your own class that has a Map as variable, implement all methods you need again and pass it on to the map.

Since you don't want the .put() method, you skip that one.

score 1 · Answer 3 · answered Sep 25 '13 at 15:39

You can create a UnmodifiableMap object that has all read methods of the Map interface but lacks the write methods. This object would wrap the map you want. It would look like :

public class UnmodifiableMap<K, V> {

    private final Map<K, V> map;

    public UnmodifiableMap(final Map<K, V> map) {
        this.map = map;
    }

    public boolean containsKey(final K key) {
        return map.containsKey(key);
    }

    public V get(final K key) {
        return map.get(key);
    }

    // And so on.

}

Beware that the values() and entrySet() methods returns modifiable collections, so you would need to wrap their returned value too.

If you want to comply to a `Map` as close as possible, the `get()` method should take `Object` as parameter, not `K` ;) Otherwise, that's the way to go. — jlordo, Sep 25 '13 at 15:42

Bohemian · Answer 4 · 2013-09-25T15:34:06.083

0

You can not do it.

An unmodifiable Collection is an implementation choice, not a type.

The best you could do is to document it with javadoc, something like:

/**
 * Gets the map
 * @returns An unmodifiable Map
 */
public Map<String, Bar> getMap(){
    return Collections.unmodifiableMap(aMap);
}

edited Sep 25 '13 at 15:34

answered Sep 25 '13 at 15:28

Bohemian

412,405
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So how do I let other developers know not to call put? – Boundless Sep 25 '13 at 15:33
@Boundless. You can document your `getMap()` method specifying that it returns an `Unmodifiable` map. You can't achieve more than that. – Rohit Jain Sep 25 '13 at 15:34
The return type of Collections.unmodifiableMap(aMap) is a concrete class http://www.docjar.com/docs/api/java/util/Collections$UnmodifiableMap.html. Just that it is package private and not visible outside java.util package – krishnakumarp Sep 25 '13 at 15:42

score 0 · Answer 5 · answered Sep 25 '13 at 15:39

You cannot do it with the standard classes. With the standard classes, the only way that you know that it is unmodifiable is to try to modify it and receive an UnsupportedOperationException.

You can achieve compile-time checking by creating your own UnmodifiableMap implementation that uses a normal Map internally (probably unmodifiable). It would have the usual "read" operations and they would be delegated to the underlying Map. It would not have the usual "write" operations. I would guess that you would want to also have a method that would return the underlying Map (or a copy of it) to enhance interoperability with other code.

OldCurmudgeon · Answer 6 · 2013-09-25T15:56:53.667

This seems to get near what you are asking for. It does not give an error but it does give a warning!

public class Solid {
  // My map has no put.
  public interface Map<K, V> extends java.util.Map<K, V> {
    /**
     * Put to the Map.
     *
     * @deprecated SolidMap is unmodifiable. put() will fail!
     */
    @Deprecated
    @Override
    V put(K key, V value);

  }

  // Could also do this for lists etc.

  // Helper wrappers from each kind of map.
  public class HashMap<K, V> extends java.util.HashMap<K, V> implements Solid.Map<K, V> {
  }

  public class TreeMap<K, V> extends java.util.TreeMap<K, V> implements Solid.Map<K, V> {
  }
  //...

  // Test only.
  public void test() {
    Solid.Map<String, String> sm = new Solid.HashMap<>();
    // Warning that put is deprecated!
    String put = sm.put("Key", "Value");
  }

  public static void main(String args[]) {
    try {
      new Solid().test();
    } catch (Throwable t) {
      t.printStackTrace(System.err);
    }
  }

}

score 0 · Answer 7 · answered Feb 03 '23 at 09:42

Currently I'm debugging an issue in Jenkins, then come across the default variable "prams" in Jenkins. With echo params.getClass().getSimpleName(), the output is UnmodifiableMap.
Then find out there is a method in groovy which will return a UnmodifiableMap class: Map map = [:], def updatedMap = Collections.unmodifiableMap(map), then the type of the "updatedMap" is "UnmodifiableMap".

How to declare that method is returning Collections.unmodifiableMap

7 Answers7