Beginner Android Development: findViewById issue

Question

I get the following error message when running my basic application. The application consists of a button called button1 and a textView called topLeft.

07-01 15:33:02.754: D/AndroidRuntime(2334): Shutting down VM 07-01 15:33:02.754: W/dalvikvm(2334): threadid=1: thread exiting with
uncaught exception (group=0x40a13300) 07-01 15:33:02.984:
E/AndroidRuntime(2334): FATAL EXCEPTION: main 07-01 15:33:02.984:
E/AndroidRuntime(2334): java.lang.RuntimeException: Unable to instantiate activity
ComponentInfo{com.example.myfirstapp/com.example.myfirstapp.MainActivity}:
java.lang.NullPointerException 07-01 15:33:02.984:
E/AndroidRuntime(2334): at  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1983)
07-01 15:33:02.984: E/AndroidRuntime(2334): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
07-01 15:33:02.984: E/AndroidRuntime(2334): at android.app.ActivityThread.access$600(ActivityThread.java:130) 07-01
15:33:02.984: E/AndroidRuntime(2334): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
07-01 15:33:02.984: E/AndroidRuntime(2334): at android.os.Handler.dispatchMessage(Handler.java:99) 07-01
15:33:02.984: E/AndroidRuntime(2334): at android.os.Looper.loop(Looper.java:137) 07-01 15:33:02.984:
E/AndroidRuntime(2334): at android.app.ActivityThread.main(ActivityThread.java:4745) 07-01
15:33:02.984: E/AndroidRuntime(2334): at java.lang.reflect.Method.invokeNative(Native Method) 07-01
15:33:02.984: E/AndroidRuntime(2334): at java.lang.reflect.Method.invoke(Method.java:511) 07-01 15:33:02.984:
E/AndroidRuntime(2334): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
07-01 15:33:02.984: E/AndroidRuntime(2334): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 07-01
15:33:02.984: E/AndroidRuntime(2334): at dalvik.system.NativeStart.main(Native Method) 07-01 15:33:02.984:
E/AndroidRuntime(2334): Caused by: java.lang.NullPointerException
07-01 15:33:02.984: E/AndroidRuntime(2334): at android.app.Activity.findViewById(Activity.java:1825) 07-01
15:33:02.984: E/AndroidRuntime(2334): at com.example.myfirstapp.MainActivity.<init>(MainActivity.java:28) 07-01
15:33:02.984: E/AndroidRuntime(2334): at java.lang.Class.newInstanceImpl(Native Method) 07-01 15:33:02.984:
E/AndroidRuntime(2334): at java.lang.Class.newInstance(Class.java:1319) 07-01 15:33:02.984:
E/AndroidRuntime(2334): at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
07-01 15:33:02.984: E/AndroidRuntime(2334): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1974)
07-01 15:33:02.984: E/AndroidRuntime(2334):     ... 11 more

When attempting to use the findViewById, this error comes up referring to line 28.

The program looks like the following:

package com.example.myfirstapp;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity implements OnClickListener{

    int a = 0;

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

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
      getMenuInflater().inflate(R.menu.activity_main, menu);
      return true;
    }


    Button button1 = (Button) findViewById(R.id.button1);
    TextView topLeft = (TextView) findViewById(R.id.textView2);
//  button1.setOnClickListener( this );

    public void onClick(View v) {
        // TODO Auto-generated method stub

    }    

}

What does this mean? Why is my code having trouble doing something along the lines of referring to the R.java file?

R.java does have
    public static final class id {
        public static final int button1=0x7f080001;
        public static final int menu_settings=0x7f080003;
        public static final int textView1=0x7f080000;
        public static final int textView2=0x7f080002;

Thanks in advance.

Answer 1

You're calling findViewById as part of your field definition, at which point the layout isn't inflated (that happens when you call setContentView). findViewById isn't finding the view, so the reference is being set to null.

Moving your findViewById calls inside of onCreate, after calling setContentView should fix it.