iterating over map and array simultaneously in a for loop

Question

I am having some trouble creating a for loop within a constructor to iterate over a map and an array at the same time. Here, it is indicated that this cannot be done with an enhanced for loop.

I have something like this, which raises a compiler error. Basically, the class has a Map which I want to populate via the constructor which takes a collection and a variable number of integers as parameters.

The var-arg expression evaluates to an array of integers, so I tried to put both enhanced iterators in the same loop, but didn't work.

private final Map<Module, Integer> modules = new HashMap<Module, Integer>();    
    AssemblyType(Collection<Module> modules, int... units) {
     int i = 0;
     for (Module module : modules, int i : units) {
      this.modules.put(module, units[i]);
     }  
    }

Thanks for any ideas on how to proceed.

Answer 1

The naive way to do this would just be to track i yourself:

private final Map modules = new HashMap();    
    AssemblyType(Collection modules, int... units) {
        int i = 0;
        for (Module module : modules) {
                this.modules.put(ingredient, units[i]);
                i++;
        }               
    }

I'm not sure if there's a better way, but I'm pretty sure you can't combine two iterators inside a single for loop like your original example.

Answer 2

It looks like your trying to pass a the modules Map as a Collection which would cause a compile error.

Code to iterate over both could be something like this

public MyMethod(Map<Object, Object> objectMap, Integer ... intArray) {
    if( intArray.length != ObjectMap.size() ) {
       //However you want to handle this case
    }
    Iterator<Object> mapKeyIterator = objectMap.keySet().iterator();
    Iterator<Integer> integerIterator = Arrays.asList(intArray).iterator();

    while(mapKeyIterator.hasNext()) { //If the array and map are the same size then you only need to check for one.  Otherwise you'll need to validate both iterators have a next
        Object keyFromMap = mapKeyIterator.next();
        Object valueFromMap = objectMap.get(keyFromMap);
        Integer intFromArray = integerIterator.next();
        //Whatever you want to do
   }
}

If you know they are the same length then you could also traverse the array with a for(int i ... loop and just use an iterator for the map if you didn't want to create a List.

Answer 3

This seems like a very error-prone API, to ask for a completely disconnected map and array. Not just error-prone, but it will be hard for readers of the code to be able to tell which int goes with which map entry. I always advise against this kind of API. Try a Map<Object, MySpecialType> where MySpecialType aggregates both an Object and an int.