Android Null Pointer Exception Button

Question

Ok this may seem like a pointless example but if I can figure this out then the program I am trying to make will work. So I have two activities test and test two each with one button.

Test 1:

package thompson.cameron.com;

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

public class Test extends Activity implements OnClickListener {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        View button = findViewById(R.id.testButton);
        button.setOnClickListener(this);
    }

    public void onClick(View v){
     Intent i = new Intent(this, Test2.class);
     startActivity(i);
     }

}

and test2

package thompson.cameron.com;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;

public class Test2 extends Activity implements OnClickListener {

 @Override
 public void onCreate(Bundle savedInstanceState){
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main2);

  View test = findViewById(R.id.testButton);
  test.setOnClickListener(this);
 }

 public void onClick(View v){
  switch (v.getId()){
  case R.id.testButton:
   System.exit(1);
  }
 }

}

When I click the button on Test it is supposed to launch test2 however it is at this point I get an null pointer exception that I have narrowed down to test.setOnClickListener(this); line of code. Below are my two xml files for the layout. I can get the button to work when I only have one activity but as soon as I add a second activity with a different layout file it all falls apart

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"
    >
<Button
 android:id="@+id/testButton"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:text="TEST TEST TEST"/>

</LinearLayout>

main2.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"
    >
<Button
 android:id="@+id/testButton"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:text="TEST2 TEST2 TEST2"/>

</LinearLayout>

I'm still new at Android programming so thanks for all your help.

score 2 · Answer 1 · edited Aug 26 '14 at 09:23

In your test.java file give:

implements View.OnClickListener

Initialize your button as:

Button testButton = (Button) findViewById(R.id.testButton);

and inside your onClick method, check whether you are clicking button:

if(v == testButton) {
//give ur intent code
}

There are different ways to perform onClick functionality. One is the above method which I have mentioned. Another one is what ankit has mentioned. Third way is through your layout.

Inside your layout for your button tag, you may give as:

<Button android:id="@+id/testButton" android:layout_height="wrap_content" android:layout_width="wrap_content" android:text="Click" android:onClick="onTestButtonClick" />

And inside your class just mention the below details for button:

public void onTestButtonClick(View view) {
     //give your intent code
}

You may refer to the link also: http://android-developers.blogspot.com/2009/10/ui-framework-changes-in-android-16.html

score 2 · Answer 2 · edited Oct 25 '13 at 09:29

2

Open the Debug perspective in Eclipse
Choose the 'Breakpoints' view tab.
Select the 'Add Java Exception Breakpoint' and choose NullPointerException.
Launch your activity, either by using 'Debug As...' or attaching the debugger to a running instance via DDMS.
Execute the offending workflow. It will break on the line that caused the NullPointerException.

edited Oct 25 '13 at 09:29

answered Oct 26 '11 at 12:46

Record413123

130
1
6

5

You're not going to spam this on every slightly related question, are you? – Oct 26 '11 at 13:43

score 1 · Answer 3 · edited Mar 07 '13 at 16:06

1

Implement OnClickListener interface

and set button.setOnClickListener(this);

and override

public void onClick(View v) {


}

edited Mar 07 '13 at 16:06

Rajneesh071

30,846
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answered Mar 07 '13 at 16:05

Pushkar Raj

99
1
3

score 1 · Answer 4 · answered Jan 13 '11 at 11:00

1

Make sure that both activities are register at the application's manifest file.

As a side note never call System.exit in your code. You can call finish() to close an Activity and this will bring at the front the previous Activity on the stack.

answered Jan 13 '11 at 11:00

kgiannakakis

103,016
27
158
194

1

if Activity is not registered then I think it should throw exception other than **NPE** – Vikas Patidar Jan 13 '11 at 11:08

score 1 · Answer 5 · answered Aug 22 '11 at 10:57

The issue here is that you haven't typecasted your views to buttons.

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; // Needed to add this import for the button casting below


public class Test extends Activity implements OnClickListener {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        // I have changed View to Button and then typecasted
        // with the "(Button)" the return of findViewById

        Button button = (Button) findViewById(R.id.testButton);
        button.setOnClickListener(this);
    }

    public void onClick(View v){
        Intent i = new Intent(this, Test2.class);
        startActivity(i);
    }
}

Let me know if you have any issues with this. I just completed my first experiment through using the onClickListener implementation through the main class instead of individual anonymous listeners.

Andrew

A Button is a View, so no need to cast. – CrackerJack9 Aug 22 '11 at 20:51 — CrackerJack9, Aug 22 '11 at 20:51

score 1 · Answer 6 · answered Jan 19 '12 at 23:57

I had the same problem, you may put the same content view that the button, setContentView(R.layout.main); if the button is in that content view, in other case, you will put:

setContentView(R.layout.buttoncontentview);
View button = findViewById(R.id.testButton);
button.setOnClickListener(this);
public void onClick(View v){ 
  Intent i = new Intent(this, Test2.class);
  startActivity(i); 
} 
setContentView(R.layout.main);

sorry for my bad english, but i'm spanish

score 0 · Answer 7 · answered Oct 23 '13 at 10:30

First of all in main.xml and main2.xml chage the button's ids like in below code.

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"
    >
<Button
 android:id="@+id/testButton"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:text="TEST TEST TEST"/>

</LinearLayout>

main2.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"
    >
<Button
 android:id="@+id/testButton1"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:text="TEST2 TEST2 TEST2"/>

</LinearLayout>

It throws nullpointerexception because of id confict with each other so in your java file use following code to find button.

In Activity 1

Button button = findViewById(R.id.testButton);
button.setOnClickListener(this);

and In Activity 2

Button button = findViewById(R.id.testButton1);
button.setOnClickListener(this);

score 0 · Answer 8 · answered Jan 13 '11 at 11:01

0

I think your buttons IDs need to be different in different activities. R.id.testButton would refer to only one button.

answered Jan 13 '11 at 11:01

Abhinav

38,516
9
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3

We can have same id names in different layout files. When we give the setContentView(layout), it take ids from that xml only. – Anju Jan 13 '11 at 11:07
1

ID's are simply mapped to an intenger, they can (and arguably should) be reused in different layouts but should be unique within the context of a single layout – Moog Aug 22 '11 at 12:01

score 0 · Answer 9 · edited Oct 23 '13 at 10:23

You can try this.it may work.

package thompson.cameron.com;

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

public class Test extends Activity{

      private Button button;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        button = findViewById(R.id.testButton);
        button.setOnClickListener(new OnClickListener() {

            public void onClick(View arg0) {

                Intent i=new Intent().setClass(Test.this,Test2.class);
                startActivity(i);

            }
        });
    }



}

score 0 · Answer 10 · edited May 23 '17 at 12:29

0

The final solution is that you may modify the AndroidManifest.xml file, i finally solved my error in this link How to register a new activity in AndroidManifest.xml?

edited May 23 '17 at 12:29

Community

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answered Jan 20 '12 at 15:09

Jose Carlos

11
1

Android Null Pointer Exception Button

10 Answers10