Solves mathematical exercises

Exercise.java

import java.util.List;
import java.util.ArrayList;
import java.util.Arrays;

class Exercise {

	public List<String> ex;

	public Exercise(String[] input) {
		ex = new ArrayList<String>(Arrays.asList(input));
	}

	public String toString() {
		String s = "" + ex.get(0);
		for (int i = 1; i < ex.size(); i++) {
			s += " " + ex.get(i);
		}
		return s;
	}

	public float solve() {
		List<String> sol = new ArrayList<String>(ex);
		while (sol.size() > 1) {

			// search for closing parenthesis
			// set close to list length if there isn't
			int indexOfClose = sol.size();
			if (sol.indexOf(")") != -1) {
				indexOfClose = sol.indexOf(")");
			}

			// search for opening parenthesis before
			int indexOfOpen = -1;
			for (int i = indexOfClose - 1; i >= 0; i--) {
				if (sol.get(i).equals("(")) {
					indexOfOpen = i;
					break;
				}
			}

			// if there is only one element in between, remove the parenthesis
			if (indexOfClose - indexOfOpen == 2) {
				sol.remove(indexOfClose);
				sol.remove(indexOfOpen);
				continue;
			}

			// else create sublist and apply calculation
			List<String> sublist = sol.subList(indexOfOpen + 1, indexOfClose);

			// look for operations to calculate (in order)
			if (sublist.indexOf("X") != -1) {
				sublistOperation(sublist, "X");
			} else if (sublist.indexOf("/") != -1) {
				sublistOperation(sublist, "/");
			} else if (sublist.indexOf("+") != -1) {
				sublistOperation(sublist, "+");
			} else if (sublist.indexOf("-") != -1) {
				sublistOperation(sublist, "-");
			}
		}

		return Float.parseFloat(sol.get(0));
	}

	private void sublistOperation(List<String> sublist, String operator) {
		int i = sublist.indexOf(operator);
		sublist.set(i, calculate(sublist.get(i - 1), sublist.get(i + 1), operator));
		sublist.remove(i + 1);
		sublist.remove(i - 1);
	}

	private String calculate(String a, String b, String operator) {
		float floatA = Float.parseFloat(a);
		float floatB = Float.parseFloat(b);
		if (operator.equals("X")) {
			return "" + (floatA * floatB);
		}
		if (operator.equals("/")) {
			return "" + (floatA / floatB);
		}
		if (operator.equals("+")) {
			return "" + (floatA + floatB);
		}
		if (operator.equals("-")) {
			return "" + (floatA - floatB);
		}
		return "" + 0;
	}

	public static void main(String[] args) {
		// exercise to solve: ((5 + 1) / 2) + 2 X (3 - 1)
		// expected solution: 7
		Exercise exercise = new Exercise(new String[]{"(", "(", "5", "+", "1", ")", "/", "2", ")", "+", "2", "X", "(", "3", "-", "1", ")"});
		System.out.println("Exercise: " + exercise);
		System.out.println("Solution: " + exercise.solve());

		// exercise to solve: (1 / 4) + 2.5 X 2.5
		// expected solution: 6.5
		exercise = new Exercise(new String[]{"(", "1", "/", "4", ")", "+", "2.5", "X", "2.5"});
		System.out.println("Exercise: " + exercise);
		System.out.println("Solution: " + exercise.solve());
	}
}