Java map.get(key) - Is there a way to automatically call put(key) and return if the key doesn't exist?

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

To achieve what you need:

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

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.

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.

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

map.getOrDefault(key, defaultValue);

javadocs

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);

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