Jpaji Rajnish Jpaji Rajnish - 5 months ago 14
Android Question

Having trouble dynamically creating buttons in Android Studio

I'm new to Android and I can't figure out how to dynamically add a button to a pre-existing layout. I patched together some example code I found from another question on SO into a hello world default project.

onCreate method:

protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

Button myButton = new Button(this);
myButton.setText("Add Me");

RelativeLayout ll = (RelativeLayout)findViewById(R.id.main_layout);
RelativeLayout.LayoutParams lp = new RelativeLayout.LayoutParams(RelativeLayout.LayoutParams.MATCH_PARENT, RelativeLayout.LayoutParams.WRAP_CONTENT);
ll.addView(myButton, lp);

setContentView(R.layout.activity_main);
}


Layout:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
android:id="@+id/main_layout"
android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">

<TextView android:text="@string/hello_world" android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/textView" />

<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="1"
android:id="@+id/cat_1"
android:layout_below="@+id/textView"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_marginTop="26dp" />

<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="2"
android:id="@+id/cat_2"
android:layout_alignBottom="@+id/cat_1"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true"
android:layout_marginRight="65dp"
android:layout_marginEnd="65dp" />

</RelativeLayout>


The project works when I comment out the adding a button part so I know that's the problem. And I would have pasted some debug information but the 'debug' filter for the logcat was extremely verbose and constantly updating even if the program wasn't running. When I ran it in debug mode the phone displayed an error for a fraction of a second before the message "The app has stopped working" came up.

Answer

setContentView should be done right after super. Otherwise anytime you call findViewById will return a null pointer.

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

    Button myButton = new Button(this);
    myButton.setText("Add Me");

    RelativeLayout ll = (RelativeLayout)findViewById(R.id.main_layout);
    RelativeLayout.LayoutParams lp = new RelativeLayout.LayoutParams(RelativeLayout.LayoutParams.MATCH_PARENT, RelativeLayout.LayoutParams.WRAP_CONTENT);
    ll.addView(myButton, lp);
}
Comments