App chrashes while changing view - android

I want to create a very simple app. The app should consists of 2 views. In both views are one button. By clicking the button the view should change.
Here is my folder structure
I have two activitys and two activity_layouts. As you can see OverviewActivity isn't inside the activity folder. When I place it into the folder I get this structur:
Why is the activity folder away? Can you give me a little explanation?
Ok, but in this question I use the first folder structur.
activity_login.xml
<?xml version="1.0" encoding="utf-8"?>
<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="company.useradministration.activity.LoginActivity">
<Button
android:layout_width="match_parent"
android:layout_height="match_parent"
android:onClick="btnOverview"
android:text="go to overview"/>
</RelativeLayout>
activity_overview.xml
<?xml version="1.0" encoding="utf-8"?>
<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=".activity.OverviewActivity">
<Button
android:layout_width="match_parent"
android:layout_height="match_parent"
android:onClick="btnLogin"
android:text="go to login"/>
</RelativeLayout>
LoginActivity
package company.useradministration.activity;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import company.useradministration.R;
public class LoginActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
}
public void btnOverview(View view){
setContentView(R.layout.activity_overview);
}
}
OverviewActivity
package company.useradministration.activity;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import company.useradministration.R;
public class OverviewActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_overview);
}
public void btnLogin(View view){
setContentView(R.layout.activity_login);
}
}
Okay. When I start the app, I see a button with the text "go to view". After pressing this button, the view changed. Now I see a button with the text "go to overview". When I press this button, the app crashes with the error:
java.lang.IllegalStateException: Could not find method btnOverview(View) in a parent or ancestor Context for android:onClick attribute defined on view class android.support.v7.widget.AppCompatButton
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.resolveMethod(AppCompatViewInflater.java:307)
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:266)
at android.view.View.performClick(View.java:4438)
at android.view.View$PerformClick.run(View.java:18439)
at android.os.Handler.handleCallback(Handler.java:733)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5085)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:611)
at dalvik.system.NativeStart.main(Native Method)

UPDATE:
Here is the explained answer:
You have a activity_overview.xml with it's class OverviewActivity.
In your activity_overview.xml you set the android:onClick="btnLogin" to the button and you have:
public void btnLogin(View view){
setContentView(R.layout.activity_login);
}
in your OverviewActivity so when user clicks this button it changes the contentview successfully.
The question here is:
Why does the app crashes when you click on the button in activity_login.xml to change the contentview back to activity_overview.xml?
Simple. Because you have set the onclick method in your LoginActivity but you aren't switching to that activity but just changing the layout. So when the button to go back to overview content is clicked the app searches for the function in you OverviewActivity because the activity didn't swithed to LoginActivity. It has just changed the content and there is no function called btnOverview in your OverviewActivity. So here you have 2 solutions:
1- Create two RelativeLayouts in just one xml file and than setVisibility of each layouts on button click.
So for that you have to:
Create only one activity instead of two different and in it's xml file use this code:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="#+id/overview"
android:visibility="gone">
<Button
android:layout_width="wrap_content"
android:layout_centerVertical="true"
android:layout_centerHorizontal="true"
android:text="go to login"
android:id="#+id/go_to_login"
android:layout_height="wrap_content" />
//Do your stuff here for overview content
</RelativeLayout>
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="#+id/login">
<Button
android:layout_width="wrap_content"
android:layout_centerVertical="true"
android:layout_centerHorizontal="true"
android:text="go to overview"
android:id="#+id/go_to_overview"
android:layout_height="wrap_content" />
//Do your stuff here for login content
</RelativeLayout>
</RelativeLayout>
This code has 2 RelativeLayouts. The first one with id:overview is of the overview content and it's visibility is gone and the second RelativeLayout with id:login has the visibility visible by default (Change the two layouts visibility as desired).
Than in your MainActivty.java (call it as you wish) you can change the visibility of these views like this:
public class MainActivity extends AppCompatActivity {
Button goToOverview, goToLogin;
RelativeLayout overview, login;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//Remember to change the content to match your xml file name
setContentView(R.layout.activity_main);
//Buttons
goToOverview = (Button) findViewById(R.id.go_to_overview);
goToLogin = (Button) findViewById(R.id.go_to_login);
//Layouts
overview = (RelativeLayout) findViewById(R.id.overview);
login = (RelativeLayout) findViewById(R.id.login);
goToOverview.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
login.setVisibility(View.GONE);
overview.setVisibility(View.VISIBLE);
}
});
goToLogin.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
overview.setVisibility(View.GONE);
login.setVisibility(View.VISIBLE);
}
});
}
In this way you can switch to two different layout without any problem.
Edit: You can achieve this also with this solution but the difference between the first solution and this solution is just that you will have 2 xml files instead of one.
2nd- You will have one java class file (in this example I will take OverviewActivityas example) and 2 different layout files (Which you already have: activity_login.xml and activity_overview.xml).
So in your activity_overview.xml change this line: tools:context=".activity.OverviewActivity" to tools:context=".OverviewActivity" as you said that you are using the first folder structure where the OverviewActivity file is out of the activity folder.
Than in your OverviewActivity make the following changes:
public class OverviewActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_overview);
}
public void btnLogin(View view){
setContentView(R.layout.activity_login);
}
public void btnOverview(View view){
setContentView(R.layout.activity_overview);
}
}
That's it. Please comment if you are having any problem with the code. I will be glad to help your further more.
Hope this can solve your issue.
Regards

I think this is your problem:
public void btnOverview(View view){
setContentView(R.layout.activity_overview);
}
This changes the layout for the current activity LoginActivity to activity_overview which contains the button thats looking for the btnLogin method
You are successfully changing the layout, but your staying in the same activity, so the new layout is looking for a method that exists in the other activity
Instead change the above code to this:
public void btnOverview(View view){
Intent intent = new Intent(this, OverviewActivity.class);
startActivity(intent);
}
And in OverviewActivity
public void btnLogin(View view){
Intent intent = new Intent(this, LoginActivity.class);
startActivity(intent);
}

I think you can use this as a help. I'm newbie myself to android java programming, but I believe you have to create intent to move to another activity. Additionally be sure to add your Activity to AndroidManifest.xml. That's all I think. If I'm wrong I'll be happy if someone more experienced will correct me :)

Related

Android Studio - Button Click Crash (Simple)

I'm brand new to Android Studio and for whatever reason am experiencing really weird errors if I could please get some help.
I have very basic code that is supposed to, on button click change the text of my button from "button" to "clicked!". However everytime I press the button, the app crashes and I get "Appname has stopped" on the emulator.
What is incredibly weird is that in my activity_main.xml Design view, the onClick dropdown shows two functions of the same name (https://puu.sh/t2h5I/42ad4379d6.png)
H owever the code only works when the bottom one is selected. AND each time I run the app, it deselects the bottom one and reselects the top, only to stop working.
Here is my MainActivity:
package com.example.john.ameladay;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends AppCompatActivity {
public Button melButtonCode;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public void buttonPress(View v){
melButtonCode = (Button) v;
((Button) v).setText("Has been clicked!");
}
}
Here is my activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/activity_main"
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.john.ameladay.MainActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/mainText"
android:id="#+id/textView" />
<ImageView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:src="#drawable/mela"
android:id="#+id/melPhoto" />
<Button
android:text="Button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_toRightOf="#+id/textView"
android:layout_toEndOf="#+id/textView"
android:layout_marginLeft="32dp"
android:layout_marginStart="32dp"
android:layout_marginBottom="43dp"
android:id="#+id/button"
android:onClick="buttonPress (MainActivity)" />
Any help would be greatly appreciated!!
Simple. You should write:
android:onClick="buttonPress"
Why happened
If you wrote buttonPress (MainActivity), Android tries to find buttonPress (MainActivity) method (not MainActivity.buttonPress()), but MainActivity doesn't have buttonPress (MainActivity) method. So the error happened.
Simply replace this Tag in button
Remove this
android:onClick="buttonPress (MainActivity)"
And Paste This
android:onClick="buttonPress"
A better way to do it is, get a reference to the button in your Java code using findViewById() method and set an OnClickListener to the button.
For your current problem, use
android:onClick="buttonPress"
instead of
android:onClick="buttonPress (MainActivity)"
According to me this is the better way to set click on button
public class MainActivity extends AppCompatActivity {
public Button melButtonCode;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
melButtonCode = (Button).findViewById(R.id.button);//find button by Id
melButtonCode.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
melButtonCode.setText("Has been clicked!");
}
});
}
}
For your problem
replace it
android:onClick="buttonPress (MainActivity)"
With
android:onClick="buttonPress"
Make the method public - protected works when you are in instantrun mode, but not when not. No idea why!

when i change my button to an icon i can't click on it to go to a new layout

this my xml file main
<?xml version="1.0" encoding="utf-8"?>
<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:background="#raw/christmas"
android:orientation="vertical">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="#+id/by_kostas"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:layout_marginBottom="83dp"
android:text="#string/by_kostas"
android:textSize="20sp"
android:textColor="#F2F3F4"/>
<ImageView
android:id="#+id/settings"
android:layout_width="50dp"
android:layout_height="50dp"
android:layout_alignParentBottom="true"
android:layout_alignParentRight="true"
android:onClick="nameOfMethod"
android:src="#raw/settings" />
</RelativeLayout>
and my java Main
package com.kostas.mytorch;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.ImageView;
public class Main extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
//start our layout
super.onCreate(savedInstanceState);
setContentView(R.layout.main);{
final ImageView diskView = (ImageView) findViewById(R.id.settings);
diskView.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v){
//my codes
}
});
diskView.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v){
// System.out.println("image clicked...");//in my logcat
startActivity(new Intent("com.kostas.standroid.settings"));
}
});
}
}
my problem is when i click in the settings icon, my program crashes instead of what i wanted to create a new layout (settings xml is just black page),can someone be kind enough to help me out
You can attach click listener to any view by two ways:
In xml, write onClick attribute of the view and pass the method
name you have implemented in the activity.
Example: android:onClick="someMethod" and in the activity code, declare a method
public void someMethod(View view)
{
// handle click here
}
In activity, use setOnClickListener() to the view.
For now, try this:
diskView.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v){
// System.out.println("image clicked...");//in my logcat
startActivity(new Intent(Main.this, settings.class));
}
});
Also remove this line from xml: android:onClick="nameOfMethod"
Some time there is contextual problem. Try this.
Intent intent = new Intent(CurrentActivity.this, UpcomingActivity.class);
startActivity(intent);
Also don't forget to define class in Manifest file
try to remove
android:onClick="nameOfMethod"
from the xml layout. I believe this is the method that it's called when you click the settings button and it doesn't exist so it crashes.

Respond to send button

I installed the Android SDK bundle today and I am following the "My First App" tutorial and I am stuck, it states:
Open the MainActivity class (located in the project's src/ directory) and add the corresponding method:
/** Called when the user clicks the Send button */
public void sendMessage(View view) {
// Do something in response to button
}
Where do I put this in the file? and is this the "MainActivity.java" file?
I have tried and I keep getting errors so I am obviously going wrong somewhere.
activity_main.xml:
<LinearLayout 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:orientation="horizontal">
<EditText android:id="#+id/edit_message"
android:layout_weight="1"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:hint="#string/edit_message" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/button_send"
android:onClick="sendMessage" />
</LinearLayout>
MainActivity.java:
package com.example.myfirstapp;
import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
public class MainActivity extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
#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;
}
}
Hope I have made my problem clear, I looked on the forum for an answer but I couldn't find anything.
If you have a button in your(say activity_main.xml) xml layout and you have the below attribute for button
android:onClick="sendMessage"
and you have the below in MainActiivty.java
setContentView(R.layout.activity_main);
You should have the below in MainActivity.java
/** Called when the user clicks the Send button */
public void sendMessage(View view) {
// Do something in response to button
}
Example:
MainActivity.java
// Your imports
public class MainActivity extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main); //setting the layout to activity
}
/** Called when the user clicks the Send button */
public void sendMessage(View view) {
// Do something in response to button
}
}
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical">
// other widgets
<Button
android:id="#+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:layout_marginBottom="146dp"
android:onClick="sendMessage"
android:text="Button" />
</RelativeLayout>
If you are a new android developer and doing your first so start from basic like launch new activity it contain hello world or any text view, button then you will clear idea about application.
create your android application
in XML layout drag the button and text view
run your first app.
you will get your output.
Put it in MainActivity.java at the top right after
public class MainActivity extends ActionBarActivity {
After you do this, you may have to import. Do this by pressing control / shift / O (not zero)

Android Null Pointer Exception Button

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.
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.
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
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.
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
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
Implement OnClickListener interface
and set button.setOnClickListener(this);
and override
public void onClick(View v) {
}
I think your buttons IDs need to be different in different activities. R.id.testButton would refer to only one button.
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?
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);
}
});
}
}
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);

Error "android (name of project) has stopped unexpectedly

package com.iperetz1.android.testbutton1;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
public class TestButton extends Activity {
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Button test2 = (Button)findViewById(R.id.test2);
test2.setOnClickListener(new OnClickListener()
{
#Override
public void onClick(View v)
{
setContentView(R.layout.test2);;
}
});
Button other = (Button)findViewById(R.id.backmain);
other.setOnClickListener(new OnClickListener()
{
#Override
public void onClick(View v)
{
setContentView(R.layout.main);;
}
});
}
}
main.xls
<?xml version="1.0" encoding="utf-8"?>
<AbsoluteLayout
android:id="#+id/widget0"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
xmlns:android="http://schemas.android.com/apk/res/android"
>
<Button
android:id="#+id/test2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="test2"
android:layout_x="24px"
android:layout_y="165px"
>
</Button>
</AbsoluteLayout>
test2.xml
<?xml version="1.0" encoding="utf-8"?>
<AbsoluteLayout
android:id="#+id/widget0"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
xmlns:android="http://schemas.android.com/apk/res/android"
>
<Button
android:id="#+id/backmain"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="backmain"
android:layout_x="24px"
android:layout_y="165px"
>
</Button>
</AbsoluteLayout>
findViewById is a lot simpler than people tend to think it is. It traverses the view hierarchy looking for a view with the given ID. If it's not found, findViewById returns null.
You started by setting the content view to your main layout but later on you tried to findViewById(R.id.backmain). Since there is no view with that ID in your main layout, it returns null. At that point attempting other.setOnClickListener will fail. You will only be able to do this when your button actually exists in the view hierarchy.
There's nothing inherently wrong with dynamically changing your view hierarchy, but you'll have to handle some things differently if you go that route. (Such as when you wire up events to views that don't exist during onCreate like you're trying to do above.)
As #Cristian Castiblanco said, changing the view dynamically is causing the problem, for these kind of scenarios, you have to create separate activities and invoke them using intents and pass data between them using bundles.

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