InfixExpr.java

package com.sprint.datastructure;

import java.util.List;
import java.util.Stack;
public class InfixExpr {
	String expr = null;

	public InfixExpr(String expr) {
		this.expr = expr;
	}
	public float evaluate() {
		TokenParser parser = new TokenParser();
		List<Token> tokens = parser.parse(this.expr);

		Stack<Token> opStack = new Stack<>();
		Stack<Float> numStack = new Stack<>();
		
		for (Token token : tokens) {
			if (token.isOperator()) {
				while (!opStack.isEmpty()
						&& !token.hasHigherPriority(opStack.peek())) {
					Token prevOperator = opStack.pop();
					Float f2 = numStack.pop();
					Float f1 = numStack.pop();
					Float result = calculate(prevOperator.toString(), f1, f2);
					numStack.push(result);
				}
				opStack.push(token);
			}

			if (token.isNumber()) {
				numStack.push(new Float(token.getIntValue()));
			}
		}

		while (!opStack.isEmpty()) {
			Token token = opStack.pop();
			Float f2 = numStack.pop();
			Float f1 = numStack.pop();
			numStack.push(calculate(token.toString(), f1, f2));
		}

		return numStack.pop().floatValue();
	}

	private Float calculate(String op, Float f1, Float f2) {
		if (op.equals("+")) {
			return f1 + f2;
		}
		if (op.equals("-")) {
			return f1 - f2;
		}
		if (op.equals("*")) {
			return f1 * f2;
		}
		if (op.equals("/")) {
			return f1 / f2;
		}
		throw new RuntimeException (op + " is not supported ");
	}
}