Java Type Witness Generic

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• Type Witness

List<String> moons = Collections.emptyList();

This statement expects to receive an instance of List<String> which is the target type. Because emptyList() returns List<T>, the compiler infers that type argument T must be String. Java 7 and Java 8 support this inference. Alternatively, you could employ a type witness and specify T's value as shown below:

List<String> moons = Collections.<String>emptyList(); // Type witness not need from Java 8

Another example:

Planet(String name, List<String> moons)

{

this.name = name;

this.moons = moons;

}

Planet mercury = new Planet("Mercury", Collections.emptyList()); // compile error under Java 7,

// which outputs an error message that's similar to the following message:List<Object> cannot be converted to List<String>