创建对象期间出现安卓 Java空指针异常

Question

一切似乎都很好，但我无法在另一个类中创建新对象。我只是"java.lang.Null.Pointer.Exception" 实际上，我有一个F16类，用于创建F16飞机。现在，当我试图在MainActivity中创建一个新对象来显示飞机时，会显示这个错误

main活动类

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);
    }
}

飞机等级

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

Answer 1

在F16类中使用上下文作为参数生成构造函数 比如

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

从主要活动传递上下文

F16 myF16=new F16(this); 

Answer 2

NullPointerException是由F16类的第3行引起的

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

字段context被隐式初始化为null。然后Image被初始化并调用context.getResources()，但是由于context是空的，所以您得到了NPE

Answer 3

试试下面的代码

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);
    }
}