Lambda Expressions for Abstract Classes

You cannot directly make a lambda expression target an abstract class, as Sleiman Jneidi pointed out in his answer. However, you can use a workaround:

public class AbstractLambda<T> extends Abstract<T>
    private final Supplier<? extends T> supplier;
    public AbstractLambda(Supplier<? extends T> supplier)
        this.supplier = supplier;

    public T getSomething()
        return this.supplier.get();

This can be used with a lambda expression:

Abstract<String> a = new AbstractLambda<>(() -> "Hello World");
System.out.println(a.getSomething()); // prints 'Hello World'

In case your getSomething(...) method has arguments, use a java.util.function.Function or the appropriate interface from the java.util.function package instead of java.util.function.Supplier.

This is also how the java.lang.Thread lets you use a Runnable lambda instead of having to subclass the class:

Thread t = new Thread(() -> System.out.println("Hello World"));

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