Develop Reference

Saving objects in cache

I am trying to save in cache response from server for certain time.
There are tne next data for saving in cache: I have a List<ProgrammeItem> which I am getting from server. While user is working, he can download up to ~230 List<ProgrammeItem> (but it is unreal to reach this, estimated is 10-50).
ProgrammeItem oblect including strings, int, int[].
That is how I am saving and getting the last downloaded List<ProgrammeItem>:
//saving / getting Programme items
public boolean saveObject(List<ProgrammeItem> obj) {
final File suspend_f=new File(android.os.Environment.getExternalStorageDirectory(), "test");
FileOutputStream fos = null;
ObjectOutputStream oos = null;
boolean keep = true;
try {
fos = new FileOutputStream(suspend_f);
oos = new ObjectOutputStream(fos);
oos.writeObject(obj);
} catch (Exception e) {
keep = false;
Log.e("catching exception ", "" + e.getMessage() + ";;;" + e);
} finally {
try {
if (oos != null) oos.close();
if (fos != null) fos.close();
if (keep == false) suspend_f.delete();
} catch (Exception e) { /* do nothing */ }
}
return keep;
}
public List<ProgrammeItem> getObject(Context c) {
final File suspend_f=new File(android.os.Environment.getExternalStorageDirectory(), "test");
List<ProgrammeItem> simpleClass= null;
FileInputStream fis = null;
ObjectInputStream is = null;
try {
fis = new FileInputStream(suspend_f);
is = new ObjectInputStream(fis);
simpleClass = (List<ProgrammeItem>) is.readObject();
} catch(Exception e) {
String val= e.getMessage();
} finally {
try {
if (fis != null) fis.close();
if (is != null) is.close();
} catch (Exception e) { }
}
return simpleClass;
}
That is how I am saving and getting object in activity:
PI = new ProgrammeItem();
List<ProgrammeItem> programmeItems = new ArrayList<>();
...
//filling programmeItems with data from server
...
boolean result = PI.saveObject(programmeItems); //Save object
ProgrammeItem m = new ProgrammeItem();
List<ProgrammeItem> c = m.getObject(getApplicationContext()); //Get object
The question is: how can I save a lot of my objects instead of only one?
I think I should done something like public boolean addObjectsInCache(List<ProgrammeItem> obj) for adding objects, not overriding them.
And change get method into public List<ProgrammeItem> getObject(Context c, String id), where id will be unique identifier, which will includes into every ProgrammeItem in the every List<ProgrammeItem>.
Am I right? And how I can achieve this? Maybe you will show me the other way to work with objects and cache?

You can use SharedPreference instead, while having a local database Android Room can also be an option. SharedPreference basically is stored in your device's cache while the local database is stored in your device's data hence in our apps we have clear cache and clear data function.
Additional Resources:
StackOverFlow: How Android SharedPreferences save/store object
Object based preference library: https://github.com/ShawnLin013/PreferencesManager I would suggest you go with this one, since it can easily save you time saving list based object and retrieving them. You can also add more to the persisted list object when needed.
Secured Preferences: https://github.com/scottyab/secure-preferences

An option could be to use Room database with inMemoryDatabaseBuilder:
db = Room.inMemoryDatabaseBuilder(context, ProgrammeDatabase::class.java)
.build()
if it all can fit in memory.

null exception when writing file to internal storage from separate class

I have two activities that I want to call the time class bellow but every time i call that class it throws up an exception. But if i put the timeWrite() method in the Activity it works well but if I put it in the time class and then try to call it to write the file it throws up exception.
public class time extends AlphabetActivity{
private int time;
public void timeWrite(int time) {
try {
String timeVal = String.valueOf(time);
FileOutputStream timeStream = openFileOutput("time_file.txt", Context.MODE_PRIVATE);
timeStream.write(timeVal.getBytes());}
timeStream.close();
catch (Exception e) {
e.printStackTrace();}}
It seems like something is wrong with inheritance and I am just not understanding what I am doing wrong.

try {
String timeVal = String.valueOf(time);
FileOutputStream timeStream = openFileOutput("time_file.txt", Context.MODE_PRIVATE);
timeStream.write(timeVal.getBytes());
}
timeStream.close();
catch (Exception e) {
e.printStackTrace();}
}
Your code will compile if you write:
try {
String timeVal = String.valueOf(time);
FileOutputStream timeStream = openFileOutput("time_file.txt", Context.MODE_PRIVATE);
timeStream.write(timeVal.getBytes());
timeStream.close();
}catch (Exception e) {
e.printStackTrace();
}
If think this sytanx is wrong, does this code compile ?
Please read this, I know, long but you have to study now or later:
https://docs.oracle.com/javase/tutorial/essential/exceptions/
same
http://www.tutorialspoint.com/java/java_exceptions.htm
if you write file to the external storge, mind to add the READ and WRITE_EXTERNAL_STORAGE permission to the manifest

Can not read text files

I am trying to write and read a text file which is full of words and add it to an ArrayList. The ArrayList later is used from another part of the program to display text in a TextView. But when i run the program and open the specific part of it, then there is nothing. The ArrayList is just empty. I don't get any exceptions but for some reason it doesn't work. Please help me.
I don't seem to have problems with the file writing:
TextView txt = (TextView)findViewById(R.id.testTxt);
safe = txt.getText().toString();
try {
FileOutputStream fOut = openFileOutput("test.txt", MODE_WORLD_READABLE);
OutputStreamWriter osw = new OutputStreamWriter(fOut);
try {
osw.write(safe);
osw.flush();
osw.close();
Toast.makeText(getBaseContext(), "Added to favorites", Toast.LENGTH_SHORT).show();
} catch (FileNotFoundException ex){
Log.e("Exception", "File write failed: ");
}
} catch (IOException e) {
Log.e("Exception", "File write failed: ");
}
But I think the problem is in the file reading. I made some "Log.d" and found out that everything works fine till the InputStreamReader line:
public favHacks() {
testList = new ArrayList<String>();
try {
//Works fine till here
InputStreamReader inputStreamReader = new InputStreamReader(openFileInput("test.txt"));
BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
String receiveString = "";
while ( (receiveString = bufferedReader.readLine()) != null )
{
testList.add(receiveString);
}
bufferedReader.close();
}
catch (FileNotFoundException ex) {
Log.d("login activity", "File not found: ");
} catch (IOException ex) {
Log.d("login activity", "Can not read file: ");
}
}

If you have a relatively small collection of key-values that you'd like to save, you should use the SharedPreferences APIs.
http://developer.android.com/training/basics/data-storage/shared-preferences.html
also if you want to write/read files, or do any kind of operations that can block the Main Thread try to use another Thread like when you are trying to save the data in a file or use a Handler if you have multiple Threads (one for saving and one for reading).
https://developer.android.com/training/multiple-threads/index.html
In your code the method called favHacks can return an ArrayList with the list of all the strings Something like
//
public ArrayList<String> readFromFile(String file){
ArrayList<String> mArrayList= new ArrayList<String>();
//read from file here
return mArrayList;
}
but as I said before, you need to the operations that can block the UI thread in a new Thread.
https://developer.android.com/training/multiple-threads/communicate-ui.html
And also I think that the best way to do this is using Asynk task
Why and how to use asynctask

Serialization of Application Object

At first I have strong Java knowledege, but however just started with Android.
My Android app is downloading some fairly complex data (texts, dates, images) which I am saving in a custom object. The data need to be refresh from time to time. However usually the data downloaded will not change.
In order to keep the data in memory I am using the Application Object. Unfortunately, it looks like the application object instance is destroyed when the app is killed.
Hence, I was wondering if it would be of good practice to serialize and save my custom object (which is contained in the application object) in the internal storage during onPause(). Obviously, I would then first read from the file in onResume() before reloading from the internet. The idea is also to enable offline viewing.
In longer term the plan is to move the code downloading the date in a background service. As there seems to be many different ways to keep application state in Android, I would like to be be sure that this is the correct way to go.

Try using those methods class to save the Object(s) (implements serialize) you need:
public synchronized boolean save(String fileName, Object objToSave)
{
try
{
// save to file
File file = new File(CONTEXT.getDir("filesdir", Context.MODE_PRIVATE) + "/file.file");
if (file.exists())
{
file.delete();
}
file.getParentFile().mkdirs();
file.createNewFile();
ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(file));
oos.writeObject(objToSave);
oos.close();
return true;
}
catch (FileNotFoundException e)
{
e.printStackTrace();
return false;
}
catch (IOException e)
{
e.printStackTrace();
return false;
}
}
public synchronized Object load(String fileName)
{
try
{
File file = new File(CONTEXT.getDir("filesdir", Context.MODE_PRIVATE) + "/file.file");
if (!file.exists())
{
return null;
}
ObjectInputStream ois = new ObjectInputStream(new FileInputStream(file));
savedObj = ois.readObject();
ois.close();
return savedObj;
}
catch (FileNotFoundException e)
{
e.printStackTrace();
return null;
}
catch (Exception e)
{
e.printStackTrace();
return null;
}
}
You'll need to cast the Object you load().
CONTEXT is an Activity or ApplicationContext to get access to the cachedir.
Your could use Environment.getExternalStorageState() instead to get a directory path. DOn't forget to add it "/filename".

Android force closes when reading from internal storage 5 times in a row

In my android app, I am reading a file from internal storage every time a new game loads.
The first 4 times I do this, it works fine, but on the fifth time it force closes.
Here is my code
private String readFromInternalStorage(String filename) {
FileInputStream fis=null;
byte[] bytes = new byte[1000000];
try {
fis=startGame.openFileInput(filename);
} catch (FileNotFoundException e) {
e.printStackTrace();
}
try {
fis.read(bytes);
} catch (IOException e) {
e.printStackTrace();
}
return new String(bytes);
}
While messing around with the code, I noticed that if I change the length of the byte array, it changes the amount of times I can read a file without it force closing. If I change the length to 2000000, it closes after the second time and if I change it to 100000 it closes after the eighth time. I'm pretty clueless as to why this would happen because I am creating a new byte array every time the method is called so I wouldn't think that the size would change anything.
Update:
After going back and doing some more testing it seems like file input has nothing to do with why my app is force closing. When this code is commented out, the app will load five levels in a row without force closing so I thought that it was the problem, but it still force closes after eight tries so clearly there's something else that's not working. Thanks for your help anyway.

I don't see a "close()" in your code:
http://docs.oracle.com/javase/6/docs/api/java/io/FileInputStream.html#close%28%29

You shouldn't hard-code the array size. Besides you should use finally, in order to make sure the FileInputStream is closed, even when failed.
Here's a code sample that shows how it should be done:
FileInputStream fis;
String info = "";
try {
fis = mContext.openFileInput(this.fileName);
byte[] dataArray = new byte[fis.available()];
if (dataArray.length > 0) {
while (fis.read(dataArray) != -1) {
info = new String(dataArray);
}
Log.i("File Reading" , "Success!");
isOk = true;
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e1) {
e1.printStackTrace();
} finally {
fis.close();
}

a safe version of what you do is e.g.:
private String readFromInternalStorage(String filename) {
FileInputStream fis = null;
File file = new File(startGame.getFilesDir(), filename);
long size = file.length();
// impossible to have more than that (= 2GB)
if (size > Integer.MAX_VALUE) {
Log.d("XXX", "File too big");
return null;
}
int iSize = (int) size;
try {
fis = new FileInputStream(file);
// part of Android since API level 1 - buffer can scale
ByteArrayBuffer bb = new ByteArrayBuffer(iSize);
// some rather small fixed buffer for actual reading
byte[] buffer = new byte[1024];
int read;
while ((read = fis.read(buffer)) != -1) {
// just append data as long as we can read more
bb.append(buffer, 0, read);
}
// return a new string based on the large buffer
return new String(bb.buffer(), 0, bb.length());
} catch (FileNotFoundException e) {
Log.w("XXX", e);
} catch (IOException e) {
Log.w("XXX", e);
} catch (OutOfMemoryError e) {
// this could be left out. Keep if you read several MB large files.
Log.w("XXX", e);
} finally {
// finally is executed even if you return in above code
// fis will be null if new FileInputStream(file) throws
if (fis != null) {
try {
fis.close();
} catch (IOException e) {
// ignored, nothing can be done if closing fails
}
}
}
return null;
}