I had convert() as a static method before, but I've changed it prior to uploading. The computer I'm on doesn't have any IDE and I can't be bothered compiling and running through a CLI, so I hope it works (it seems like it should :P).

It's a simple Base Convert for Java, you create a new BaseConverter object with parameters inBase - the base the strings you want to convert are in - and outBase - the base you want the strings to be converted to - (they can be changed with the set methods). You then call the method convert(String numString) with numString being the number you want to convert (must be in base-inBase). There are also get methods to obtain the current in and out bases.

Comments are welcome.

import java.util.ArrayList;
public class BaseConverter {

    private int inBase;
    private int outBase;

    public BaseConverter(int iB, int oB) {
        inBase = iB;
        outBase = oB;
    }

    public int getInBase() {
        return inBase;
    }

    public int getOutBase() {
        return outBase;
    }

    public void setInBase(int iB) {
        inBase = iB;
    }

    public void setOutBase(int oB) {
        outBase = oB;
    }

    public String convert(String numString) {
        String answer = "";
        int[] numReversed = new int[numString.length()];
        int decimalValue = 0;
        int arrayIndex = 0;
        for (int currentCharIndex = numString.length() - 1; currentCharIndex >= 0; currentCharIndex--) {
            switch (numString.charAt(currentCharIndex)) {
                case 'a':
                case 'A': numReversed[arrayIndex] = ((int) 'A') - 55; break;
                case 'b':
                case 'B': numReversed[arrayIndex] = ((int) 'B') - 55; break;
                case 'c':
                case 'C': numReversed[arrayIndex] = ((int) 'C') - 55; break;
                case 'd':
                case 'D': numReversed[arrayIndex] = ((int) 'D') - 55; break;
                case 'e':
                case 'E': numReversed[arrayIndex] = ((int) 'E') - 55; break;
                case 'f':
                case 'F': numReversed[arrayIndex] = ((int) 'F') - 55; break;
                default: numReversed[arrayIndex] = Integer.parseInt(Character.toString(numString.charAt(currentCharIndex)));
            }
            if (numReversed[arrayIndex] > inBase - 1) {
                return numString + " is not a valid base-" + inBase + " number.";
            }
            numReversed[arrayIndex] *= Math.pow(inBase,arrayIndex);
            arrayIndex++;
        }
        for (int b = 0; b < numReversed.length; b++)
            decimalValue += numReversed[b];
        if (outBase == 10)
            return Integer.toString(decimalValue);
        ArrayList<Integer> nums = new ArrayList<Integer>();
        while (decimalValue != 0) {
            nums.add(decimalValue % outBase);
            decimalValue /= outBase;
        }
        for (int currentArrayIndex = nums.size() - 1; currentArrayIndex >= 0; currentArrayIndex--) {
            if (nums.get(currentArrayIndex) >= 10) {
                answer += Character.toString((char) (nums.get(currentArrayIndex) + 55));
                continue;
            }
            answer += Integer.toString(nums.get(currentArrayIndex));
        }
        return answer;
    }

}