JAVA作业题提问,关于字符串和静态方法.

Question

老师布置了一个作业,大致如下.
在一篇文章中,例如:
Abc.Def.Ghk.Gml.Zkb.
需要用户输入一个字符.
然后输出前后文.
如用户输入e, 则输出前一个句子,自身句子,和后面的句子.即输出
Abc.DEf.Ghk. (同时需要把用户输入的字符大写)
如果前面或者后面没有句子,则可以忽略.

老师说需要用至少一个静态方法完成.

在此请教各位.

public static String foo(String text, String find) {
    String lowertext=text.toLowerCase();
    String arr[] = text.split("\\.");
    String lowerArr[] = text.split("\\.");
    String lowerfind = find.toLowerCase();
    for (int i = 0; i < arr.length; i++) {
        if (lowerArr[i].indexOf(lowerfind) != -1) {
            if (i == 0)
                return arr[0] + "." + arr[1] + ".";
            else if (i > 0 && i < (arr.length) - 1)
                return arr[i - 1] + "." + arr[i] + "." + arr[i + 1] + ".";
            else if (i == (arr.length) - 1)
                return arr[i - 1] + "." + arr[i] + ".";
        }
        if (arr[i].indexOf(find) != -1)
            break;
    }
    return null;
}

public static void main(String[] args) {
    String oriText = searchText();
    Scanner input = new Scanner(System.in);
    String cusInput = "";
    while (cusInput.length() < 1) {
        System.out.println("Please enter a word or a phrase and I will show its context");
        cusInput = input.nextLine();
    }
    String lowerCaseInput = cusInput.toLowerCase();
    String lowerCaseText = oriText.toLowerCase();
    int a = lowerCaseText.indexOf(lowerCaseInput);
    if (a == -1)
        System.out.println("The text does not contain the specified phrase.");
    else {
        String result = foo(oriText, cusInput);
        String upperCaseInput = lowerCaseInput.toUpperCase();
        System.out.println("The phrase " + upperCaseInput + " appears in the following context:");
        System.out.println("----------------------------------------------------------------");
        System.out.println((result.replaceFirst(lowerCaseInput, upperCaseInput)));
    }
}

}

以上是我的代码,输入某些结果的时候有时候能正确输出,但有时候会显示
context:Exception in thread "main"
java.lang.NullPointerException
at assignment4.main(assignment4.java:55)
这个错误代码,请问该如何改正?

Answer 1

 public static String foo(String s, String find)
    {
        String arr[] = s.split("\\.");
        for (int i = 1; i < arr.length - 1; i++)
        {
                     int pos =arr[i].indexOf(find);
            if ( pos != -1)
                            arr[i] = arr[i].substring(0,pos) + (char)(arr[i].charAt(pos)+32)+arr[i].substring(pos+1);
                                if(pos == 0){
                                    return arr[i] + "." + arr[i + 1];
                                }
                return arr[i - 1] + "." + arr[i] + "." + arr[i + 1];
        }
        return "";
    }