当DOM元素作为参数传入构造函数时抛出NullPointer异常

我使用两个类来使用DOM解析器解析XML文档。样本元素“对象”是这样的：当DOM元素作为参数传入构造函数时抛出NullPointer异常

<Object type="0x632" id="Fire Golem"> 
    <Group>Golems</Group> 
    <Enemy/> 
    <Class>Character</Class> 
    <AnimatedTexture><File>chars8x8rMid</File><Index>0x0e</Index></AnimatedTexture> 
    <HitSound>monster/golems_hit</HitSound> 
    <DeathSound>monster/golems_death</DeathSound> 
    <Size>140</Size> 
    <MaxHitPoints>220</MaxHitPoints> 
    <Defense>3</Defense> 
    <Terrain>MidSand</Terrain> 
    <XpMult>0.7</XpMult> 
    <SpawnProb>1.0</SpawnProb> 

    <Projectile id="0"> 
     <ObjectId>Red Fire</ObjectId> 
     <Damage>35</Damage> 
     <Speed>45</Speed> 
     <LifetimeMS>4500</LifetimeMS> 
     <Size>100</Size> 
    </Projectile> 
    <Projectile id="1"> 
     <ObjectId>Red Fire</ObjectId> 
     <Damage>45</Damage> 
     <Speed>60</Speed> 
     <LifetimeMS>2500</LifetimeMS> 
     <Size>220</Size> 
    </Projectile> 


</Object>

我使用的是2类解析XML，一个拿到这样的元素，而另一个解析这些元素。

eList = root.getElementsByTagName("Object"); 
     for(int i = 0, len = eList.getLength();i<len;i++){ 
      Element elem = (Element)eList.item(i); 
      Element sub=(Element)elem.getElementsByTagName("Class").item(0); 

      //Doesn't Contain Element 'Class'? No Entry! 
      if(sub.equals(null)) 
       continue; 

      //Get Object Attributes 
      String Class = sub.getTextContent(); 
      short type = (short)Util.getInt(elem.getAttribute("type")); 
      String id = elem.getAttribute("id"); 
      System.out.println(Class+" "+type+" "+id); 
      //Categorizing 
      //Sellable 
      if(Class.equalsIgnoreCase("Equipment")||Class.equalsIgnoreCase("Dye")||Class.equalsIgnoreCase("Pet")){ 
       ItemDescs.put(type, Desc.new Item(elem)); 
       //Set Up Shop Locations and Prices 
       if((sub = (Element)elem.getElementsByTagName("Shop").item(0))!=null){ 
        ItemShops.put((int)type, sub.getElementsByTagName("Name").item(0).getTextContent()); 
        ItemPrices.put(type, Util.getInt(sub.getElementsByTagName("Price").item(0).getTextContent())); 
       } 
      } 

      //Game Pieces 
      if(Class.equalsIgnoreCase("Character")||Class.equalsIgnoreCase("GameObject")||Class.equalsIgnoreCase("Wall")|| 
        Class.equalsIgnoreCase("ConnectedWall")||Class.equalsIgnoreCase("CaveWall")||Class.equalsIgnoreCase("Portal")){ 
       Descriptors.ObjectDesc Piece = Desc.new ObjectDesc(elem); //NullPointerException Thrown Here 
       ObjectDescs.put(type, Piece); 
      } 


      //Portals 
      if(Class.equalsIgnoreCase("Portal")){ 
       PortalDescs.put(type, Desc.new PortalDesc(elem)); 
      } 

      //Keys 
      if((sub = (Element)elem.getElementsByTagName("Key").item(0))!=null){ 
       Keys.add(type); 
       KeyPrices.put(type, Util.getInt(sub.getTextContent())); 
      } 
     }

和类描述符内的另一个类用于解析上述目的：

class Descriptors{ 
     public class ObjectDesc{ 
    public short ObjectType; 
    public String ObjectId; 
    public String DisplayId; 
    public String Group; 
    public String Class; 
    public boolean Player; 
    public boolean Enemy; 
    public boolean OccupySquare; 
    public boolean FullOccupy; 
    public boolean EnemyOccupySquare ; 
    public boolean Static; 
    public boolean NoMiniMap ; 
    public boolean ProtectFromGroundDamage; 
    public boolean ProtectFromSink; 
    public boolean Flying; 
    public boolean ShowName; 
    public boolean DontFaceAttacks; 
    public int MinSize; 
    public int MaxSize; 
    public int SizeStep; 
    public ProjectileDesc[] Projectiles; 

    public int MaxHP; 
    public int Defense; 
    public String Terrain; 
    public float SpawnProbability; 
    public SpawnCount Spawn; 
    public boolean Cube; 
    public boolean God; 
    public boolean Quest ; 
    public int Level; 
    public boolean StasisImmune; 
    public boolean Oryx ; 
    public boolean Hero ; 
    public Integer PerRealmMax ; 
    public Float ExpMultiplier ; //Exp gained = level total/10 * multi 

    public ObjectDesc(Element elem){ 
     Element sub; 
     ObjectType = (short)Util.getInt(elem.getAttribute("type")); 
     ObjectId = elem.getAttribute("id"); 
     Class = elem.getElementsByTagName("Class").item(0).getTextContent(); 
     if((sub =(Element)elem.getElementsByTagName("Group").item(0))!=null) 
      Group = sub.getTextContent(); 
     else 
      Group = null; 
     if((sub = (Element)elem.getElementsByTagName("DisplayId").item(0))!=null) 
      DisplayId = sub.getTextContent(); 
     else 
      DisplayId = null; 

     Player = elem.getElementsByTagName("Player").getLength()!=0; 
     Enemy = elem.getElementsByTagName("Enemy").getLength()!=0; 
     OccupySquare = elem.getElementsByTagName("OccupySquare").getLength()!=0; 
     FullOccupy = elem.getElementsByTagName("FullOccupy").getLength()!=0; 
     EnemyOccupySquare = elem.getElementsByTagName("EnemyOccupySquare").getLength()!=0; 
     Static = elem.getElementsByTagName("Static").getLength()!=0; 
     NoMiniMap = elem.getElementsByTagName("NoMiniMap").getLength()!=0; 
     ProtectFromGroundDamage = elem.getElementsByTagName("ProtectFromGroundDamage").getLength()!=0; 
     ProtectFromSink = elem.getElementsByTagName("ProtectFromSink").getLength()!=0; 
     Flying = elem.getElementsByTagName("Flying").getLength()!=0; 
     ShowName = elem.getElementsByTagName("ShowName").getLength()!=0; 
     DontFaceAttacks = elem.getElementsByTagName("DontFaceAttacks").getLength()!=0; 

     if ((sub = (Element)elem.getElementsByTagName("Size").item(0)) != null) 
     { 
      MinSize = MaxSize = Util.getInt(sub.getTextContent()); 
      SizeStep = 0; 
     } 
     else 
     { 
      if ((sub = (Element)elem.getElementsByTagName("MinSize").item(0)) != null) 
       MinSize = Util.getInt(sub.getTextContent()); 
      else 
       MinSize = 100; 
      if ((sub = (Element)elem.getElementsByTagName("MaxSize").item(0)) != null) 
       MaxSize = Util.getInt(sub.getTextContent()); 
      else 
       MaxSize = 100; 
      if ((sub = (Element)elem.getElementsByTagName("SizeStep").item(0)) != null) 
       SizeStep = Util.getInt(sub.getTextContent()); 
      else 
       SizeStep = 0; 
     } 

     List<ProjectileDesc> prj = new ArrayList<ProjectileDesc>(); 
     NodeList nSub; 
     nSub=elem.getElementsByTagName("Projectile"); 
     for(int i=0;i<nSub.getLength();i++){ 
      prj.add(new ProjectileDesc((Element)nSub.item(i))); 
     } 
     Projectiles = (ProjectileDesc[])prj.toArray(); 

    if((sub = (Element)elem.getElementsByTagName("MaxHitPoints").item(0)) != null) 
     MaxHP = Util.getInt(sub.getTextContent()); 
    if ((sub = (Element)elem.getElementsByTagName("Defense").item(0)) != null) 
     Defense = Util.getInt(sub.getTextContent()); 
    if ((sub = (Element)elem.getElementsByTagName("Terrain").item(0)) != null) 
     Terrain = sub.getTextContent(); 
    if ((sub = (Element)elem.getElementsByTagName("SpawnProbability").item(0)) != null) 
     SpawnProbability = Float.parseFloat(sub.getTextContent()); 
    if ((sub = (Element)elem.getElementsByTagName("Spawn").item(0)) != null) 
     Spawn = new SpawnCount(sub); 

    StasisImmune = elem.getElementsByTagName("StasisImmune").getLength() !=0; 
    Oryx = elem.getElementsByTagName("Oryx").getLength() !=0; 
    Hero = elem.getElementsByTagName("Hero").getLength() !=0; 

    if ((sub = (Element)elem.getElementsByTagName("PerRealmMax").item(0)) != null) 
     PerRealmMax = Util.getInt(sub.getTextContent()); 
    else 
     PerRealmMax = null; 

    if ((sub = (Element)elem.getElementsByTagName("XpMult").item(0)) != null) 
     ExpMultiplier = Float.parseFloat(sub.getTextContent()); 
    else 
     ExpMultiplier = null; 

    }  
} 
    }

上面的代码抛出一个空指针异常，当“对象”元素作为参数传递给发送ObjectDesc的构造函数，但是，我可以获得First Class中的属性和字段。 JAXB似乎是解析这些文档的正确选择，但是我花了2天的时间编写了这个Horror后发现了它们。堆栈跟踪：

java.lang.NullPointerException 
at com.azuro.ultraserve.db.data.XMLData.ProcessXml(XMLData.java:150) 
at com.azuro.ultraserve.db.data.XMLData.ReadXMLs(XMLData.java:69) 
at com.azuro.ultraserve.db.data.XMLData.<clinit>(XMLData.java:59)

来源

2013-08-17 UserNotFound

NullPointer发生在哪里？显示堆栈跟踪（并标记该行）。 – Kayaman

我在代码中添加了断点作为注释。 – UserNotFound

我评论过'// NullPointerException抛出这里'在第二个代码块（看近端）。 – UserNotFound

 if(sub.equals(null)) 
      continue;

是不是要检查空的方式。如果sub为null，则此代码将引发NullPointerException。

你想要的这里是

 if(sub == null) 
      continue;

现在，我看到这行你NullPointerException异常在发生，虽然，这可能不是你所看到的问题的原因。导致空指针异常的线路是：

  Descriptors.ObjectDesc Piece = Desc.new ObjectDesc(elem);

这很可能意味着你的Desc变量为空，而不是实际Descriptors对象。由于ObjectDesc是一个非静态的内部类Descriptors，您需要这样的对象才能调用内部构造函数。

来源

2013-08-17 12:51:28

谢谢，但问题仍然存在，因为它的父元素作为参数传递时引发NullPointer。 – UserNotFound

当DOM元素作为参数传入构造函数时抛出NullPointer异常

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