@ExtensionMethod

Overview

You can make a class containing a bunch of public, static methods which all take at least 1 parameter. These methods will extend the type of the first parameter, as if they were instance methods, using the @ExtensionMethod feature.

For example, if you create public static String toTitleCase(String in) { ... }, you can use the @ExtensionMethod feature to make it look like the java.lang.String class has a method named toTitleCase, which has no arguments. The first argument of the static method fills the role of this in instance methods.

All methods that are public, static, and have at least 1 argument whose type is not primitive, are considered extension methods, and each will be injected into the namespace of the type of the first parameter as if they were instance methods. As in the above example, a call that looks like: foo.toTitleCase() is replaced with ClassContainingYourExtensionMethod.toTitleCase(foo);. Note that it is actually not an instant NullPointerException if foo is null - it is passed like any other parameter.

You can pass any number of classes to the @ExtensionMethod annotation; they will all be searched for extension methods. These extension methods apply for any code that is in the annotated class.

Lombok does not (currently) have any runtime dependencies which means lombok does not (currently) ship with any useful extension methods so you'll have to make your own. However, here's one that might spark your imagination:

public class ObjectExtensions {
	public static <T> or(T object, T ifNull) {
		return object != null ? object : ifNull;
	}
}

With the above class, if you add @ExtensionMethod(ObjectExtensions.class) to your class definition, you can write:

String x = null;
System.out.println(x.or("Hello, World!"));

The above code will not fail with a NullPointerException; it will actually output Hello, World!

Small print