抽象类的 Lambda 表达式 [英] Lambda Expressions for Abstract Classes

问题描述

我有一个带有一个抽象方法的抽象类.我如何使用 lambda 表达式来实例化它.它不能被做成接口，因为它扩展了一个类.

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
            }
        });
    }
}

推荐答案

正如 Sleiman Jneidi 在他的回答中指出的那样，您不能直接使 lambda 表达式目标为抽象类.但是，您可以使用一种解决方法:

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();
    }
}

这可以与 lambda 表达式一起使用:

This can be used with a lambda expression:

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

如果您的 getSomething(...) 方法有参数，请使用 java.util.function.Function 或 java 中的相应接口.util.function 包而不是 java.util.function.Supplier.

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.

这也是 java.lang.Thread 让您使用 Runnable lambda 而不必子类化类的方式:

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();

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