Android Community

May 24, 2013 No Comments

Android programming crash null pointer exception

 

hey everyone at the moment im trying to display all the files in a certain folder on the android devices sd card in a

ListView

and so far im having bad luck,It crashes with a nullpointer exception so i was wondering if anyone here could shine some lite on my proble ill post my activity and logcat below and thanks.

Activity

package com.mkyong.android;

import java.io.File;

import java.util.ArrayList;

import android.app.ListActivity;
import android.os.Bundle;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;
import android.view.View;

public class ListMobileActivity extends ListActivity {

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

    }
        public ArrayList<String> GetFiles(String DirectoryPath) {
            ArrayList<String> MyFiles = new ArrayList<String>();
            File f = new File(DirectoryPath);

            f.mkdirs();
            File[] files = f.listFiles();
            if (files.length == 0)
                return null;
            else {
                for (int i=0; i<files.length; i++)
                    MyFiles.add(files[i].getName());
            }

            return MyFiles;
        }
        {
        ListView lv;
        ArrayList<String> FilesInFolder = GetFiles("/sdcard/RootBox");
        lv = (ListView)findViewById(R.id.listView1);

        lv.setAdapter(new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_1, FilesInFolder));

        lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v, int position, long id) {

             }
        });
    }

    @Override
    protected void onListItemClick(ListView l, View v, int position, long id) {

        //get selected items
        String selectedValue = (String) getListAdapter().getItem(position);
        Toast.makeText(this, selectedValue, Toast.LENGTH_SHORT).show();

    }

}

and logcat
05-24 06:42:28.992: E/AndroidRuntime(796): FATAL EXCEPTION: main
05-24 06:42:28.992: E/AndroidRuntime(796): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mkyong.android/com.mkyong.android.ListMobileActivity}: java.lang.RuntimeException: Your content must have a ListView whose id attribute is ‘android.R.id.list’
05-24 06:42:28.992: E/AndroidRuntime(796): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
05-24 06:42:28.992: E/AndroidRuntime(796): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
05-24 06:42:28.992: E/AndroidRuntime(796): at android.app.ActivityThread.access$600(ActivityThread.java:141)
05-24 06:42:28.992: E/AndroidRuntime(796): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
05-24 06:42:28.992: E/AndroidRuntime(796): at android.os.Handler.dispatchMessage(Handler.java:99)

 

Answer

If you have asked for permission in android manifest for reading storage, then this might be the problem.

It seems from the code that you are querying a instance of list view in the instance block.
This might be called before the setContentView is called.
You should instantiate a UI component in XML after the layout is inflated or else you will run into exception.

Try this.

    public class ListMobileActivity extends ListActivity {

    ListView lv;

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

        ArrayList<String> FilesInFolder = GetFiles("/sdcard/RootBox");
        lv = (ListView) findViewById(R.id.listView1);

        lv.setAdapter(new ArrayAdapter<String>(this,
                android.R.layout.simple_list_item_1, FilesInFolder));

        lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            public void onItemClick(AdapterView<?> parent, View v,
                    int position, long id) {

            }
        });
    }

    public ArrayList<String> GetFiles(String DirectoryPath) {
        ArrayList<String> MyFiles = new ArrayList<String>();
        File f = new File(DirectoryPath);

        f.mkdirs();
        File[] files = f.listFiles();
        if (files.length == 0)
            return null;
        else {
            for (int i = 0; i < files.length; i++)
                MyFiles.add(files[i].getName());
        }

        return MyFiles;
    }

}

It should work.

May 24, 2013 No Comments

how to get the files of assest subfolder in android?

 

How can i get the files of assets sub folder.in my Assets folder i have another sub folder(sample).I want to get all files of sample folder and store it into sdcard.Please can any one help me?

Thanking in Advance.

 

Answer

You can do it this way..

AssetManager assetManager = getAssets();
String[] files = assetManager.list("sample");
for(String filename : files) {
.....
}

May 24, 2013 No Comments

How to change the background button of image dynamically

 

I have a

Button

name is

Start

actually

Start

is a image which i have placed in my button as a background

android:background=”@drawable/btn_stop

what i want to do is By pressing the

Start

button it should replace the

Start

image with

Stop

I am able to do it in Java Code by using

btnStartStop.setBackgroundResource(R.drawable.btn_stop);

but i want to do it

XML

by using

selector

i have used this code

<Button
        android:id="@+id/button_start_stop"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="10dip"
        android:layout_marginTop="10dip"
        android:background="@drawable/btn_selector"/>

Here btn_selector is

XML

file which i have created inside the

drawable

folder
which contain
btn_selector.xml

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android" >
    <item android:drawable="@drawable/btn_stop"
          android:state_pressed="true" />
    <item android:drawable="@drawable/btn_start"/>
</selector>

but its only creating a hovering effect for ex:it only showing

Stop

when i am pressing the

Start

for a second and then after its showing

Start

.

Note:I Don’t want to use Toggle Button

 

Answer

Your selector shoul be like,

    <?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
        <item android:state_pressed="true" android:drawable="@drawable/switch_on" />
        <item android:state_selected="true" android:drawable="@drawable/switch_on" />
        <item android:drawable="@drawable/switch_off" /> <!-- default -->
    </selector>

And your listener should be like,

 button.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View arg0) {
                    // TODO Auto-generated method stub

                    if(button.isSelected())
                     {
                        button.setSelected(false);
                     }
                    else
                     {
                        button.setSelected(true);
                     }

                }
            });

May 24, 2013 No Comments

How can I align text of the spinner to the centre?

 

Here is my XML of the spinner but it did not work.

  <Spinner
        android:id="@+id/frequency"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@drawable/spinner_voices"
        android:gravity="center|center_horizontal"
         />

 

Answer

You need to set your own layout for spinner item.

ArrayAdapter adap = new ArrayAdapter<String>(this, R.layout.spinner_item, new String[]{"ABC", "DEF", "GHI"});
spriner.setAdapter(adap);

Where R.layout.spinner_item is a layout with content:

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_height="fill_parent"
    android:layout_width="fill_parent"
    android:gravity="center"/>

May 24, 2013 No Comments

Auto cut background image edges

 

I would like to improve my androids app design with background image but its resizing/spreading it on screens and make the app uglier as you can imagine,

I googled the issue and the only thing I found is putting many images for smartphone, tablet etc… to get always the best sized image but I m finding a way to put a huge image cut edges when screen is too small without spreading all the image because I don’t need to see the whole image

Someone know is it possible and how?

 

Answer

What you need is to place an image view to your layout.Set height and width as match parent.Then set scaleType as centreCrop.And place all other views above this image view.By doing so, Android will crop the image according to screen size of devices.