java - Lambda Expressions for Abstract Classes

Question

I have an abstract class with one abstract method. How can I use lambda expressions to instantiate it. It cannot be made into an interface because it extends a class.

public class Concrete<T> {
    // Has a bunch of predefined methods.
}


public abstract class Abstract<T> extends Concrete<T> {
    public T getSomething();
    // Uses inherited methods from Concrete class
}

public class Driver {
    public static void main(String[] args) {
        System.out.println(new Abstract<String>() {
            public String getSomething() {
                // Returns something using inherited methods from Abstract              
                // Class and Concrete Class
            }
        });
    }
}

Answer 1

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;
    }

    @Override
    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"));
t.start();