Question

Here’s another question of “How would I do this in Java?” In Python, I can use the ‘*’ symbol to unpack arguments like so:

>>> range(3, 6)             # normal call with separate arguments
[3, 4, 5]
>>> args = [3, 6]
>>> range(*args)            # call with arguments unpacked from a list
[3, 4, 5]

Java supports getting a list of args with ...args syntax, but is there a way (perhaps using the Reflection libraries?) to unpack those for some other function?

Answer 1

public void printStrings(String... strings)
{
   // the strings parameter is really a String[].
   // You could do anything to it that you normally
   // do with an array.
   for(String s : strings){
      System.out.println(s);
   }
}

Can be called like this:

String[] stringArray = new String[10];
for(int i=0; i < stringArray.length; i++){
   stringArray[i] = "String number " + (i+1);
}

printStrings(stringArray);

The ... syntax is really syntactic sugar for arrays.

Java doesn’t have the facility that you describe, but you could fake it several ways.

I think the closest approximation means overloading any function that you want to use in that fashion using varargs.

If you have some method:

public void foo(int a, String b, Widget c) { ... }

You can overload it:

public void foo(Object... args) {
    foo((Integer)args[0], (String)args[1], (Widget)args[2]);
}

But this is really clumsy and error prone and hard to maintain.

More generically, you could use reflection to call any method using any arguments, but it’s got a ton of pitfalls, too. Here’s a buggy, incomplete example of how it gets ugly really fast:

public void call(Object targetInstance, String methodName, Object... args) {
    Class<?>[] pTypes = new Class<?>[args.length];
    for(int i=0; i < args.length; i++) {
        pTypes[i] = args[i].getClass();
    }
    Method targetMethod = targetInstance.getClass()
              .getMethod(methodName, pTypes);
    targetMethod.invoke(targetInstance, args);
}