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.

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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();