Java Map implementation that returns a default value instead of null

@Jon's answer is a good way to do what you are asking directly.

But is strikes me that what you might be trying to implement is a "multimap"; i.e. a mapping from a key to a collection of values. If that is the case, then you should also look at the multimap classes in Guava or Apache commons collections.

Look at:

• the com.google.common.collect.Multimap interface and its implementations, or
• the org.apache.commons.collections.MultiMap interface and its implementations.
• the org.apache.commons.collections4.MultiMap interface and its implementations (a new version of the previous MultiMap; introduced in v4).