Java: Using recursion with ArrayLists

Question

This calculates a chain of numbers.

biggestList and upperBound are initialized on top.

Even if I set i to some non 1 number, it only prints 1-> That's 1 numbers long!

The code was working fine with long tallys, but once I replaced them with ArrayLists that held the chains, it broke. Am I doing something wrong in the ArrayList code?

private void calculateIterations(){
    for (long i = 1; i <= upperBound; i++){
        calculateAnswer(i);
    }
}

private void calculateAnswer(long i) {
    long number = i;
    ArrayList <Long> tempList = new ArrayList<>();

    if (number == 1 ){
        tempList.add(number);
        if (tempList.size() > biggestList.size()){
            biggestList.clear();
            biggestList.addAll(tempList);
            printLargest();
        }
    }
    else if (number % 2 == 0){
        tempList.add(number);
        number /= 2;
        calculateAnswer(number);

    }
    else {
        tempList.add(number);
        number = number * 3 + 1;
        calculateAnswer(number);
    }
}
private void printLargest(){
    biggestList.stream().forEach((n) -> {
        System.out.print(n + " → ");
    });
    System.out.println("");
    System.out.println("That's " + biggestList.size() + " numbers long!");
}

It shows no compiler errors. I'm new to recursions and copying ArrayLists so I'm not even sure where to look. AFAIK calculateIterations only runs once, but print prints 1 even if i = whatever. So confused. — stravinsky7
– stravinsky7, Commented Aug 26, 2014 at 19:32
Hint, your method shouldn't be void and to use recursion it seems like it would need to take a List. — Elliott Frisch
– Elliott Frisch, Commented Aug 26, 2014 at 19:33

sitting_duck · Accepted Answer · 2014-08-26 19:39:11Z

1

I looks like every time you enter calculateAnswer() you create a new instance of the ArrayList and assign it to the tempList variable that is only in scope within that method. Seems like you need to have access to a tempList that is persistent across multiple calculateAnswer() calls.

answered Aug 26, 2014 at 19:39

sitting_duck

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1 Comment

stravinsky7 Over a year ago

That was exactly the problem. I put it up top and it worked like a charm. Don't know why a locally declared tally worked before, come to think of it.

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