PrefixExpr.java

package com.sprint.datastructure;

import java.util.List;
import java.util.Stack;
public class PrefixExpr {
	String expr = null;
	
	public PrefixExpr(String expr) {
		this.expr = expr;	
	}

	public float evaluate() {
		TokenParser parser = new TokenParser();
		List<Token> tokens = parser.parse(this.expr);
		
		Stack<Token> exprStack = new Stack<>();
		Stack<Float> numStack = new Stack<>();

		for (Token token : tokens) {
			exprStack.push(token);
		}
		
		while (!exprStack.isEmpty()) {
			Token t = exprStack.pop();
			if (t.isNumber()) {
				numStack.push(new Float(t.getIntValue()));		
			} else {
				Float f1 = numStack.pop();
				Float f2 = numStack.pop();
				numStack.push(calculate(t.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 ");
	}	
}