如何防止我的程序由于用户输入而崩溃

Question

我正在尝试实现“识别语言字符串”的算法

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

我的问题是，每当我输入任何没有$的内容时，它就会在while循环中崩溃。 防止它崩溃的最佳方法是什么？

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; 
        }


}

线程“主”中的异常java.lang.StringIndexOutOfBoundsException：字符串索引超出范围：java.lang.String.charAt（未知源）为4，assignmnet5.StackReferenceBased.isInLanguage（StackReferenceBased.java:87）为Assignmnet5.Question3.main （Question3.java:19）

这是我的主要：

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

听起来这可能就足够了：

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

Answer 2

可能的问题是空指针异常，请尝试在函数顶部添加此行

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

如果仍然崩溃，请完成代码，然后需要提供运行代码时遇到的错误。