Need to have HashMap add(sum) multiple values within a key - jsf

Question

I am attempting to have a single button process multiple inputs in the same input boxes. I have 16 input boxes, each with its own id# (YfProduct) which I am using as the key for my hashmap. For the input value, I have Weight. A user will enter in whatever double weight they want, in however many input boxes they wish, and click a button (an a4j:commandButton) which activates the method below.
private HashMap<Integer, Double> storeWeight = new HashMap<Integer, Double>();
public void storeWeight(Yieldfl yieldfl){
for (YieldItem row : yielditem) {
storeWeight.put(row.getYfProduct(), row.getWeight());
System.out.print(storeWeigt)}
}
Right now this code will set the appropriate values with the right key, and replace those values with new input entered and another button click. However what I am trying to do is have the bean save the previous values, and sum up the next values entered with the previous entry(s) that have the same key. So at the end of the user's input, the HashMap will contain 16 keys with the sum of the individual values added up for each key. I havn't been able to come up with a way to do this without some serious hardcoding. Help much appreciated.

Answer 1

So this actually turned out to be a simple fix, I'll put the solution on here just incase somebody runs into a similar problem. It's my first time working with a HashMap so I had a colleague help me out. I needed the value to call the key.
storeWeight.put(row.getYfProduct(), storeWeight.get(row.getYfProduct() + (row.getWeight()));
and to avoid null-pointers:
public void storeWeight(Yieldfl yieldfl){
for (YieldItem row : yielditem) {
Double oldValue = storeWeight.get(row.getYfProduct());
if (oldValue == null)
oldValue = 0.0;
storeWeight.put(row.getYfProduct(), oldValue + (row.getWeight()));
row.setWeight(0.0);
System.out.print(storeWeight);}
}