空指针除外，但为什么？ [英] Null Pointer Except, but why?

问题描述

  public static String getType（int x ）
 {
 Contact_Type Type; 
 String strType =; 
 
 Type = AddyBook.get（x）.getContactType（）; 
 strType = Type.toString（）; 
 return strType; 
} 
 
 public static String displayByType（String type）
 {
 int i = 0; 
 String strTypeList =; 
 
 if（type.equals（null））
 strTypeList =什么？什么？什么？LOLOLOLOLOLOopechucktesta; 
 while（i< AddyBook.size（））
 {
 if（getType（i）.equalsIgnoreCase（type））
 {
 strTypeList + = getType一世）; 
 strTypeList + =\\\
; 
} 
 i ++; 
} 
 return strTypeList; 
} 
  

我被指向的行是

  strType = Type.toString（）; 
  

和

  if（getType（i）.equalsIgnoreCase（type））
  

我也得到其他方法中的异常除了它们正在操作的变量之外是相同的。

我的问题是在我的驱动程序类中，我直接测试了getType并且它正常工作。然后，在displayByType方法中，我甚至说如果Type为null，这样做，但它仍然只是抛出异常。我不知道为什么，但我觉得这可能很简单明了，我刚刚在这个工作太久了，现在看到它。 ：/

编辑1：Type = AddyBook.get（x）的内容是一个对象，而.getContactType（）的结果是枚举。 >

解决方案

我相信这一行是错误的：

  if（type.equals（null））
 strTypeList =什么？什么？什么？LOLOLOLOLOLOopechucktesta; 
  

应该是：

  if（type == null）
 strTypeList =什么？什么？什么？LOLOLOLOLOLOopechucktesta; 
  

没有看到方法调用的内容：

  Type = AddyBook.get（x）.getContactType（）; 
  

这将很难调试，但如果我猜这将是getContactType（ ）正在返回null。

编辑：

尝试取消链接呼叫：

而不是：

  Type = AddyBook.get（x）.getContactType（）; 
 strType = Type.toString（）; 
  

尝试：

 code>入场条目; //我实际上并不知道什么.get（）返回
 entry = AddyBook.get（x）; 
 if（entry == null）
 throw new RuntimeException（entry in null at+ x +book size =+ AddyBook.size（））; 
 
类型type = entry.getContactType（）; 
 if（type == null）
 throw new RuntimeException（type from null from entry =+ entry.toString（））; 
 
 strType = Type.toString（）; 
  

I'm getting a nullpointerexception when I run my program using these two methods:

public static String getType(int x)
{
    Contact_Type Type;
    String strType = "";

    Type = AddyBook.get(x).getContactType();
    strType = Type.toString ( );
    return strType;
}

public static String displayByType(String type)
{
    int i = 0;
    String strTypeList = "";

    if(type.equals(null))
        strTypeList = "What?What?What?LOLOLOLOLOLnopechucktesta";
    while(i < AddyBook.size())
    {
        if(getType(i).equalsIgnoreCase(type))
        {
            strTypeList += getType(i);
            strTypeList += "\n";
        }
        i++;
    }
    return strTypeList;
}

The lines that I'm being pointed to are

strType = Type.toString ( );

and

if(getType(i).equalsIgnoreCase(type))

I'm also getting the exception in other methods that are identical except for the variable they're operating on.

My issue is that in my driver class I tested the "getType" method directly, and it worked properly. Then, in the "displayByType" method I even say "if Type is null, do this," but it still just throws the exception. I have no clue why, but I feel like it's probably really simple and obvious; I've just been working on this for too long to see it right now. :/

Edit 1: The contents of Type = AddyBook.get(x) is an object, and the result of .getContactType() is an enumeration.

解决方案

I believe this line is wrong:

if(type.equals(null))
    strTypeList = "What?What?What?LOLOLOLOLOLnopechucktesta";

it should be:

if(type == null)
    strTypeList = "What?What?What?LOLOLOLOLOLnopechucktesta";

Without seeing the contents of the method calls here:

Type = AddyBook.get(x).getContactType();

it'll be hard to debug but if I were to guess it would be the method "getContactType()" is returning null.

EDIT:

Try unchaining the calls:

Instead of:

Type = AddyBook.get(x).getContactType();
strType = Type.toString ( );

Try:

Entry entry; // I don't actually know what .get() returns
entry = AddyBook.get(x);
if (entry == null)
    throw new RuntimeException("entry is null at "+x+" book size="+AddyBook.size());

Type type = entry.getContactType();
if (type == null)
    throw new RuntimeException("type is null from entry="+entry.toString());

strType = Type.toString ( );

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