使用堆栈的后缀转换中缀的错误

Question

我正在使用堆栈执行从后缀表达式到后缀表达式的转换。 但是，执行时会发生以下错误：

Exception in thread "main" java.lang.NullPointerException
      at Stack.pop(InfixReader.java:108)
      at InfixReader.doConversion(InfixReader.java:46)
      at InfixReader.main(InfixReader.java:6)

这是一个空指针异常，似乎起源于pop（）方法。但是，我无法修复此错误。这是否意味着return语句将返回空值，或者顶部将指向空值。

这是我的代码：

import java.io.*;

public class InfixReader {

    public static void main(String[] args) {
        InfixReader myAnswer = new InfixReader();
        myAnswer.doConversion();
    }

    public void doConversion() {
        String inp[] = readInfix();
        Stack post = new Stack();
        String n = "";
        String ope = "";
        int l = inp.length;
        for (int i = 0; i < l; i++) {
            if (inp[i] != "+" || inp[i] != "-" || inp[i] != "*" || inp[i] != "/" || inp[i] != "^") {
                n += inp[i];
            } else if (post.empty() == true)
                post.push(inp[i]);
            else {
                boolean id = false;
                while (id == false)
                    ope = post.peek();
                if (ope == "^")
                    n += post.pop();
                if (ope == "*" || ope == "/")
                    if (inp[i] == "*" || inp[i] == "/" || inp[i] == "+" || inp[i] == "-")
                        n += post.pop();
                if (ope == "+" || ope == "-")
                    if (inp[i] == "+" || inp[i] == "-")
                        n += post.pop();
                if (inp[i] == "^") {
                    post.push(inp[i]);
                    id = true;
                }
                if (inp[i] == "*" || inp[i] == "/")
                    if (ope == "+" || ope == "-") {
                        post.push(inp[i]);
                        id = true;
                    }
            }
        }
        while (post.empty() != true) {
            n += post.pop();
        }
        evalPostfix(n);
    }

    public void evalPostfix(String postfix) {
        Stack eval = new Stack();
        int l = postfix.length();
        for (int i = 0; i < l; i++) {
            char ch = postfix.charAt(i);
            if (ch != '+' || ch != '-' || ch != '*' || ch != '/' || ch != '^') {
                eval.push(Character.toString(ch));
            } else {
                String ch1 = eval.pop();
                String ch2 = eval.pop();
                int o1 = Integer.parseInt(ch1);
                int o2 = Integer.parseInt(ch2);
                if (ch == '+')
                    eval.push(Integer.toString(o1 + o2));
                if (ch == '-')
                    eval.push(Integer.toString(o1 - o2));
                if (ch == '*')
                    eval.push(Integer.toString(o1 * o2));
                if (ch == '/')
                    eval.push(Integer.toString(o1 / o2));
                if (ch == '^')
                    eval.push(Integer.toString(o1 ^ o2));
            }
        }
        System.out.println(eval.pop());
    }

    public String[] readInfix() {
        BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
        String inputLine;
        try {
            System.out.print("Input infix: ");
            inputLine = input.readLine();
            return inputLine.split(" ");
        } catch (IOException e) {
            System.err.println("Input ERROR.");
        }
        // return empty array if error occurs
        return new String[]{};
    }
}

class Stack {
    private int maxSize;
    private String[] stackArray;
    private int top;

    public void MyStack(int s) {
        maxSize = s;
        stackArray = new String[maxSize];
        top = -1;
    }

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

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

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

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

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