Is there anything like an Objective-C block in Java?

问题

I'm just learning Java, and I've developed apps in Objective-C before. I like the concept of "blocks", because they allow code to be ran after something happens.

For example, to execute a block after a certain amount of time in a SpriteKit app that calls a method helloWorld from scene myScene:

[myScene runAction:[SKAction sequence:@[[SKAction waitForDuration:5], [SKAction runBlock:^{
    [myScene helloWorld];
}]]]];

Is there anything like a block in Java? If so, how would I use it? What's the syntax to...

• use it as a function parameter?
• call said block in the function?
• assign a value to the block?

I've heard a little bit about "closures," but I'm not so sure what they are or how to use them.

回答1:

In Java, the way to do this is to use an interface. The most basic interface for this is Runnable which has a method run.

What's the syntax to...

use it as a function parameter?

void foo(Runnable r)

call said block in the function?

r.run();

assign a value to the block?

Before Java 8 you had to do something like this:

Runnable r = new Runnable() {
    @Override
    public void run() {
        // do something
    }
}

Now you can just do:

Runnable r = () -> { // do something }

An expression involving -> is called a lambda. The brakets () are where you would write any parameters, but the method run of Runnable doesn't have any parameters. You can also pass a Runnable to a method without mentioning Runnable:

foo(() -> { // do something });

Java 8 introduced many, many new interfaces where the method can return a value or accept parameters.

来源：https://stackoverflow.com/questions/32997887/is-there-anything-like-an-objective-c-block-in-java