为什么我最后的a不能return呢？

import java.util.EmptyStackException;
import java.util.Stack;
import *;
import java.util.Scanner;
import java.util.StringTokenizer;

public class ITP
{
   //public ITP();
   
    public static String infixToPostfix(String str)
    {
       int i=0, j=0;
        String[] input=new String[100];
        String a;
        
       ArrayStack<String> S = new ArrayStack<String>();
        ArrayStack<String> A = new ArrayStack<String>();
        
        StringTokenizer strTokenizer=new StringTokenizer(str, " ");
        
        while(strTokenizer.hasMoreTokens())
        {
           input[i]=strTokenizer.nextToken();
            i++;
        }
        
        while(j<input.length)
        {
           if(input[j]!="+" || input[j]!="-" || input[j]!="*" ||input[j]!="/")
           {
              S.push(input[j]);
                i++;
            }
            else if(input[j]=="+")
            {
                A.push(input[j]);
                a=S.pop()+A.pop()+input[j+1];
                j++;
            }
        }
        return a;
    }
}

        

请高手回答一下