PostfixExpr.java

package week07.expr;

import java.util.List;
import java.util.Stack;

public class PostfixExpr {
	String expr = null;

	public PostfixExpr(String expr) {
		this.expr = expr;
	}

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

		Stack<Float> numStack = new Stack<>();
		for (Token token : tokens) {
			if (token.isNumber()) {
				numStack.push(new Float(token.getIntValue()));
			} else if (token.isOperator()) {
				Float f2 = numStack.pop();
				Float f1 = numStack.pop();
				Float result = calculate(token.toString(), f1, f2);
				numStack.push(result);
			}
		}

		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");
	}

}