Android Activity findviewbyid() 为空

Question

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.widget.Button;

public class FileExplorerActivity extends Activity 
{
    public static final String TAG="ricky";
    Button button;
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        button = (Button) findViewById(R.id.but);<<<------------------
        if(button == null)
        Log.d(TAG, "FileExplorerActivity: button is null");
    }
    public FileExplorerActivity()
    {
       Log.d(TAG, "FileExplorer: constructor()");
    }
}

This is a simple Activity which is brought in front by another activity using Intent

Intent openFileBrowser = new Intent(this, FileExplorerActivity.class);
try
{
    startActivity(openFileBrowser); 
}

After running the code my LogCat file says "button is null" Why ???

Answer 1

As others stated, you didn't set a layout via setContentView() before calling findViewById() .

Why do I need that?

Because Activity.findViewById() searches in the view hierachy of the current activity. If you set no view hierachy, there is nothing to find. And when this method finds nothing, it returns null.

Therefore you should add a layout after the call to super.onCreate()

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

button = (Button) findViewById(R.id.but);
// ... 

Answer 2

你需要设置你的布局

setContentView(R.layout.main_layout);

Answer 3

Next code solves the issue (Xamarin Android)

public class MainActivity : WearableActivity
{
    ...

    OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);

        **SetContentView(Resource.Layout.RoundMain);**

        // Does not return null anymore:

        FindViewById<Button>(Resource.Id.ButtonVgOk).LongClick += (s, e) => ButtonVgOkOnLongClick();

        ...
   }