运行应用程序时出现java.lang.NullPointerException

Question

I am new to android development. I downloaded source file from the following link

" http://android-er.blogspot.in/2012/07/implement-custom-linearlayout-for.html " , but while trying to run in emulator it shows

java.lang.NullPointerException at com.example.androidhorizontalscrollviewgallery.MainActivity.onCreate(MainActivity.java:27)

My MainActivity.java code is below:

package com.example.androidhorizontalscrollviewgallery;

import java.io.File;
import android.os.Bundle;
import android.app.Activity;
import android.content.Context;

public class MainActivity extends Activity {

MyHorizontalLayout myHorizontalLayout;

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

    myHorizontalLayout = (MyHorizontalLayout)findViewById(R.id.mygallery);


    File targetDir=getDir("Pictures",Context.MODE_PRIVATE);

    String targetPath=targetDir+ "/homepage/";
    File targetDirector = new File(targetPath);

    File[] files = targetDirector.listFiles();

    for(File f : files){

        myHorizontalLayout.add(f.getAbsolutePath());
    }


    }
}

Here I have stored my images in my computer in "Libraries\\Pictures\\homepage" path. I wanted to get the images in gallery but I could not. I don know what causes this exception can anyone please tell the solution

Answer 1

try this..

File targetDir=getDir("Libraries\Pictures\homepage",Context.MODE_PRIVATE);
 String targetPath=targetDir.toString();

Answer 2

From the exception, you have a null pointer on line 27, which means that the files object that you are iterating is null. You use the listFiles method to obtain that object, but the listFiles method can return null. You have to check whether you have null or not to avoid your app crashing. See example below. Also, refer to the File class documentation .

if (files == null) {
    // handle case where the file object is not a directory
}
else {
for(File f : files){
        myHorizontalLayout.add(f.getAbsolutePath());
    }
}

Answer 3

I think you can't directly get images from your computer path .

You have to copy that images in your drawable or assests folder of the application.

Or your images should reside in either your device or emulator gallery or any other folders . But it should be in device or emulator in whichever you are testing.

See this line in tutorial you used.

String ExternalStorageDirectoryPath = Environment
      .getExternalStorageDirectory()
      .getAbsolutePath();

String targetPath = ExternalStorageDirectoryPath + "/test/";

So this is your device's sdcard path - external storage path.