Output Parameters in Java

Question

With a third party API I observed the following.

Instead of using,

public static string getString(){
   return "Hello World";
}

it uses something like

public static void getString(String output){

}

and I am getting the "output" string assigned.

I am curious about the reason of implementing such functionality. What are the advantages of using such output parameters?

Answer 1

Something isn't right in your example.

class Foo {

    public static void main(String[] args) {
        String x = "foo";
        getString(x);
        System.out.println(x);
    }

    public static void getString(String output){
        output = "Hello World"
    }
}

In the above program, the string "foo" will be output, not "Hello World".

Some types are mutable, in which case you can modify an object passed into a function. For immutable types (such as String), you would have to build some sort of wrapper class that you can pass around instead:

class Holder<T> {
    public Holder(T value) {
        this.value = value;
    }
    public T value;
}

Then you can instead pass around the holder:

public static void main(String[] args) {
    String x = "foo";
    Holder<String> h = new Holder(x);
    getString(h);
    System.out.println(h.value);
}

public static void getString(Holder<String> output){
    output.value = "Hello World"
}

Answer 2

That example is wrong, Java does not have output parameters.

One thing you could do to emulate this behaviour is:

public void doSomething(String[] output) {
    output[0] = "Hello World!";
}

But IMHO this sucks on multiple levels. :)

If you want a method to return something, make it return it. If you need to return multiple objects, create a container class to put these objects into and return that.