I'm trying to make a simple app that opens the time picker dialog when you click the "Check" button, and once you set the time and tap "OK", it checks the system time and outputs whether it is correct. However, I am given a null pointer exception on line 21 of MainActivity.java. At the moment the code is simply set up to bring up the dialog once "Check" is tapped.
MainActivity.java
package com.example.timechecker;
import android.app.Activity;
import android.os.Bundle;
import android.support.v4.app.DialogFragment;
import android.support.v4.app.FragmentManager;
import android.view.View;
public class MainActivity extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public FragmentManager getSupportFragmentManager() {
return null;
}
public void checkTime(View view) {
DialogFragment newFragment = new TimePickerFragment();
newFragment.show(getSupportFragmentManager(), "timePicker");
}
}
TimePickerFragment.java
package com.example.timechecker;
import java.util.Calendar;
import android.app.Dialog;
import android.app.TimePickerDialog;
import android.os.Bundle;
import android.support.v4.app.DialogFragment;
import android.text.format.DateFormat;
import android.widget.TimePicker;
public class TimePickerFragment extends DialogFragment
implements TimePickerDialog.OnTimeSetListener {
#Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
// Use the current time as the default values for the picker
final Calendar c = Calendar.getInstance();
int hour = c.get(Calendar.HOUR_OF_DAY);
int minute = c.get(Calendar.MINUTE);
// Create a new instance of TimePickerDialog and return it
return new TimePickerDialog(getActivity(), this, hour, minute,
DateFormat.is24HourFormat(getActivity()));
}
public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
// Do something with the time chosen by the user
}
}
Manifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.timechecker"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="17" />
<application
android:allowBackup="true"
android:icon="#drawable/ic_launcher"
android:label="#string/app_name"
android:theme="#style/AppTheme" >
<activity
android:name="com.example.timechecker.MainActivity"
android:label="#string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name="com.example.timechecker.TimePickerFragment"
android:label="#string/app_name" >
</activity>
</application>
</manifest>
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="vertical"
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=".MainActivity" >
<TextView
android:id="#+id/whatTime"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="What time is it?"
android:textAppearance="?android:attr/textAppearanceLarge" />
<Button
android:id="#+id/check"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:onClick="checkTime"
android:text="Check" />
</LinearLayout>
Ah i see what is happening here.
public FragmentManager getSupportFragmentManager() {
return null;
}
public void checkTime(View view) {
DialogFragment newFragment = new TimePickerFragment();
newFragment.show(getSupportFragmentManager(), "timePicker");
}
You are overriding getSupportFragmentManager to return null, and after that you call the method to show the dialog, looks like you created the nullpointer on purpose? =D
Related
I am trying to use intents but on clicking any of the buttons,the app stops working and terminates.I am a beginner and I couldn't find the reason.I am providing my xml file,java file and menifest file.Please someone help.
here is my xml file:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context="com.example.shreya.intents.MainActivity">
<Button
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Set Alarm"
android:id="#+id/alarm"
android:onClick="setAlarm"/>
<Button
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Map"
android:id="#+id/map"
android:onClick="seeMap"/>
<Button
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Mail"
android:id="#+id/mail"
android:onClick="email"/>
<EditText
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="Email ID"
android:inputType="textCapWords"
android:id="#+id/address"/>
</LinearLayout>
And here is my .java file:
package com.example.shreya.intents;
import android.content.Intent;
import android.net.Uri;
import android.provider.AlarmClock;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import java.util.Locale;
import static com.example.shreya.intents.R.id.alarm;
public class MainActivity extends AppCompatActivity {
private Button mail;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public void setAlarm(){
String message="Wake Up";
int hour=7;
int minutes=0;
Intent intent = new Intent(AlarmClock.ACTION_SHOW_ALARMS);
if (intent.resolveActivity(getPackageManager()) != null) {
startActivity(intent);
}
}
public void seeMap(){
String uri = String.format(Locale.ENGLISH, "geo:%f,%f", 28.699884, 77.273075);
Intent intent = new Intent(Intent.ACTION_VIEW);
intent.setData(Uri.parse(uri));
if(intent.resolveActivity(getPackageManager())!=null)
startActivity(intent);
}
public void email(){
EditText mail=(EditText)findViewById(R.id.address);
String add=mail.getText().toString();
composeEmail(add,"shreya");
}
public void composeEmail(String addresses, String subject) {
Intent intent = new Intent(Intent.ACTION_SENDTO);
intent.setData(Uri.parse("mailto:")); // only email apps should handle this
intent.putExtra(Intent.EXTRA_EMAIL, addresses);
intent.putExtra(Intent.EXTRA_SUBJECT, subject);
intent.putExtra(Intent.EXTRA_TEXT,"hello");
if (intent.resolveActivity(getPackageManager()) != null) {
startActivity(intent);
}
}
}
And finally this is my menifest file:
<?xml version="1.0" encoding="utf-8"?>
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
You have to add parameter View view to your onClick methods.
For example:
public void setAlarm(View view){
}
Add that parameter to other onClick methods also: email and seeMap.
Under the hood, the system uses reflection to figure out the exact onClick method in the Activity and the system uses the following exact pattern to find the method: a public void method with the specified method name in the xml onClick attribute and has a single parameter View.
I am trying to get the ABOUT button to listen for a click, in order to go from a screen displaying three buttons and a picture, to another screen just displaying some text.
This is my Main java file (MainActivity.java)
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
public class MainActivity extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
addListenerOnButton();
}
public void addListenerOnButton() {
final Context cont = this;
Button aButton = (Button) findViewById(R.id.ab_button);
aButton.setOnClickListener(
new Button.OnClickListener(){
public void onClick(View v){
Intent intent = new Intent(cont, AboutActivity.class);
startActivity(intent);
}
});
}
This is my second java file for the about button (AboutActivity.java):
import android.app.Activity;
import android.os.Bundle;
import android.widget.Button;
import android.widget.RelativeLayout;
import android.widget.TextView;
public class AboutActivity extends Activity {
public void onCreate(Bundle about){
super.onCreate(about);
setContentView(R.layout.activity_about);
TextView briText = (TextView) findViewById(R.id.about_b2);
briText.setText("Goodie");
}
}
This is the main xml file that displays the picture and the buttons (activity_main.xml):
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/about_b"
android:id="#+id/ab_button"
android:shadowColor="#8e2300"
android:background="#drawable/oval_button"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/share_button"
android:id="#+id/share_button"
android:shadowColor="#8e2300"
android:onClick="shareActivity"
android:background="#drawable/oval_button"
And this is the xml file that displays the text (activity_about.xml):
<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:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
android:paddingBottom="#dimen/activity_vertical_margin"
android:id="#+id/back_to_work_rel">
<TextView android:text="#string/about_bac"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="#+id/about_b2" />
</RelativeLayout>
Manifest file (AndroidManifest.xml):
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.brian.eggalong" >
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:theme="#style/AppTheme" >
<activity
android:name=".MainActivity"
android:label="#string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".AboutActivity"
android:parentActivityName=".MainActivity"
android:label="#string/about_bac">
</activity>
</application>
</manifest>
Every time I click the ABOUT button, nothing happens. Any help would be greatly appreciated.
Try to remove the background attribute of your button.
Otherwise you can try this
MainActivity.java
public void moveToAboutActivity(){
Intent i = new Intent(getApplicationContext(), AboutActivity.class);
startActivity(i);
}
activity_main.xml
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/about_b"
android:id="#+id/ab_button"
android:onClick="moveToAboutActivity"/>
good Luck
Ok, so, definetly the problem is that you need to put the activity in your manifest file like:
<activity
android:name=".activity.SplashScreen"
android:screenOrientation="portrait"
android:label="#string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
I am trying to make a simple quiz app
The user can answer the question and press "next question" button until he reaches the final question. When he press the button then the Android should show a new Activity containing the result.
But the problem here. When I tried the app, it shows an empty Activity instead.
And that's my code:
AndroidManifest.xml :
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.quizapp"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="21" />
<application
android:allowBackup="true"
android:icon="#drawable/ic_launcher"
android:label="#string/app_name"
android:theme="#style/AppTheme" >
<activity
android:name=".Quiz"
android:label="#string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".Result"
android:label="#string/app_name" >
<intent-filter>
<action android:name="MySecond" />
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>
</activity>
</application>
</manifest>
Quiz.java :
package com.example.quizapp;
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;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;
public class Quiz extends Activity {
TextView tv;
Button nxt ;
RadioGroup rg;
RadioButton r1 , r2 , r3 ;
String question [] = {"your name ?","your age?"};
String ans[] = {"Tareq","22"};
String opt []= {"moh","Tareq","khaled","22","21","20"};
int flag = 0 ;
public static int marks , correct , wrong ;
public void onCreate(Bundle b) {
super.onCreate(b);
setContentView(R.layout.quiz);
tv=(TextView)findViewById(R.id.question);
nxt=(Button)findViewById(R.id.button1);
rg=(RadioGroup)findViewById(R.id.radioGroup1);
r1=(RadioButton)findViewById(R.id.radio1);
r2=(RadioButton)findViewById(R.id.radio2);
r3=(RadioButton)findViewById(R.id.radio3);
tv.setText(question[flag]);
r1.setText(opt[0]);
r2.setText(opt[1]);
r3.setText(opt[2]);
nxt.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
RadioButton selectedAns = (RadioButton)findViewById(rg.getCheckedRadioButtonId());
String selectedAnsText = selectedAns.getText().toString();
if (selectedAnsText.equals(ans[flag])){
correct ++ ;
}
else {
wrong ++ ;
}
flag ++ ;
if ( flag < question.length)
{
tv.setText(question[flag]);
r1.setText(opt[flag*3]);
r2.setText(opt[flag*3]+1);
r3.setText(opt[flag*3]+2);
}
else {
marks = correct - wrong ;
startActivity (new Intent("MySecond"));
}
}
});
}
}
Result.java :
package com.example.quizapp;
import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
public class Result extends Activity{
TextView tv ;
public void OnCreate(Bundle b){
super.onCreate(b);
setContentView(R.layout.result);
tv = (TextView) findViewById(R.id.result);
tv.setText("your final result is :");
}
}
quiz.xml : is fine
result.xml :
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<TextView
android:id="#+id/result"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Large Text"
android:textAppearance="?android:attr/textAppearanceLarge" />
<Button
android:id="#+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button" />
</LinearLayout>
So where the problem?
Thx in advance ..
In your Result.java change
public void OnCreate(Bundle b){ to public void onCreate(Bundle b){
Notice the o in onCreate. It should be in small case.
I’m following the android tutorial First App in the link below:
https://developer.android.com/training/basics/firstapp/starting-activity.html
The app can be launched in the emulator but the problem is when I click on the button “send, the application crashed with the message “unfortunately, my fist app has stopped”.
I think I did follow the instructions in the tutorial and can’t figure out where is the problem from.
Logcat send this message:
02-12 16:19:36.031: E/AndroidRuntime(786): java.lang.RuntimeException:
Unable to instantiate activity
ComponentInfo{com.example.myfirstapp/com.example.myfirstapp.DisplayMessageActivity}:
java.lang.NullPointerException
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.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.EditText;
public class MainActivity extends Activity {
public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
/** Called when the user clicks the Send button */
public void sendMessage(View view) {
Intent intent = new Intent(this, DisplayMessageActivity.class);
EditText editText = (EditText) findViewById(R.id.edit_message);
String message = editText.getText().toString();
intent.putExtra(EXTRA_MESSAGE, message);
startActivity(intent);
// Do something in response to button
}
#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;
}
}
activity_display_message.xml
<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=".DisplayMessageActivity" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/hello_world" />
</RelativeLayout>
DisplayMessageActivity.java
package com.example.myfirstapp;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.support.v4.app.NavUtils;
import android.view.MenuItem;
import android.widget.TextView;
public class DisplayMessageActivity extends Activity {
Intent intent = getIntent();
String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
#SuppressLint("NewApi")
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Get the message from the intent
Intent intent = getIntent();
String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
// Create the text view
TextView textView = new TextView(this);
textView.setTextSize(40);
textView.setText(message);
// Set the text view as the activity layout
setContentView(textView);
// Make sure we're running on Honeycomb or higher to use ActionBar APIs
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
// Show the Up button in the action bar.
getActionBar().setDisplayHomeAsUpEnabled(true);
}
}
#Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case android.R.id.home:
NavUtils.navigateUpFromSameTask(this);
return true;
}
return super.onOptionsItemSelected(item);
}
}
MyFirstApp Manifest
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.myfirstapp"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="18" />
<application
android:allowBackup="true"
android:icon="#drawable/ic_launcher"
android:label="#string/app_name"
android:theme="#style/AppTheme" >
<activity
android:name="com.example.myfirstapp.MainActivity"
android:label="#string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name="com.example.myfirstapp.DisplayMessageActivity"
android:label="#string/title_activity_display_message"
android:parentActivityName="com.example.myfirstapp.MainActivity" >
<meta-data
android:name="android.support.PARENT_ACTIVITY"
android:value="com.example.myfirstapp.MainActivity" />
</activity>
</application>
</manifest>
Thank you for your help
I believe the problem is
Intent intent = getIntent();
String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
before onCreate(). Intent needs a Context which isn't available until the Activity has been created. Remove those lines since you already have it in onCreate().
Also, when it crashes, post your full logcat because it will tell us, usually, exactly where and what the problem is.
Okay, this is really ticking me off. I ran it on both my emulator and on an android device. The code does NOT display "Helloworld, Android -mykong.com". I start the app, find it on the emulator, click on it, and it goes to interface of the app. However, it is just a blank screen! This error is not an issue of me finding the app, its not with the emulator or the android phone, the issue has to lie either in the code or in the way the code is structured/built. I know the the helloworld code is 100% correct because its from a reputable tutorial site. These are my three pairs of code. Please help, I am in desperate need!
AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.test123"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="17" />
<application
android:allowBackup="true"
android:icon="#drawable/ic_launcher"
android:label="#string/app_name"
android:theme="#style/AppTheme" >
<activity
android:name="com.example.test123.MainActivity"
android:label="#string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
MainActivity.java
package com.example.test123;
import android.app.Activity;
import android.content.Intent;
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);
}
public void activity()
{
Intent helloWorld = new Intent(getApplicationContext(), HelloWorldjavaactivity.class );
startActivity( helloWorld );
}
#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;
}
}
HelloWorldjavaactivity.java
package com.example.test123;
import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
public class HelloWorldjavaactivity extends Activity {
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
TextView text = new TextView(this);
text.setText("Hello World, Android - mkyong.com");
setContentView(text);
}
}
Activity_main.xml
<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=".MainActivity" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:layout_marginLeft="35dp"
android:layout_marginTop="57dp"
android:text=""
/>
</RelativeLayout>
You're not starting the HelloWorldjavaactivity.java activity. You need something like the following in your MainActivity:
Intent helloWorld = new Intent(getApplicationContext(), HelloWorldjavaactivity.class );
startActivity( helloWorld );
Of course, you need that code to be triggered by some even, a button press for example.
It would be easier to just eliminate the second activity (since your MainActivity isn't doing anything anyway) and put the code in your MainActivity instead. You'll also need to edit the layout for your MainActivity so that it includes the appropriate views.