Java—在Try/Catch过程中出现随机“null”

我有一个脚本，测试输入的字符的有效性，以便学习Try/Catch。必须以严格的$123.45格式输入数字，否则Try/Catch会打印错误。它需要能够捕获所有整数错误并打印它们。目前，如果输入的一美元和一美分不正确，脚本将正常工作并打印这两个错误。但是，如果在输入美分时仅发现错误，则打印行显示以null开头

工作示例： 输入： 请在表格$中输入销售金额。q退出：12天

打印： 无效的美元格式-用于输入字符串：12d

无效的美分格式-用于输入字符串：de

示例不起作用： 输入： 请在表格$中输入销售金额。q退出：$123.de

打印： Null无效的美分格式-用于输入字符串：de

错误在myCent的Catch中的+=中：

myBad+=无效的美分格式-用于输入字符串：\+mySale.substringmySale.indexOf'.+1，mySale.length+\\n

如何让Catch在不丢失多错误打印功能的情况下不打印null？非常感谢所有帮助我们的人

该程序由以下两个脚本组成，DKSaleCheck.java是我的问题孩子： DKUnit6Ch15.java

DKSaleCheck.java

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恐怖袋熊有最好的答案。我改变了：

public DKSaleCheck(String mySale) {    
   myBad = null; 
   //rest of class
}

public void print(){ 
   if(myBad != null)
   //rest of class
}

致：

---

像冠军一样工作

您有myBad+=需要myBad=的地方。因为它最初是空的，所以会得到一个空值。但是如果我删除+它将不再找到倍数。它在只打印带有美分的错误后结束。例如，如果我输入$12d.de，它只显示美分部分的错误，而不是美元部分的错误。请格式化代码。在调试时，您可以看到在您的情况下什么是null；到myBad=；倍数是多少？一次只能得到一个异常。

    class DKSaleCheck{ //Begin Class DKSaleCheck

    int myDollar; //Define a new Variable
    int myCent; //Define a new Variable
    String myBad; //Define a new String Variable

    public DKSaleCheck(String mySale) { //Begin Method DKSaleCheck and receive sale as a string
        myBad = null; //Define a new Variable

        if(!mySale.startsWith("$")) { //Begin If Statement (if mySale does NOT start with a "$")
            myBad = "Invalid sale format missing \"$\" - " + mySale + "\n"; //Fill the Variable with the String data
        } //End If Statement

        else if(mySale.indexOf('.') == -1) { //Begin ElseIf Statement (if mySale does NOT contain a ".")
            myBad = "Invalid sale format missing \".\" - " + mySale + "\n"; //Fill the Variable with the String data
        } //End ElseIf Statement

        else{ //Begin Else Statement
            try{ //Begin Try Statement
                myDollar = Integer.parseInt(mySale.substring(1, mySale.indexOf('.'))); //Fill the Variable with the data if ONLY integers are detected from Index 1 to the "."
            } //End Try Statement

            catch(Exception myError) { //Begin Catch Statement (if the subString does not contain ONLY integers)
                myBad = "Invalid dollar format - For input string: \"" + mySale.substring(1,mySale.indexOf('.')) + "\"\n"; //Fill the Variable with the String data
            } //End Catch Statement

            try{ //Begin Try Statement
                myCent = Integer.parseInt(mySale.substring(mySale.indexOf('.') + 1,mySale.length())); //Fill the Variable with the data if ONLY integers are detected after the "."
            } //End Try Statement
            catch(Exception myError) { //Begin Catch Statement (if the subString does not contain ONLY integers)
                myBad += "Invalid cents format - For input string: \"" + mySale.substring(mySale.indexOf('.') + 1,mySale.length()) + "\"\n"; //Fill the Variable with the String data
            } //End Catch Statement
        } //End Else Statement

    } //End Method DKSaleCheck

    public void print(){ //Begin Print Method

        if(myBad != null){ //Begin If Statement (if the error variable is NOT null)
            System.out.println(myBad); //Print the String Variable     
        } //End If Statement

        else{ //Begin Else Statement
            System.out.println("$" + myDollar + "." + myCent); //Print the text
            System.out.println(myDollar + " dollars and " + myCent + " cents\n"); //Print the text
        } //End Else Statement

    } //End Print Method

} //End Class DKSaleCheck

public DKSaleCheck(String mySale) {    
   myBad = null; 
   //rest of class
}

public void print(){ 
   if(myBad != null)
   //rest of class
}

public DKSaleCheck(String mySale) {    
   myBad = ""; 
   //rest of class
}

public void print(){ 
   if(myBad != "")
   //rest of class
}