Android：仅在父活动中的子活动上调用函数

Question

My ParentActivity.java is like this

public class ParentActivity extends Activity{

    public void childOnlyMethod(){
        Log.d(TAG,"child only method triggered in parent activity");
    }

    public void startChildActivtityButton(){
    startActivity(new Intent(this, ChildActivity.class));
   }
    public void childOnlyMethodButton(){
        childOnlyMethod();
   }
}

And my ChildActivity.java is like this

public class ChildActivity extends ParentActivity{
    @Override
    public void childOnlyMethod(){
       Log.d(TAG,"child only method triggered in child activity");
    }   
}

The problem is when I press childOnlyMethodButton, childOnlyMethod() in both parent and child activity gets invoked I want it to be invoked on child only how can I achieve that?

Answer 1

@Override annotation does nothing, it is only used to tell the compiler and IDE that this method overrides its super class. Non-static methods are associated to objects, not classes. Overriding means completely replace the method in its super class. So if you invoke childOnlyMethod on ChildActivity, only child version will be invoked.

I'm guessing you were actually clicking on the parent activity instance. I don't really get the point why you want to invoke a child method on parent reference. If you can post the real code, I can give you more precise answer.

However, you can try the following code. This example will only invoke child version childOnlyMethod on ChildActivity instance. But it will still invoke parent's childOnlyMethod if you click the button on ParentActivity.

public class ParentActivity extends Activity implements View.OnClickListener {
    private static final String TAG = "ParentActivity";

    private Button mStartChileButton;
    private Button mButton;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mStartChileButton = (Button) findViewById(R.id.startChileButton);
        mButton = (Button) findViewById(R.id.button);

        mStartChileButton.setOnClickListener(this);
        mButton.setOnClickListener(this);
    }

    public void onClick(View v) {
        switch(v.getId()) {
            case R.id.R.id.startChileButton:
                startActivity(new Intent(this, ChildActivity.class));
                break;
            case R.id.R.id.button:
            default:
                childOnlyMethod();
                break;
        }
    }

    public void childOnlyMethod() {
        Log.d(TAG, "Called from ParentActivity");
    }
}

public class ChildActivity extends ParentActivity {
    private static final String TAG = "ChildActivity";

    @Override
    public void childOnlyMethod() {
        Log.d(TAG, "Called from ChildActivity");
    }
}

Answer 2

If you are extending a parent class, all the methods in the parent class are passed down somewhat to the child class, if you don't want the method in the parent class to be implemented, first start by removing that @Override annotation in the child class above the method. Then use this snipped below, all it does is make the Method in the parent class either private or protected so that it can't be accessed in the child class.

public class ParentActivity extends Activity {

    private void childOnlyMethod(){
        Log.d(TAG,"child only method triggered in parent activity");
    }
    //OR
    protected void childOnlyMethod(){
        Log.d(TAG,"child only method triggered in parent activity");
    }
}