Java map.get(key) - automatically do put(key) and return if key doesn't exist?

Question

I am sick of the following pattern:

value = map.get(key);
if (value == null) {
    value = new Object();
    map.put(key, value);
}

This example only scratches the surface of the extra code to be written when you have nested maps to represent a multi-dimensional structure.

I'm sure something somewhere exists to avoid this, but my Googling efforts yielded nothing relevant. Any suggestions?

Out of curiosity, the Object you want to put, is it just an Object, or will the type vary? Also, is it already created or should it only be created if no object already exists? — Roger Lindsjö, Dec 02 '11 at 07:34
The type is known at compile time. Usually it's a String to Map (to Map)* to Integer. — Sridhar Sarnobat, Dec 16 '11 at 16:47

Roger Lindsjö · Accepted Answer · 2022-03-06T13:54:20.867

76

The

java.util.concurrent.ConcurrentMap

and from Java 8

Java.util.Map

has

putIfAbsent(K key, V value)

which returns the existing value, and if that is null inserts given value. So if no value exists for key returns null and inserts the given value, otherwise returns existing value

If you need lazy evaluation of the value there is

computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)

edited Mar 06 '22 at 13:54

answered Nov 30 '11 at 22:48

Roger Lindsjö

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Returns: the previous value associated with the specified key, or null if there was no mapping for the key – user802421 Nov 30 '11 at 23:31
Perfect! That's what I was looking for. Thank you Roger. – Sridhar Sarnobat Dec 01 '11 at 00:09
7

Note that this forces you to create a new object, even if there is already one available in the map. If you really only create a `new Object()`, this may be negligible. – Christian Semrau Dec 01 '11 at 21:45
1

@user802421 is right--this will return _null_ if the value is inserted. – Saad Malik Jul 05 '14 at 01:47
3

java.util.Map has putIfAbsent from JDK 8, not only ConcurrentMap. – yelliver Nov 13 '15 at 04:12
3

it doesn't work is the value itself is supposed to be a collection. as what I wanted is add an empty collection AND RETURN IT so that I can add items into that collection. – efinal Nov 21 '16 at 12:15
You probably want `computeIfAbsent` as shown by @itake – Roger Lindsjö Nov 21 '16 at 15:50

score 72 · Answer 2 · answered Apr 15 '16 at 07:00

72

Java 8 adds nice method to the Map: compute, computeIfPresent, computeIfAbsent

To achieve what you need:

Object existingOrCreated = map.computeIfAbsent(key, (k) -> new Object());

answered Apr 15 '16 at 07:00

iTake

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The accepted answer points to this same method. However, this answer makes it clearer. – Renato Back Apr 11 '18 at 18:14

Thomas · Answer 3 · 2021-06-02T13:56:57.360

The problem with this pattern is that you'd have to somehow define the value that should be used in case the get() returns null.

There certainly are libraries out there and IIRC there are also some newer collections that do that, but unfortunately I don't remember which those were.

However, you could write a utility method yourself, provided you have a standard way of creating the new values. Something like this might work:

public static <K, V> V safeGet(K key, Map<K,V> map, Class<V> valueClass) throws /*add exceptions*/ {
  V value = map.get(key);
  if( value == null ) {
    value = valueClass.newInstance();
    map.put( key, value );
  }   

  return value;
}

Note that you'd either have to throw the reflection exceptions or handle them in the method. Additionally, this requires the valueClass to provide a no-argument constructor. Alternatively, you could simply pass the default value that should be used.

Java 8 update

It has already been mentioned in other answers but for the sake of completeness I'll add the information here as well.

As of Java 8 there is the default method computeIfAbsent(key, mappingFunction) which basically does the same, e.g. if the value class was BigDecimal it could look like this:

BigDecimal value = map.computeIfAbsent(key, k -> new BigDecimal("123.456"));

The implementation of that method is similar to the safeGet(...) defined above but more flexible, directly available at the map instance and better tested. So when possible I'd recommend using computeIfAbsent() instead.

That's a useful code snippet. Thank you. I didn't know how to do it with generics. — Sridhar Sarnobat, Dec 01 '11 at 00:12

score 4 · Answer 4 · answered Mar 26 '16 at 01:35

You can use MutableMap and getIfAbsentPut() from Eclipse Collections which returns the value mapped to the key or inserts the given value and returns the given value if no value is mapped to the key.

You can either use a method reference to create a new Object:

MutableMap<String, Object> map = Maps.mutable.empty();
Object value = map.getIfAbsentPut("key", Object::new);

Or you can directly create a new Object:

MutableMap<String, Object> map = Maps.mutable.empty();    
Object value = map.getIfAbsentPut("key", new Object());

In the first example, the object will be created only if there is no value mapped to the key.

In the second example, the object will be created regardless.

Note: I am a contributor to Eclipse Collections.

letthefireflieslive · Answer 5 · 2017-07-13T05:17:25.660

2

If in any case you need to get a default data in your map if it's not existing

map.getOrDefault(key, defaultValue);

javadocs

edited Jul 13 '17 at 05:17

answered Jul 13 '17 at 05:03

letthefireflieslive

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Interesting, but this doesn't put it in the map as the OP asked – Sridhar Sarnobat Jul 13 '17 at 05:09
Note: MAP.getOrDefault(key, defaultValue) is *supported only on java 8 and above.* – Arun Joseph Sep 15 '20 at 05:47

Christian Semrau · Answer 6 · 2014-07-30T15:50:43.330

1

EDIT : Note that the feature mentioned below is long deprecated, and a CacheBuilder should be used instead.

The Guava library has a "computing map", see MapMaker.makeComputingMap(Function).

Map<String, Object> map = new MapMaker().makeComputingMap(
    new Function<String, Object>() {
      public String apply(Stringt) {
        return new Object();
      }
  });

If you need the Function several times, extract it into a utility class, and then create the Map like this (where MyFunctions.NEW_OBJECT is the static Function instance):

Map<String, Object> map = new MapMaker()
    .makeComputingMap(MyFunctions.NEW_OBJECT);

edited Jul 30 '14 at 15:50

answered Nov 30 '11 at 23:03

Christian Semrau

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`MapMaker` from Google Guava is dead. Ref: http://code.google.com/p/guava-libraries/wiki/MapMakerMigration – kevinarpe Jul 30 '14 at 13:54
I understand the Guava people think an autocomputing `Map.get()` is bad, because it breaks the `Map` contract. If you use the computing map only internally, it should be easy to use a `LoadingCache` instead. I can see their point of a catastrophe waiting to happen when sharing a computing map with unsuspecting code. For that use case, one should use a `safeGet()` utility function like the one given by Thomas in his answer. – Christian Semrau Jul 30 '14 at 15:47

Trein · Answer 7 · 2011-12-01T12:22:03.263

0

Maybe I'm not seeing the whole problem, but how about using inheritance or composition to add this behavior to the Map object?

edited Dec 01 '11 at 12:22

answered Nov 30 '11 at 23:47

Trein

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That was the next best thing I found, yes. Thanks for the reply. – Sridhar Sarnobat Dec 01 '11 at 00:09

Java map.get(key) - automatically do put(key) and return if key doesn't exist?

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