Android development beginner - error

Question

Well, I'm doing this app just for trial. And there's this error. It says:

04-21 04:11:41.618: E/AndroidRuntime(333): FATAL EXCEPTION: main
04-21 04:11:41.618: E/AndroidRuntime(333): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.windteste/com.example.windteste.MainActivity}: java.lang.NullPointerException
04-21 04:11:41.618: E/AndroidRuntime(333): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585)
04-21 04:11:41.618: E/AndroidRuntime(333): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
04-21 04:11:41.618: E/AndroidRuntime(333): at android.app.ActivityThread.access2300ドル(ActivityThread.java:125)

..and a lot more lines of error. So, these are the two methods (Dfig and MainActivity):

package com.example.windteste;
import android.app.Activity;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.Bundle;
public class Dfig extends Activity{
 @Override
 protected void onCreate(Bundle blablu) {
 // TODO Auto-generated method stub
 super.onCreate(blablu);
 setContentView(R.layout.activity_main);
 MediaPlayer windsound = MediaPlayer.create(Dfig.this, R.raw.windsound);
 windsound.start();
 Thread timer = new Thread(){
 public void run(){
 try{
 sleep(4000);
 }catch (InterruptedException e){
 e.printStackTrace();
 }finally{
 Intent openMain = new Intent("com.example.windteste.MAIN");
 startActivity(openMain);
 }
 }
 };
 timer.start();
 windsound.release();
 }
}
 package com.example.windteste;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.app.Activity;
import android.text.InputType;
----------------------------------------------------------------------------------
public class MainActivity extends Activity {
 Button check = (Button) findViewById(R.id.calculator);
 final EditText UserInput = (EditText) findViewById(R.id.windspeed);
 final TextView P = (TextView) findViewById(R.id.potencia);
 @Override
 protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.layout2);
 UserInput.setInputType(InputType.TYPE_CLASS_NUMBER);
 check.setOnClickListener(new View.OnClickListener() {
 public void onClick(View v) {
 Number Ws = (Number) UserInput.getText();
 P.setText("Your total is " + Ws);
 }
 });
 }
 @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;
 } 
}

Well, I hope you can help me.
Thank you!

• Matt Clark: Is the activity declared in your Manifest?
• paulsm4: HINT: always look at the part of the error message with a line number for your code. That's usually a good way to zero in on the problem. SUGGESTION: Don't initialize "check", "UserInput" and "P" outside of a method. All then inside a method, after you've called "setContentView".
• user2303633: Yeah, but it had some weird numbers... It showed java:1249, but my code doesn't even have so many lines. Thank you.

Answer 1

Try:

public class MainActivity extends Activity {
 Button check;
 final EditText UserInput;
 final TextView P;
 @Override
 protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.layout2);
 check = (Button) findViewById(R.id.calculator);
 UserInput = (EditText) findViewById(R.id.windspeed);
 P = (TextView) findViewById(R.id.potencia);
 //Rest is same 

You cannot use findViewById() until after setContentView() has been called.

• user2303633: Thank you, it helped! But know it has another problem in the line Number Ws = (Number) UserInput.getText(); P.setText("Your total is " + Ws);.
• Raghunandan: @user2303633 getText() will return a string. String s= UserInput.getText(); then set P.setText("Your total is " + s);.
• user2303633: Yes, but I wanted I number input from user. But I found it how to do it. You saved dude.
• Raghunandan: you can use Integer.parseInt(s) for the same. put it in a try catch block of the input is not a number will throw a numberformatexception or android:inputType="number" specify the attribute for edittext

Answer 2

Make sure both the activities are listed in the manifest. What it looks like is MainActivity is not listed in your manifest file.

• user2303633: Yes, they're listed. Thank you.