What are curly braces after function call for?

Question

In the following code, what does Type type mean, and what are the curly brackets are used for?

Type type = new TypeToken<List<String>>(){}.getType();
List<String> list = converter.fromJson(jsonStringArray, type ); 

Answer 1

Type is a class.

new TypeToken<List<String>>() {
}.getType();

Is creating an anonymous inner class and invoking getType() on the object created.

Answer 2

That's not after a function call, but after a constructor call. The line

Type type = new TypeToken<List<String>>(){}.getType();

is creating an instance of an anonymous subclass of TypeToken , and then calling its getType() method. You could do the same in two lines:

TypeToken<List<String>> typeToken = new TypeToken<List<String>>(){};
Type type = typeToken.getType();

The Java Tutorial Anonymous Subclasses has more examples of this. This is a somewhat peculiar usage, since no methods are being overridden, and no instance initialization block is being used. (See Initializing Fields for more about instance initialization blocks.)

Answer 3

The curly brackets are the anonymous class constructor and are use after a constructor call. Inside you can override or create a method.

Example:

    private static class Foo {

    public int baz() {
        return 0;
    }
}

public static void main(final String[] args) {
    final Foo foo = new Foo() {
        @Override
        public int baz() {
            return 1;
        }
    };

    System.out.println(foo.baz());
}

Output:

1