Android：Nullpointer问题

Question

我的Nullpointer预感让我难过。我在FindViewByID上得到nullpointer异常。这是我的一段代码

public class numberActivity extends Activity {

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.number);

}    

ImageView portrait = (ImageView) findViewById(R.id.numberImage);
public void numberClick(View v) {   
    switch(v.getId()){
            case R.drawable.numberone:
                portrait.setImageResource(R.drawable.numbertwo);
                break;

            case R.drawable.numbertwo:
                portrait.setImageResource(R.drawable.numberthree);
                break;

        }

}

}

我的number.xml在这里

<ImageView
    android:id="@+id/numberImage"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:src="@drawable/numberone" 
    android:onClick="numberClick" />

我认为非常简单的代码，但自从昨晚以来，空指针异常令我感到沮丧。任何帮助表示赞赏。

Answer 1

正如其他人所指出的那样，在onCreate方法中获得ImageView。

 您获得空指针异常的原因是您不能initialize java中任何函数之外的对象。必须在函数中进行初始化，在本例中为onCreate()。 在function或constructors之外声明的任何非静态内容都不会被初始化。就在那里。要开始使用它，请在函数中初始化它。

Answer 2

问题是由这条线引起的。

 ImageView portrait = (ImageView) findViewById(R.id.numberImage);

将'portrait'变量移动到类成员并在setContentView（）之后在onCreate（）中初始化它，这将解决您的问题。

或更改您的代码

public class numberActivity extends Activity {

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.number);

} 



  public void numberClick(View v) {   
     switch(v.getId()){
        case R.drawable.numberone:
            ((ImageView)v).setImageResource(R.drawable.numbertwo);
            break;

        case R.drawable.numbertwo:
            ((ImageView)v).setImageResource(R.drawable.numberthree);
            break;

    }

  }
 }

Answer 3

试试这个

 public class numberActivity extends Activity {

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.number);


ImageView portrait = (ImageView) findViewById(R.id.numberImage);
}    

public void numberClick(View v) {   
    switch(v.getId()){
            case R.drawable.numberone:
                portrait.setImageResource(R.drawable.numbertwo);
                break;

            case R.drawable.numbertwo:
                portrait.setImageResource(R.drawable.numberthree);
                break;

        }

}
}

Answer 4

把这个

 ImageView portrait = (ImageView) findViewById(R.id.numberImage);

在onCreate方法..

Answer 5

ImageView portrait = (ImageView) findViewById(R.id.numberImage);

这一行必须在onCreate方法中。 喜欢这个

  public class numberActivity extends Activity {

    ImageView portrait;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.number);
//This element will be found once your contentview is set
        portrait = (ImageView) findViewById(R.id.numberImage);

    }    


    public void numberClick(View v) {   
        switch(v.getId()){
                case R.drawable.numberone:
                    portrait.setImageResource(R.drawable.numbertwo);
                    break;

                case R.drawable.numbertwo:
                    portrait.setImageResource(R.drawable.numberthree);
                    break;

            }

    }