Question

每件事似乎都很好我仍然无法在另一个类中创建一个新对象。我得到了"java.lang.Null.Pointer.Exception" 实际上我有一个用于制作F16飞机的F16级。现在当我尝试在MainActivity中创建一个新对象来显示飞机时，会显示此错误。

MainActivity Class

public class MainActivity extends Activity {    
    LinearLayout Map=null;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Map=(LinearLayout)findViewById(R.id.Map);
        try{
        F16 myF16=new F16();        
        myF16.CreateAirCraft(myF16.CoordinateX,myF16.CoordinateY, this, Map);
        }
        catch(Exception ex){
        Log.d("Er", ex.toString());     
        }
    }

F16班级

public class F16 extends AirCraft {
Context context;
private int Image=context.getResources().getIdentifier("F16" , "drawable", context.getPackageName());
LinearLayout PlaneBody=null;//a container used to hold All information of Aircraft
TextView PlaneName=null;// mentions to AirCraft name
ImageView PlaneImg=null;//mentions to AirCraft Image
public void CreateAirCraft(byte X , byte Y,Context Ctx,LinearLayout Map){
        Name="F-16";    
        PlaneBody=new LinearLayout(Ctx);
        PlaneName=new TextView(Ctx);
        PlaneImg=new ImageView(Ctx);
        PlaneName.setText(Name);
        PlaneImg.setImageResource(Image);
        PlaneBody.addView(PlaneName);
        PlaneBody.addView(PlaneImg);
        Map.addView(PlaneBody);
    }
}

AirCraft Class

public abstract class AirCraft {
   public String Name="";
   public byte CoordinateX=0;
   public byte CoordinateY=0;
   public byte Weight=0;
}

Answer 1

使用context作为参数在F16类中创建构造函数例如

public F16(Context context)
{
Image=context.getResources().getIdentifier("F16" , "drawable", context.getPackageName());
}

并从主要活动传递上下文

F16 myF16=new F16(this);

Answer 2

尝试以下代码。

public class F16 extends AirCraft {

Context context;
private int Image;
LinearLayout PlaneBody=null;//a container used to hold All information of Aircraft
TextView PlaneName=null;// mentions to AirCraft name
ImageView PlaneImg=null;//mentions to AirCraft Image

public F16(Context context)
{
    this.context = context;
    Image=context.getResources().getIdentifier("F16" , "drawable", context.getPackageName());
}

public void CreateAirCraft(byte X , byte Y,Context Ctx,LinearLayout Map){
        Name="F-16";    
        PlaneBody=new LinearLayout(Ctx);
        PlaneName=new TextView(Ctx);
        PlaneImg=new ImageView(Ctx);
        PlaneName.setText(Name);
        PlaneImg.setImageResource(Image);
        PlaneBody.addView(PlaneName);
        PlaneBody.addView(PlaneImg);
        Map.addView(PlaneBody);
    }
}

Answer 3

NullPointerException由F16类的第3行引起。

Context context;
private int Image=context.getResources().getIdentifier("F16" , "drawable", context.getPackageName());

字段context已隐式初始化为null。然后正在初始化Image并调用context.getResources()，但由于context为空，因此您获得了NPE。

创建对象期间的Java空指针异常

3 个答案: