multiply 2 arraysLists together in java

Question

So I have an two arrayLists that represent two numbers. This is so I don't have to use BigInt. so for example

ArrayList<Integer> LargeInt = [2,3,6,4] would really equal 2,364
ArrayList<Integer> LargeInt2 = [8,7,9,4,6] would be 87,946

my goal is to figure out a way to multiply the two numbers and make the answer to a string. I know that multiplying two numbers would need to be put into another array before put into a string so it won't crash with larger numbers. I also know it will be one for loop put into another. But i am finding it difficult to make a code that multiplies the two numbers. the two arrays being multiplied can be any number.

BigInteger is going to be much faster than whatever you end up creating. — , Sep 14 '12 at 02:41
in this situation i can't use bigInt. that makes it more difficult. I know bigInt is better but this situations its not an option. — user1670252, Sep 14 '12 at 02:42
You remember how to do multiplication by hand right? From elementary school? Now do it in code. — Mark Peters, Sep 14 '12 at 02:43
yes i know how the issue is making a code that multiplies each number together then adding them. — user1670252, Sep 14 '12 at 02:44
Unless you have been forced to do this as an exercise, don't reinvent the wheel... use `BigInteger` — Bohemian, Sep 14 '12 at 02:44
@user: The best way to start is by writing code. Show us what you've tried. — Mark Peters, Sep 14 '12 at 02:44
@Bohemian: It's homework. He's been asked to reinvent the wheel by a teacher. — Don Roby, Sep 14 '12 at 02:48

score 5 · Answer 1 · answered Sep 14 '12 at 02:44

Assuming that this is homework, here is a no-code explanation of what you need to do:

Define a class that wraps ArrayList<Integer>; let's say you call it ArrayInt
Define an operation that adds two ArrayInts together, and returns a third ArrayInt that equals their sum. You can do it digit by digit, taking care of a possible carry into an extra digit, so you need to size your result accordingly.
Define an operation that multiplies your number by a power of ten by adding zeros to the array list. Again, the operation should return a new ArrayInt, rather than modifying the current one
Define an operation that multiplies a number by a single digit. You can use multiplication, or a simple loop that uses addition. The loop would not run more than nine times, so it shouldn't be too bad.
Combine the three operations that you have (addition, multiplication by a digit, and multiplication by a power of ten) into a simple multiplication algorithm that you learned in the elementary school.

I would agree on this. But there might be approach to save some runtime, follow the rules of Bitwise Multiplication. You may want to take a look the code snippet presented [here](http://stackoverflow.com/questions/4456442/interview-multiplication-of-2-integers-using-bitwise-operators) to give you an idea how it is done. — David B, Sep 14 '12 at 04:59
@Dr.Java I seriously doubt that saving time is a goal here: after all, it's a homework exercise. — Sergey Kalinichenko, Sep 14 '12 at 09:36

multiply 2 arraysLists together in java

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