Store instance of constantly changing variable in another variable in java

Question

I have a small program with an algorithm to iterate through n number of parcels to find a certain weight numWeight. After each iteration, the weight of a parcel is increased. When a solution is found, the solution should be stored in the solutions variable. As such, when a solution is found, (when currentWeight == numWeight) I want the current instance of comb to be stored in an index of the solutions arraylist. However, when try and store comb in solutions, the instance stored continues to change with comb. How would I get it so that solutions could store an instance of comb as it is at the time that line executes? The code snippet in question is as follows:

    public void solution2(){
    BookCombination comb = new BookCombination(numParcels, mailClass);
    ArrayList<BookCombination> solutions = new ArrayList<>();
    int currentWeight;
    while (!stopCalc){
        currentWeight = comb.getWeight();
        if (currentWeight == 0){
            break;
        } else if (currentWeight == numWeight){
            solutions.add(comb);
            comb.increaseWeight();
        } else {
            comb.increaseWeight();
        }
    }
}

Thanks!

davidxxx · Accepted Answer · 2017-08-06 17:23:41Z

1

You have to create a new BookCombination object at each time you add one in the list.
Otherwise you use the same for next iterations.

Creating exactly the same object would not make really sense :

 comb = new BookCombination(numParcels, mailClass);

I think that you should increment maybe the number of parcels or something in this way to test other combinations.

Try it :

public void solution2(){
    BookCombination comb = new BookCombination(numParcels, mailClass);
    ArrayList<BookCombination> solutions = new ArrayList<>();
    int currentWeight;
    while (!stopCalc){
        currentWeight = comb.getWeight();
        if (currentWeight == 0){
            break;
        } else if (currentWeight == numWeight){
            solutions.add(comb);
            comb = new BookCombination(numParcels++, mailClass); // change here               
        } else {
            comb.increaseWeight();
        }
    }
}

edited Aug 6, 2017 at 17:23

answered Aug 6, 2017 at 17:18

davidxxx

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