java - PhoneDirectory程序中的NullPointerException

我在PhoneDirectory程序中遇到错误。当我尝试编译ArrayPhoneDirectoryTester时，它会成功构建，但是返回错误：

Exception in thread "main" java.lang.NullPointerException
at ArrayPhoneDirectory.addChangeEntry(ArrayPhoneDirectory.java:60)
at ArrayPhoneDirectoryTester.main(ArrayPhoneDirectoryTester.java:17)
Java Result: 1

通过一些在线研究，我了解到这意味着我要使用的变量已设置为null，并且该变量出现在第60行的ArrayPhoneDirectory Java文件中以及第17行的ArrayPhoneDirectoryTester Java文件中（都在下面的代码中进行了注释。）。我仍然不确定它指向哪个变量，因此，对解决此异常的任何帮助将不胜感激。

ArrayPhoneDirectory代码：

import java.io.*;
import java.util.*;

public class ArrayPhoneDirectory implements PhoneDirectory {

private static final int INIT_CAPACITY = 100;
private int capacity = INIT_CAPACITY;

//holds telno of directory entries
private int size = 0;

//Array to contain directory entries
private DirectoryEntry[] theDirectory = new DirectoryEntry[capacity];

//Holds name of data file to be read
private String sourceName = null;

/**
 * Flag to indicate whether directory was modified since it was last loaded
 * or saved.
 */
private boolean modified = false;

// PUBLIC INTERFACE METHODS

public void loadData(String sourceName) {
    Scanner scan = new Scanner(sourceName).useDelimiter("\\Z");

    while (scan.hasNextLine()) {
        String name = scan.nextLine();
        String telno = scan.nextLine();

        add(name, telno);
    }
}

/**
 * find method is called, returning the position in the array of the given
 * name.
 */
public String lookUpEntry(String name) {
    find(name);
    return null;

}

/**
 * for loop that checks every DirectoryEntry inside theDirectory and then
 * checks it against the name and telno given in the parameter, if both are
 * equal, the telno is upda ted, else it is added to theDirectory
 *
 */
public String addChangeEntry(String name, String telno) {
    for (DirectoryEntry x : theDirectory) {
        if (x.getName().equals(name)) {              //LINE 60
            x.setNumber(telno);
            return x.getNumber();
        } else {
            add(name, telno);
        }
    }
    return null;
}

//TO COMPLETE
public String removeEntry(String name) {
    return null;
}

/**
 * A new PrintWriter object is created, and a for loop is used to print the
 * name and number of each DirectoryEntry to the console.
 */
public void save() {
    PrintWriter pw = null;
    try {
        pw = new PrintWriter(new FileWriter("directory.txt", true));

        for (DirectoryEntry x : theDirectory) {
            pw.write(x.getName());
            pw.write(x.getNumber());
        }
    }
    catch(Exception e) {
        e.printStackTrace();
    }
    finally {
        pw.close();
    }
}




//Private helper methods

private void reallocate() {
    capacity = capacity * 2;
    DirectoryEntry[] newDirectory = new DirectoryEntry[capacity];
    System.arraycopy(theDirectory, 0, newDirectory,
            0, theDirectory.length);

    theDirectory = newDirectory;
}

private void add(String name, String telno) {
    if (size >= capacity) {
        reallocate();
    }
    theDirectory[size] = new DirectoryEntry(name, telno);
    size = size + 1;
}

private int find(String name) {
    int i = 0;
    for (DirectoryEntry x : theDirectory) {
        if (x.getName().equals(name)) {
            return i;
        }
        i++;
    }
    return -1;
}

@Override
public String format() {
    return null;
}



}

ArrayPhoneDirectoryTester代码：

public class ArrayPhoneDirectoryTester {
public static void main (String[] args)
{
    //creates a new PhoneDirectory
    PhoneDirectory newdir = new ArrayPhoneDirectory();

    newdir.addChangeEntry("Joe Perkins", "999999");  //LINE 17

    System.out.println(newdir);
}

}

最佳答案

当您尝试访问为空的对象时（例如，在空对象上调用方法），将引发NullPointerException。在第60行中：

x.getName().equals(name)

您有两个方法调用。为了使该行抛出NPE，“ x”为空，或者“ x.getName（）”为空。如果将其重构为两行，您将知道：

String xName = x.getName();
xName.equals(name)

理想情况下，您应该检查它们是否为空：

String xName = x.getName();
if(xName == null) { } // do some error handling here
xName.equals(name) ...

编辑

这是一个例子：

public String addChangeEntry(String name, String telno) {

for (DirectoryEntry x : theDirectory) {
    if (x.getName() == null) {
        log.error("Found a user with no name!!);
        continue; // or throw an exception if this is really bad for you
    }

    if (x.getName().equals(name)) {              //LINE 60
        x.setNumber(telno);
        return x.getNumber();
    } else {
        add(name, telno);
    }
}
return null;

}