Java 后缀表达式计算器

Question

I really need help with my post-fix expression calculator. I don't really know what's wrong with the code that I've written, however when I run the program, it just prints whatever number is on the top. For example, if I input "7 2 +", the output is 2. If I input "2 7 +", the output is 7. Could somebody please point me in the right direction on how to fix this? I think (not sure) that the problem is that my program is not able to properly detect the operands "+" and "*". However, I can't tell why.

File #1:

import java.io.*;
import java.util.*;

public class ProblemTwo {

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);
        System.out.println("Enter a post value expression: ");
        String input = scan.nextLine();
        StringTokenizer st = new StringTokenizer(input);
        Stack hello = new Stack(st.countTokens());

        for (int i = 0; i <= st.countTokens(); i++) {
            String inputToken = st.nextToken();
            if (inputToken.trim().contains("+")) {
                int sum = Integer.parseInt(hello.pop() + Integer.parseInt(hello.pop()));
                System.out.println(sum);
                hello.push(Integer.toString(sum));
            }
            else if (inputToken.trim().contains("*")){
                int product = Integer.parseInt(hello.pop()) * Integer.parseInt(hello.pop());
                hello.push(Integer.toString(product));
            }
            else {
                hello.push(inputToken);
            }
        }
        System.out.println(hello.pop());
    }
}

File #2:

public class Stack {

        private String[] stackArray;
        private int arraySize;
        private int top;

        public Stack(int capacity) {
            arraySize = capacity;
            stackArray = new String[arraySize];
            top = -1;
        }

        public void push(String i) {
            stackArray[++top] = i;
        }

        public String pop() {
            return stackArray[top--];
        }

        public boolean isEmpty() {
            return top == -1;
        }

        public boolean isFull() {
            return top == arraySize - 1;
        }
    }

Answer 1

Problem is you are using st.countTokens() in for loop but each subsequent call to countTokens() returns count of the number of times this tokenizer's nextToken method can be called . From StringTokenizer 's doc:

Calculates the number of times that this tokenizer's nextToken method can be called before it generates an exception. The current position is not advanced.

Use another variable to capture st.countTokens() before starting loop or better use st.hasMoreTokens() for terminating loop. Like:

while (st.hasMoreElements()) {
 // same logic
}

Also modify pop() method to return stackArray[top--]; .

Answer 2

Your code has at least 2 problems: Your took not all inputed value in for loop and your stack has broken implementation. Here is working version.

File #1:

import java.io.*;
import java.util.*;

public class ProblemTwo {

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);
        System.out.println("Enter a post value expression: ");
        String input = scan.nextLine();
        StringTokenizer st = new StringTokenizer(input);
        Stack hello = new Stack(st.countTokens());

        for (int i = 0; i <= st.countTokens()+1; i++) {
            String inputToken = st.nextToken();
            if (inputToken.trim().contains("+")) {
                int sum = Integer.parseInt(hello.pop()) + Integer.parseInt(hello.pop());
                System.out.println(sum);
                hello.push(Integer.toString(sum));
            }
            else if (inputToken.trim().contains("*")){
                int product = Integer.parseInt(hello.pop()) * Integer.parseInt(hello.pop());
                hello.push(Integer.toString(product));
            }
            else {
                hello.push(inputToken);
            }
        }
        System.out.println(hello.pop());
    }
} 

File #2:

public class Stack {

    private String[] stackArray;
    private int arraySize;
    private int top;

    public Stack(int capacity) {
        arraySize = capacity;
        stackArray = new String[arraySize];
        top = -1;
    }

    public void push(String i) {
        stackArray[++top] = i;
    }

    public String pop() {
        return stackArray[top--];
    }

    public boolean isEmpty() {
        return top == -1;
    }

    public boolean isFull() {
        return top == arraySize - 1;
    }
}

Answer 3

import java.util.Stack;
public class Postfix_Eva {
public static void main(String[] args)
{
    String exp="231*+9-";//giving input
    System.out.println(eva(exp));//calling eva method with argument exp(String)
}

static int eva(String e)
{
    Stack stk=new Stack();//creating object for Stack
    for(int i=0;i<e.length();i++)
    {
        char c=e.charAt(i); //retriving each character from String using .charAt() method
        if(Character.isDigit(c)) //checking whether the character is digit or not
        {
            //Casting is not done
            stk.push(c-'0');

        }
        else
        {
            int a=(int) stk.pop(); //casting popped value into int
            int b=(int) stk.pop();
            switch(c)
            {
                case '+': 
                stk.push(a+b); 
                break; 

                case '-': 
                stk.push(a-b); 
                break; 

                case '/': 
                stk.push(a/b); 
                break; 

                case '*': 
                stk.push(a*b); 
                break; 
            }
        }
    }
    return (int) stk.pop();//returning int casted value of stk.pop() 
  }
}