Question

What might be the reasons why the button is NULL?

Button press;
//...

press = (Button) findViewById (R.id.enter);

if (press != null)
press.setOnClickListener(this);

I checked the manifest, the xml file, the code… I don’t have any duplicate of R.id.enter, still the button is NULL… It all happened after I introduced a new Activity…

public void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    press = (Button) findViewById (R.id.enter);

    if (press != null)
    press.setOnClickListener(this);
}

the XML here (main.xml):

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="@drawable/gift2"
>

<Button
android:id="@+id/enter"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="415dp"
android:background="@drawable/pattern"
android:layout_gravity="center_horizontal"
android:text="Open your box now!"
android:textSize="20sp"
/>

</LinearLayout>

Answer 1

One possibility is that the auto-generated R.java file has somehow got out of sync or has invalid values in it.

In this case, R.id.enter may no longer be valid and as a result findViewById(R.id.enter) can no longer find a valid object to return.

If using Eclipse, try using “Project -> Clean…” – this will delete all auto-generated files and re-create them.