My app is unable to start activity?

Question

When i try to run the app on my phone,it force closes when i try to do activity 3. Logcat says:

01-13 17:53:25.368: E/AndroidRuntime(3235): Caused by: java.lang.NullPointerException 01-13 17:53:25.368: E/AndroidRuntime(3235): at android.app.activity3.onCreate(activity3.java:18)

where line 18 is

Button wg = (Button) findViewById(R.id.Back);

heres my full code for activity3.java:

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


public class activity3 extends Activity{

    /** Called when the activity is first created. */
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main3);

        Button wg = (Button) findViewById(R.id.Back);
        wg.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                Intent intent = new Intent();
                setResult(RESULT_OK, intent);
                finish();
            }

        });
    }
}

Thanks in advance

score 1 · Answer 1 · answered Jan 14 '12 at 02:09

1

I don't see a button in the xml. And once you have that, it needs to have:

android:id="@+id/Back"

answered Jan 14 '12 at 02:09

Jere

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O so i should add a button to xml? – user1148715 Jan 14 '12 at 02:09
Yup. When you say "findViewById" it is looking for a view in the XML. It's not finding it and throwing a null pointer. – Jere Jan 14 '12 at 02:10

score 1 · Answer 2 · answered Jan 14 '12 at 02:09

1

This is happening because you don't have a button declared in your view. Only a Textview. You must create the button in the view. You're referencing nothing hence the null.

You can find a nice example of how to make a button here.

answered Jan 14 '12 at 02:09

Aidanc

6,921
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score 1 · Answer 3 · answered Jan 14 '12 at 02:10

1

You don't have button with id "Back" in xml posted, that is why you are getting null there. Add button entry in your xml.

answered Jan 14 '12 at 02:10

kosa

65,990
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score 1 · Accepted Answer · answered Jan 14 '12 at 02:20

You need to modify your 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"
    >

    <!-- your textview -->
    <TextView
        android:id="@+id/text1"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/text1"
        android:textAppearance="?android:attr/textAppearanceMedium"
        />

    <!-- your back button -->
    <Button
        android:id="@+id/back"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/Back"
        />

</RelativeLayout>

What I did:

Removed text and orientation from your root RelativeLayout, these do nothing.
Modified your height and width, feel free to change these as you please.
Added a Button with id of R.id.back and text of @string/Back.

You will now be able to reference your button using findViewById(R.id.back) and set your click listener.

Gee thanks! This gave the most effort even though you were last i like this answer. Every Else, Thankyou <33 — user1148715, Jan 14 '12 at 02:33

My app is unable to start activity?

4 Answers4