Android findViewById在Eclipse生成的项目中返回Null

Question

I created a new Android project in Eclipse with one Blank Activity. In the fragment, I added a TextView called textHello. All I want to do for this test is to modify textHello's text value. But I'm my findViewById is giving me null.

Here is the fragment code:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.gcuitest.MainActivity$PlaceholderFragment" >
    <TextView
        android:id="@+id/textHello"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />
</RelativeLayout>

Here is the MainActivity.java

package com.example.gcuitest;

import android.app.Activity;
import android.app.ActionBar;
import android.app.Fragment;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.os.Build;

public class MainActivity extends Activity {
    private TextView helloText;

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

        if (savedInstanceState == null) {
            //getFragmentManager().beginTransaction()
            //      .add(R.id.container, new PlaceholderFragment()).commit();
            FragmentManager fragmentManager = getFragmentManager(); 
            FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
            fragmentTransaction.add(R.id.container, new PlaceholderFragment());
            fragmentTransaction.commit();
        }

        helloText = (TextView) findViewById(R.id.textHello);
        helloText.setText("Gerard");
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
            return rootView;
        }
    }

}

Why is findViewById giving me null? Is it in the right place? I've looked at other code samples, and they place findViewById in onCreate, after setContentView, which is what this does. What did I miss? Thanks.

Answer 1

Why is findViewById giving me null?

Because textHello TextView is inside PlaceholderFragment layout instead of Activity which is activity_main .

if you want to access TextView from Fragment layout then use onCreateView as :

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_layout, container, false);
    helloText = (TextView)view. findViewById(R.id.textHello);
    helloText.setText("Gerard");

    return view;
}