Java: Passing a Function as a Parameter like in JavaScript? [duplicate]

Question

I'm trying to write a function, so I can pass a function as a parameter, such as

public class HashFunction {
    private Function f;
    public HashFunction(Function f) {
        this.f=f;
    }
    public Integer hash(String s){
        return f(s);
    }
}

So I can write code like

new HashFunction(function(String s){ return s.charAt(0)+0; });

Like in javascript. How can I do this?

Answer 1

Unlike many other modern languages, currently java doesn't syntactically support "floating chunks of code" (known as closures).

However, the concept may be achieved through the use of anonymous classes, which are "on the fly" implementation declarations that typically implement an interface, but can also extend a class.

Here's how you would code your example in java:

public interface Hasher {
    int getHash(String s);
}

public class HashFunction {
    private Hasher f;
    public HashFunction(Hasher f) {
        this.f=f;
    }
    public Integer hash(String s){
        return f(s);
    }
}

then to use:

new HashFunction(new Hasher() {
    public int getHash(String s) {return s.charAt(0)+0;}
});

Answer 2

Passing functions as parameters is not possible in Java, unless they added it in a recent language change.

The Java pattern is to use so-called anonymous classes, which implement a member method which has the desired behavior.

For example, see:

http://docstore.mik.ua/orelly/java-ent/jnut/ch03_12.htm

or

How are Anonymous (inner) classes used in Java?

Answer 3

I think you can use interface, the same way like Comparable or Comparator interface, or use annotation to mark some functions, and then use reflection to invoke them

Please check this

Answer 4

I can only second the other answers. Basically, it is not possible to pass real references to functions, like in JavaScript or Haskell, where "everything is a function".

However, if you want to have a little bit of "functional"-style programming in your code, take a look at the "Functional Java" library at http://functionaljava.org/

Maybe taking a look at Scala also can be helpful, as it runs in the JVM and is a very mature, upcoming and modern programming language. In Scala, you can pass functions and it would interoperate with your existing Java code, too. (There are functions, functors, monads, list comprehensions, ...)

Answer 5

functions are called methods in java. And you can pass them!!!
But this is advanced java. Use java.lang.reflection.Method to pass a method.
But you will not happy with that technique.