InfixExpr.java

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

/**
 * Created by Lxx on 2017/4/29.
 */
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()){
                if(opStack.isEmpty()){
                    opStack.push(token);
                }else {
                    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;
        }
        return null;
    }
}