在其他类中实现的调用接口方法上的空指针异常

Question

I am trying to call the method getFailureDialog() of the interface OnSelectedListener . The method is implemented in MainActivity.java . But when I call the method, I am getting the null pointer exception.

I know that its because OnSelectedListener is still not initialized and you are calling getFailureDialog() on uninitialized object. Obviously, interface methods are never initialized. But then how do I call the method getFailureDialog() from my class Common.java ?

I am placing only the relevant source code below-

Source code:

SharedFragment.java

public class SharedFragment extends DialogFragment 
{       
    Bundle bundle = getArguments();
    final String email = bundle.getString("email");

    Thread t=new Thread(new Runnable() 
    {
        public void run() {
        common.myRecord(email);
    }
    });   t.start(); 
}

Common.java

public class Common
{
OnSelectedListener mCallback;


    public interface OnSelectedListener 
    {
        public void getFailureDialog();
    }

    public void myRecord(String email)
    {
        mCallback.getFailureDialog();  //null pointer exception here
    }
}

MainActivity.java

public class MainActivity implements Common.OnSelectedListener
{

@Override
    public void getFailureDialog()
    {

        RecordFailure fd = new RecordFailure(); 
        fd.show(getSupportFragmentManager(), "dialog");
    }
}

Error Log

03-22 15:50:39.032: W/dalvikvm(20796): threadid=16: thread exiting with uncaught exception (group=0x4204c450)
03-22 15:50:39.052: E/AndroidRuntime(20796): FATAL EXCEPTION: Thread-30126
03-22 15:50:39.052: E/AndroidRuntime(20796): java.lang.NullPointerException
03-22 15:50:39.052: E/AndroidRuntime(20796):    at com.cornmail.util.Common.myRecord(Common.java:2062)

Answer 1

OnSelectedListener mCallback;

is never getting initialized, or is being initialized with a null value.

public class Common
{
    OnSelectedListener mCallback = new OnSelectedListener(){
        public void getFailureDialog(){
            JOptionPane.showMessageDialog(null, "An Error Has Occurred.");
        }
    };


    public interface OnSelectedListener 
    {
        public void getFailureDialog();
    }

    public void myRecord(String email)
    {
        mCallback.getFailureDialog();  //now this works.
    }
}

Answer 2

here is the source code for the OnSelectedListener . Since that is an interface, you have to initialize it using new and overriding the onSelected() method OR let your class implement this listener

Answer 3

add a method in

public class Common
{
    OnSelectedListener mCallback;

    public void setOnSelectedListener(OnSelectedListener listener){
        mCallback = listener;
    }

    public interface OnSelectedListener 
    {
        public void getFailureDialog();
    }

    public void myRecord(String email)
    {
        mCallback.getFailureDialog();  //null pointer exception here
    }
}

now use the setOnSelectedListener() to initialize your listener

but from you code you might need to implement another listener in your SharedFragment too.

Answer 4

You have to redesign your code man, make Common class implement OnSelectedListener interface. So separate OnSelectedListener as outer interface not as inner interface.

i will code it like this.

public interface OnSelectedListener 
{
    public void getFailureDialog();
}

then Common class should be like this

public class Common implements OnSelectedListener
{
    public void getFailureDialog()
    {
        RecordFailure fd = new RecordFailure(); 
        fd.show(getSupportFragmentManager(), "dialog");
    }

    public void myRecord(String email)
    {
        getFailureDialog();
        //do something more rather than just call existing method
    }
}

and this code will run smoothly. if you need to implement the getFailureDialog in MainActivity, make this Common class as abstract class.

Common class will be like this

public abstract class Common implements OnSelectedListener
{
    public abstract void getFailureDialog();

    public void myRecord(String email)
    {
        getFailureDialog();
        //do something more rather than just call existing method
    }
}

now your MainActivity class can extend from this class to implement the missing part.

Answer 5

You need to modify Common and MainActivity. In Common add a basic constructor. Then activate the callback as shown in startMyCallback.

Common.java

public class Common
{

 public Common() {}

 OnSelectedListener mCallback;


 public interface OnSelectedListener 
 {
    public void getFailureDialog();
 }

 public void myRecord(String email)
 {
    mCallback.getFailureDialog();  //null pointer exception here
 }
}

MainActivity.java

public class MainActivity implements Common.OnSelectedListener
{

    Common common = new Common();

    public MainActivity()
    {

    }

    public void startMyCallback()
    {
        common.mCallback = this;
    }

    @Override
    public void getFailureDialog()
    {
        RecordFailure fd = new RecordFailure(); 
        fd.show(getSupportFragmentManager(), "dialog");
    }
}