如何防止我的程序由於用戶輸入而崩潰

Question

我正在嘗試實現“識別語言字符串”的算法

L = {'w $ w'：w是$以外的可能的空字符串，w'= reverse（w）}

我的問題是，每當我輸入任何沒有$的內容時，它就會在while循環中崩潰。 防止它崩潰的最佳方法是什么？

public boolean isInLanguage(String inputString)
{
        StackReferenceBased stack1 = new StackReferenceBased(); 
        StackReferenceBased stack2 = new StackReferenceBased(); 
        Object qItem;
        Object sItem;
        int index = 0; 

        if (inputString.length() == 0)
        {
            return false; // empty string not in L  
        }

        else if (inputString.length() == 1)
        {
            return true; 
        }

        **while (inputString.charAt(index) != '$')**
        { 
            // save the first half of the string
            stack1.push(inputString.charAt(index));

            ++index;
        }  

        // index points to '$' or its value > than inputString.length()
        while (index < inputString.length()-1)
        {
            // save the second half of the string
            ++index;
            stack2.push(inputString.charAt(index));
        } 

        do
        {
            // match the first half of the string with the second half
        if ((stack1.isEmpty() && !stack2.isEmpty()) ||(!stack1.isEmpty() && stack2.isEmpty()))
        {
            return false;
        }
        qItem = stack1.peek();
        sItem = stack2.peek();

        if (qItem != sItem)
        {
            return false;
        }

        if (!stack1.isEmpty())
        {
            stack1.pop();
        }

        if (!stack2.isEmpty())
        {
            stack2.pop();
        }

        }while (!stack1.isEmpty() || !stack2.isEmpty());


        if (stack1.isEmpty() && stack2.isEmpty())
        {
            return true;
        }
        else
        {
            return false; 
        }


}

線程“主”中的異常java.lang.StringIndexOutOfBoundsException：字符串索引超出范圍：java.lang.String.charAt（未知源）為4，assignmnet5.StackReferenceBased.isInLanguage（StackReferenceBased.java:87）為Assignmnet5.Question3.main （Question3.java:19）

這是我的主要：

public static void main(String[]args)
    {
        StackReferenceBased stack = new StackReferenceBased(); 

        String str;
         boolean bool;

         Scanner kb = new Scanner(System.in);
         System.out.println( "Enter a string to be checked by the algorithm : ");
         str = kb.next();

        **bool = stack.isInLanguage(str);**
        if (bool == true)
           System.out.println( "The string is in language");
        else 
            System.out.println("The string is not in language");
    }

Answer 1

聽起來這可能就足夠了：

    if (inputString == null || !inputString.contains("$")) {
        return false; // empty string not in L  
    }

Answer 2

可能的問題是空指針異常，請嘗試在函數頂部添加此行

public boolean isInLanguage(String inputString)
{
    if(inputString == null){
        return false;
    }
...
...

如果仍然崩潰，請完成代碼，然后需要提供運行代碼時遇到的錯誤。