Develop Reference

Save a variable when the server is off

In fact I am making a Minecraft plugin and I was wondering how some plugins (without using DB) manage to keep information even when the server is off.
For example if we make a grade plugin and we create a different list or we stack the players who constitute each. When the server will shut down and restart afterwards, the lists will become empty again (as I initialized them).
So I wanted to know if anyone had any idea how to keep this information.

If a plugin want to save informations only for itself, and it don't need to make it accessible from another way (a PHP website for example), you can use YAML format.
Create the config file :
File usersFile = new File(plugin.getDataFolder(), "user-data.yml");
if(!usersFile.exists()) { // don't exist
usersFile.createNewFile();
// OR you can copy file, but the plugin should contains a default file
/*try (InputStream in = plugin.getResource("user-data.yml");
OutputStream out = new FileOutputStream(usersFile)) {
ByteStreams.copy(in, out);
} catch (Exception e) {
e.printStackTrace();
}*/
}
Load the file as Yaml content :
YamlConfiguration config = YamlConfiguration.loadConfiguration(usersFile);
Edit content :
config.set(playerUUID, myVar);
Save content :
config.save(usersFile);
Also, I suggest you to make I/O async (read & write) with scheduler.
Bonus:
If you want to make ONE config file per user, and with default config, do like that :
File oneUsersFile = new File(plugin.getDataFolder(), playerUUID + ".yml");
if(!oneUsersFile.exists()) { // don't exist
try (InputStream in = plugin.getResource("my-def-file.yml");
OutputStream out = new FileOutputStream(oneUsersFile)) {
ByteStreams.copy(in, out); // copy default to current
} catch (Exception e) {
e.printStackTrace();
}
}
YamlConfiguration userConfig = YamlConfiguration.loadConfiguration(oneUsersFile);
PS: the variable plugin is the instance of your plugin, i.e. the class which extends "JavaPlugin".

You can use PersistentDataContainers:
To read data from a player, use
PersistentDataContainer p = player.getPersistentDataContainer();
int blocksBroken = p.get(new NamespacedKey(plugin, "blocks_broken"), PersistentDataType.INTEGER); // You can also use DOUBLE, STRING, etc.
The Namespaced key refers to the name or pointer to the data being stored. The PersistentDataType refers to the type of data that is being stored, which can be any Java primitive type or String. To write data to a player, use
p.set(new NamespacedKey(plugin, "blocks_broken"), PersistentDataType.INTEGER, blocksBroken + 1);

Save JList into Txt File

After searching for an answer for hours I decided to ask it here, since the solutions I found didn't work.
I have a simple GUI to register a persons first/last name and date of birth. After entering the values, the data is listed in a JList. Now I want to save the data from the JList into a Txt file. But I can't find a way to get the data from the JList.
public void save(){
try(BufferedWriter bw = new BufferedWriter(new FileWriter("jlist.txt")))
{
/* Here should be the part, where I get the data from the JList */
bw.write(person.getNachname() + " ; " + person.getVorname() + " ; " + person.getDate() + "\n");
} catch (Exception speichern) {
speichern.printStackTrace();
}
}
Later I want to take the created Txt file and load it back into the same JList.
Maybe there is even a better way to do this but I haven't found something.
Some tips would be helpful :)

There is no JList method that does this for you.
You need to get the data from the ListModel.
You get the ListModel from the JList using the getModel() method.
You need to write a loop to:
get each element from the ListModel using the getElementAt(...) method.
convert the element to a String and write the data to your file.
Some tips would be helpful
Not related to your question, but typically data like this would be displayed in a JTable. Then you have a separate column for each of the first name, last name and date. Read the section from the Swing tutorial on How to Use Tables for more information.

As camickr point out there is no method implemented for what you a trying to achieve, instead there is a combination of things that you could do for archiving your goal.
You are facing the problem of data persistence. In now-a-days for small|medium|big size industrial applications the recommended approach is to relay on databases. I guess that is out the scope for one person that is starting to code, so using files for storing info is OK but is not straightforward.
In your case, if your application is for non-commercial purposes I would suggest to use the default mechanism for serializing and deserializing objects that comes bundled with the platform. With this you could write an entire object (including its data, a.k.a. its state) to a file on a disk, and later retrieve it with few lines codes. There are details about how the object gets serialize ("translate object to bits") and deserialized ("translate bits to object") that doesn't comes into place right now, but is well to advice to study them in the future if you planning to use this method in a commercial application.
So I suggest that you load and store the information of your application on start-up and shutdown respectively, thus only one load and store per application instance, while the application is active work with the data on memory. THIS is the simplest approach you could have in any application, and for that reason I suggest to start with this ideal scenario.
So, I say a lot of things but let's goes to the code that shows an example of storing (serialize) and loading (deserialize)
import java.io.*;
import java.util.*;
class Person implements Serializable {
String name;
int birthDate;
public Person(String name, int birthDate) {
this.name = name;
this.birthDate = birthDate;
}
}
class Main {
public static void main(String[] args) {
Collection<Person> collection = createExampleCollection();
System.out.println(collection);
storeCollection(collection, "persons.data");
Collection<Person> otherCollection = loadCollection("persons.data");
System.out.println(otherCollection);
}
private static Collection<Person> createExampleCollection() {
Collection<Person> collection = new ArrayList<Person>();
collection.add(new Person("p1",0));
collection.add(new Person("p2",10));
collection.add(new Person("p2",20));
return collection;
}
// here I'm doing two separated things that could gone in separate functions, 1) I'm converting into bytes and object of an specific class, 2) saving those bytes into a file on the disk. The thing is that the platform offers us convenient objects to do this work easily
private static void storeCollection(Collection<Person> collection, String filename) {
try {
FileOutputStream fos = new FileOutputStream(filename);
ObjectOutputStream out = new ObjectOutputStream(fos);
out.writeObject(collection);
out.close();
fos.close();
} catch (IOException i) {
i.printStackTrace();
}
}
// again there two things going on inside, 1) loading bytes from disk 2) converting those bits into a object of a specific class.
private static Collection<Person> loadCollection(String filename) {
try {
FileInputStream fis = new FileInputStream(filename);
ObjectInputStream in = new ObjectInputStream(fis);
Collection<Person> persons = (Collection<Person>) in.readObject();
in.close();
fis.close();
return persons;
} catch (Exception i) {
i.printStackTrace();
return null;
}
}
}
You should try to use the functions of loadCollection and storeCollection on start-up and shutdown respectively.

I made this code with comments for jButton and jList in jFrame, Button saves text Items to File from jList.
private void btnSaveActionPerformed(java.awt.event.ActionEvent evt) { //jButton name: "btnSave"
try { //trying to save file
BufferedWriter bw = new BufferedWriter(new FileWriter("data.txt")); //file where I store the data of jList1 (file will be stored at: C:\Users\%username%\Documents\NetBeansProjects\<ThisProjectName>\data.txt) (if You use NetBeans)
for (int i=0; i<jList1.getModel().getSize(); i++){ //opens a cycle to automatically store data of all items
bw.write(jList1.getModel().getElementAt(i)); //writing a line from jList1
bw.newLine(); //making a new line for the next item (by removing this line, You will write only one line of all items in file)
} //cycle closes
bw.close(); //file writing closes
} catch (IOException ex) { //catching the error when file is not saved
Logger.getLogger(NewJFrame.class.getName()).log(Level.SEVERE, null, ex); //showing the error
} //Exception closes
} //Action closes

Android How can i easily read and add to the List data from txt file?

My code: https://pastebin.com/KudWYD8W
I don't know how write method loadData.
I wanna have list with only names of Games and when I press on the element should show the details.
My file looks like:
Witcher
1996
free
66h
Please help. I tried in many ways but there was always something wrong.

Try this.
1.Save the string to be read as different list item separated with a comma(,).
2.Call this method to get the list by passing the file path.
private List<String> populateAutoComplete(String path)
{
try
{
String content = readFile(path);
Log.d(TAG,
"populateAutoComplete: " + content);
List<String> listToPopulate = Arrays.asList(content.split("\\s*,\\s*"));
return listToPopulate;
}
catch (IOException e)
{
e.printStackTrace();
return Collections.emptyList();
}
}
Ask in case of any doubts. Cheers :)

Java binary search array list

I'm trying to search for strings in an array list as below which is working fine. But, the challenge is when the string i'm searching for is embedded in between some text example "XXXXTESTXXXX".
The binary search what i have so far seems to not find it. I have also tried "contains" method but didn't work either. Not sure where i'm i going wrong. Please suggest.
Array Content :[PIGEON, XXXBEARXXX , XXXCAT, XXXDOG, XXXELEPHANTXXX , XXXHORSEXXX , XXXLIONXXX , XXXMOUSEXXX , XXXOWLXXX , XXXPARROTXXX , XXXTIGERXXX ]
Example search string:-
Search String = "BEAR"
Code
ArrayList File_F1_Array = new ArrayList();
// Read the lines of the Source file (File_1) in to Arraylist
try {
BufferedReader File_F1_Br = new BufferedReader (new FileReader(File_F1));
while ((File_F1_Line = File_F1_Br.readLine()) !=null ) {
File_F1_Array.add(File_F1_Line);
}
File_F1_Br.close();
} catch (Exception e) {
e.printStackTrace();
}
// Sort the array list
Collections.sort(File_F1_Array);
// Search lines from Refernce file (File_2) in Arraylist
try {
BufferedReader File_F2_br = new BufferedReader (new FileReader(File_F2));
while ((File_F2_Line = File_F2_br.readLine()) !=null) {
int index = Collections.binarySearch(File_F1_Array, File_F2_Line);
boolean StringCheck = File_F1_Array.contains(File_F2_Line);
}

your algorithm is not correct! you are searching in array which has "xx..." and you want to get rid of "xx..."s! you have to create another arraylist without "xx.." and put some binding key between 2 arrays in order to keep the first array with Xs.

Add comment to an ARFF file

this is my first question in this forum....
I'm making adata-mining application in java with the WEKA API.
I make first a pre-processing stage and when I save the ARFF file i would like to add a couple of lines (as comments) specifing the preprocessing task that i have done to the file...
the problem is that i don't know how to add comments to an ARFF file from the java WEKA API.
To save the file i use the class ArffSaver like this...
try {
ArffSaver saver = new ArffSaver();
saver.setInstances(dataPost);
saver.setFile(arffFile);
saver.writeBatch();
return true;
} catch (IOException ex) {
Logger.getLogger(Preprocesamiento.class.getName()).log(Level.SEVERE, null, ex);
return false;
}
I would be really greatfull if someone could give some idea...
thanks!

You should AVOID writting comments on an .arff file, even more when writting it from Java. These files are very "parser-sensitive". The Weka API to create these files is restrictive for this particular reason.
Even though, you can always add your comments manually with the % symbol. This said, I wouldn't recommend you writting anything more than instances, attributes and values into an .arff file. ;-)

I don't see a reason to not write comments into the header of an ARFF file. The specification clearly says:
Lines that begin with a % are comments.
So while it is technically valid, it can be difficult if you want to use the ArffSaver#setFile method. This method does a lot of (convenient, but somewhat arbitrary and unspecified) work internally, until it finally calls
setDestination(new FileOutputStream(m_outputFile));
If this is not required, the easiest option is to write directly to an OutputStream, which then can simply be set as the destination for the ArffSaver. This can be wrapped in a small helper method, for example, like this:
static void writeArff(
Instances instances,
List<String> commentLines,
OutputStream outputStream) throws IOException
{
ArffSaver saver = new ArffSaver();
saver.setInstances(instances);
if (commentLines != null && !commentLines.isEmpty())
{
BufferedWriter bw = new BufferedWriter(
new OutputStreamWriter(outputStream));
for (String commentLine : commentLines)
{
bw.write("% " + commentLine + "\n");
}
bw.write("\n");
bw.flush();
}
saver.setDestination(outputStream);
saver.writeBatch();
}
When calling it like this
List<String> comments = Arrays.asList("A comment", "Another one");
writeArff(instances, comments, outputStream);
then the given comments will be inserted at the top of the ARFF file.