Java 中的箭头运算符“->”有什么作用？

Question

While hunting through some code I came across the arrow operator, what exactly does it do? I thought Java did not have an arrow operator.

return (Collection<Car>) CollectionUtils.select(listOfCars, (arg0) -> {
        return Car.SEDAN == ((Car)arg0).getStyle();
});

Details : Java 6, Apache Commons Collection, IntelliJ 12

Update/Answer: It turns out that IntelliJ 12 supports Java 8, which supports lambdas, and is "folding" Predicates and displaying them as lambdas. Below is the "un-folded" code.

return (Collection<Car>) CollectionUtils.select(listOfCars, new Predicate() {
    public boolean evaluate(Object arg0) {
        return Car.SEDAN == ((Car)arg0).getStyle();
    }
});

Answer 1

That's part of the syntax of the new lambda expressions, to be introduced in Java 8. There are a couple of online tutorials to get the hang of it, here's a link to one. Basically, the -> separates the parameters (left-side) from the implementation (right side).

The general syntax for using lambda expressions is

(Parameters) -> { Body } where the -> separates parameters and lambda expression body.

The parameters are enclosed in parentheses which is the same way as for methods and the lambda expression body is a block of code enclosed in braces.

Answer 2

This one is useful as well when you want to implement a functional interface

Runnable r = ()-> System.out.print("Run method");

is equivalent to

Runnable r = new Runnable() {
        @Override
        public void run() {
            System.out.print("Run method");
        }
};

Answer 3

I believe, this arrow exists because of your IDE. IntelliJ IDEA does such thing with some code. This is called code folding. You can click at the arrow to expand it.

Answer 4

It's a lambda expression.

It means that, from the listOfCars, arg0 is one of the items of that list. With that item he is going to do, hence the ->, whatever is inside of the brackets.

In this example, he's going to return a list of cars that fit the condition

Car.SEDAN == ((Car)arg0).getStyle();

Answer 5

New Operator for lambda expression added in java 8

Lambda expression is the short way of method writing.
It is indirectly used to implement functional interface

Primary Syntax : (parameters) -> { statements; }

There are some basic rules for effective lambda expressions writting which you should konw.

Answer 6

runOnUiThread() is used when you want to update your UI from a Non-UI Thread. For eg- If you want to update your UI from a background Thread. You can also use Handler for the same thing.