java - 增强的 for 循环因 NullPointerException 而停止

Question

我的增强型 for 循环似乎没有正确迭代。目的是使用搜索类遍历 Contact 类型的 ArrayList 并找到特定名称，但由于某种原因，它仅遍历第一个联系人并在显示之后因错误而停止:

Exception in thread "main" java.lang.NullPointerException
    at client.AddressBook.search(AddressBook.java:17)
    at Main.main(Main.java:31)

我的主类如下:

import client.AddressBook;
import client.Contact;

public class Main {

    public static void main(String[] args) {

        AddressBook ab = new AddressBook();

        Contact c1 = new Contact("jeffm@engr.uconn.edu");
        ab.add(c1);

        Contact c2 = new Contact("jeffm@engr.uconn.edu", "Jeff Meunier", "jeff");
        ab.add(c2);

        Contact c3 = new Contact("billgates@engr.uconn.edu", "Bill Gates", "bill");
        ab.add(c3);

        System.out.println(ab.search("jeff"));
    }

}

AddressBook 和 Contact 类也在下面列出:

package client;
import java.util.ArrayList;

public class AddressBook {

    ArrayList<Contact> al = new ArrayList<Contact>();

    public void add(Contact contactAdd) {

        al.add(contactAdd);
    }

    public Contact search(String searchName) {

        for(Contact obj: al) {

            if(obj.getNickName().equals(searchName)) {
                return obj;
            }
        }
        return null;
    }

    public String remove(String nickname) {

        search(nickname);
        al.remove(nickname);

        return nickname;
    }

    public void show() {

        int x = 1;
        for(Contact obj: al) {

            System.out.println(x + ". " + obj.toString());
            x++;
        }
    }
}



package client;

public class Contact {

    public String _emailAddress = null;
    public String _fullName = null;
    public String _nickName = null;

    public Contact(String emailaddress, String fullname, String nickname) {

        _emailAddress = emailaddress;
        _fullName = fullname;
        _nickName = nickname;
    }

    public Contact(String emailaddress) {

        _emailAddress = emailaddress;
    }

    @Override
    public String toString() {

        if(_fullName == null & _nickName == null) {

            //System.out.println("<" + _emailAddress + ">");
            return _emailAddress;
        }
        else {
            //System.out.println(_fullName + " (" + _nickName + ") " + "<" + _emailAddress + ">");
            return _fullName + " (" +  _nickName + ") " + "<" + _emailAddress + ">";
        }
    }

    public String getNickName() {

        return _nickName;
    }
}

如有高人指点，将不胜感激。最终，现在我只是在测试搜索类是否可以搜索指定的昵称，然后打印出它的返回值。显然它应该返回第二个联系人(或者至少这是意图)。

Answer 1

问题发生在这个验证中:

 if(obj.getNickName().equals(searchName)) {
     return obj;
 }

似乎 obj.getNickName() 有时可能为空。

更改验证顺序:

public Contact search(String searchName) {

    for(Contact obj: al) {
        //I assume that searchName will never be null
        if(searchName.equals(obj.getNickName()) {
            return obj;
        }
    }
    return null;
}