如何从android库类中调用方法

Question

I want to call a method from a android library class which i have imported as a androidlib.jar. As i am able to call a whole class of library but i dont want it, but i want to call a particular method of library class.

I tried something like this, but it is showing java.lang.Nullpointer exception

This is my library class (AndroidLiB.class), where i have imported its jar file

  public class AndroidLiB extends Activity  {

   @Override
   protected void onCreate(Bundle savedInstanceState) 
    {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.android_li_b);
    startGPS();
    }

    public void startGPS()
    {
     Toast.makeText(getApplicationContext(),"Your GPS started",Toast.LENGTH_LONG).show();
    }

}

This is my application class where i want to call a method from above class

  public class AndLib1 extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.and_lib1);
    AndroidLiB abc = new AndroidLiB();
    abc.startGPS();

    }

}

But it is not working

Answer 1

I would do something like this:

public class Tool {
    private Tool() {
        // no direct instantiation
    }    

    public static void startGPS(final Context context) {
         Toast.makeText(context, "Your GPS started", Toast.LENGTH_LONG).show();
    }
}

then

public class AndroidLiB extends Activity  {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.android_li_b);
        Tool.startGPS(this);
    }
}

and

public class AndLib1 extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.and_lib1);
        Tool.startGPS(this);
    }
}

Answer 2

You can extend library class. For example:

public class YourActivity extends AndroidLib
{
  super.onCreate(savedInstanceState);
  setContentView(R.layout.and_lib1);
  startGPS();
}

Answer 3

If you want simply a method from jar, then why you need to extends Activity . My suggestion is remove extends Activity will fix the NPE error.

Try this,

 public class AndroidLiB {
  Activity activity;
  AndroidLiB(Activity activity){ 
        this.activity = activity;
   }


  public void startGPS()
  {
    Toast.makeText(activity,"Your GPS started",Toast.LENGTH_LONG).show();
  }

 }

And In your main class call like

 AndroidLiB lib = new AndroidLiB (this);
 lib.startGPS();

Answer 4

What you're trying to do isn't really the best way to do things, but I'm assuming that the question being asked is how to import the jar correctly. If so:

If using eclipse,

1. Make sure the androidlib.jar file is in the libs folder.
2. Right click on androidlib.jar and select Build Path > Add to Build Path
3. Right click on your project folder, go to Properties > Java Build Path > Order and Export and then make sure androidlib.jar is selected.

The problem with your current code is that when you call getApplicationContext() , the Activity hasn't been started yet, therefore there is no context. A quick and dirty solution would be to rewrite the startGPS() method like this:

public static void startGps(Context context, String message) {
    Toast.makeText(context, message, Toast.LENGTH_LONG).show();
}

But I would much rather put that method inside some sort of Utilities class or even inside a parent Activity class.