Null Pointer Exception when using a button to change layouts

[pocnib]

*SOLVED*

Solved, I just needed to setContentView before getting the Button from the xml file



I am new to android development, experienced with Java. I am trying, basically, to play around with OnClick events, using a button click to change Layouts. I started by making two .xml files in my Layouts folder of the project and just setting setContentView to the first xml in onCreate and to the other xml in onClick. Through some searching I found that another Activity and an Intent object. I tried this with the following:

package org.Android.HelloAndroid;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class NextActivity extends Activity{
	
	public void onCreate(Bundle bundle){
		super.onCreate(bundle);
		Button button = (Button) findViewById(R.id.goback);
		button.setOnClickListener(new OnClickListener(){
			public void onClick(View v){
				Intent myIntent = new Intent(v.getContext(), Hello_Android.class);
				startActivityForResult(myIntent,0);
			}
		});
		setContentView(R.layout.onclick);
	}
}

package org.Android.HelloAndroid;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.*;

public class Hello_Android extends Activity{
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle created) {
        super.onCreate(created);
        TextView tv = (TextView) findViewById(R.id.textview);
        Button button = (Button) findViewById(R.id.button);
        button.setOnClickListener(new OnClickListener(){
        	public void onClick(View v){
        		   Intent myIntent = new Intent(v.getContext(),NextActivity.class);
        		   startActivityForResult(myIntent,0);
        	   }
        });
        this.setContentView(R.layout.main);
    }
}

When I run this I get the error:

"The application Hello_Android (process org.android.HelloAndroid) has stopped unexpectedly. Please try again. Force Close."

I used logcat, which I will be the first to admit I am unfamiliar with how to read, but what stood out was the java Stack trace,

E/AndroidRuntime(  180): Uncaught handler: thread main exiting due to uncaught exception

E/AndroidRuntime(  180): java.lang.RuntimeException: Unable to start activity ComponentInfo{org.Android.HelloAndroid/org.Android.HelloAndroid.Hello_Android}: java.lang.NullPointerException

E/AndroidRuntime(  180): 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496)

E/AndroidRuntime(  180): 	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)

E/AndroidRuntime(  180): 	at android.app.ActivityThread.access$2200(ActivityThread.java:119)

E/AndroidRuntime(  180): 	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)

E/AndroidRuntime(  180): 	at android.os.Handler.dispatchMessage(Handler.java:99)

E/AndroidRuntime(  180): 	at android.os.Looper.loop(Looper.java:123)

E/AndroidRuntime(  180): 	at android.app.ActivityThread.main(ActivityThread.java:4363)

E/AndroidRuntime(  180): 	at java.lang.reflect.Method.invokeNative(Native Method)

E/AndroidRuntime(  180): 	at java.lang.reflect.Method.invoke(Method.java:521)

E/AndroidRuntime(  180): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)

E/AndroidRuntime(  180): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)

E/AndroidRuntime(  180): 	at dalvik.system.NativeStart.main(Native Method)

E/AndroidRuntime(  180): Caused by: java.lang.NullPointerException

E/AndroidRuntime(  180): 	at org.Android.HelloAndroid.Hello_Android.onCreate(Hello_Android.java:17)

E/AndroidRuntime(  180): 	at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)

E/AndroidRuntime(  180): 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459)

What is the most interesting to me is the null pointer exception at line 17 of Hello_Android.java, the line in question being:

button.setOnClickListener(new OnClickListener(){

I am not sure how this is throwing a null pointer exception though... any help would be appreciated.