PrefixExpr.java

package com.github.HarryHook.coding2017.stack.expr;

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);
	}
	System.out.println(tokens);
	while (!exprStack.isEmpty()) {
	    Token t = exprStack.pop();
	    if (t.isNumber()) {
		numStack.push(new Float(t.getIntValue()));
	    } else if(t.isOperator()){
		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");
    }

    public static void main(String[] args) {
	PrefixExpr expr = new PrefixExpr("-++6/*2 9 3 * 4 2 8");
	System.out.println("The result of the expression: " + expr.evaluate());
    }

}