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How to apply for loop on first n vlalues of arraylist

I have an arraylist in which I have ESSID, BSSID, Strenght of access Point on first three indexes, and from Index 4 to 6 I have again ESSID, BSSID, Strength of another AccessPoint. I want to store this list in database like first three values save in one row of table. and next three values save in 2nd row of table.
String[] namesArr = new String[arrayList2.size()]; //conver arraylist to array
for (int j = 0; j < arrayList2.size(); j++){
namesArr[j] = arrayList2.get(j);
int length = namesArr[j].length();
for (int k = 0; k < length; k += 3) {
ssid = namesArr[k];
bssid = namesArr[k + 1];
rssid = namesArr[k + 2];
}
insertValues(this);
}
public void insertValues(View.OnClickListener view){
SendData send = new SendData(this);
send.execute(bssid,ssid,rssid);}
I have made a class to store this data in database that works fine.
public class SendData extends AsyncTask<String, Void, String> {
AlertDialog dialog;
Context context;
public SendData(Context context) {
this.context = context;
}
#Override
protected void onPreExecute() {
dialog = new AlertDialog.Builder(context).create();
dialog.setTitle("Message");
}
#Override
protected void onPostExecute(String s) {
dialog.setMessage(s);
dialog.show();
}
#Override
protected String doInBackground(String... voids) {
String data = "";
String result = "";
String MAC = voids[0];
String Name = voids[1];
String Strength = voids[2];
String con_Str = "http://10.5.48.129/Webapi/accesspoints_data/create.php";
try{
URL url = new URL(con_Str);
HttpURLConnection http = (HttpURLConnection) url.openConnection();
http.setRequestMethod("POST");
http.setDoInput(true);
http.setDoOutput(true);
OutputStream out_Stream = http.getOutputStream();
BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(out_Stream, "UTF-8"));
JSONObject obj = new JSONObject();
try {
obj.put("BSSID", MAC);
obj.put("ESSID", Name);
obj.put("RSSID", Strength);
} catch (JSONException e) {
e.printStackTrace();
}
data = obj.toString();
writer.write(data);
writer.flush();
writer.close();
out_Stream.close();
InputStream in_Stream = http.getInputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(in_Stream, "ISO-8859-1"));
String line = "";
while ((line = reader.readLine()) != null)
{
result += line;
}
reader.close();
in_Stream.close();
http.disconnect();
return result;
} catch (MalformedURLException e){
result = e.getMessage();
} catch (IOException e){
result = e.getMessage();
}
return result;
}
}
SendData class is perfectly working but problem is with for loop.

I think this is result that you are expecting :
List<String> arrayList2 = new ArrayList<>();
arrayList2.add("1");
arrayList2.add("2");
arrayList2.add("3");
arrayList2.add("4");
arrayList2.add("5");
arrayList2.add("6");
arrayList2.add("6");
arrayList2.add("7");
arrayList2.add("8");
arrayList2.add("9");
arrayList2.add("10");
List<String[]> sarrayList = new ArrayList<>();
String[] arr = new String[3];
int i = 0;
for (int j = 0; j < arrayList2.size(); j++)
{
arr[i] = arrayList2.get(j);
i++;
if((j+1)%3==0)
{
sarrayList.add(arr);
i = 0;
arr = new String[3];
}
}
for(String [] sa:sarrayList)
{
for(String s:sa)
{
System.out.println(s);
}
System.out.println("=========");
}
This might not be the most efficient way of doing it. But it splits the ArrayList in to String arrays of length=3 and stores them in a new ArrayList named sarrayList

I would advise to use a datastructure to hold the record. See the code below this is a small example how you could do it
ArrayList<Record> records;
for (int i = 2; i < inputArrayList.size(); i = i + 3){
string ssid = namesArr.get(i - 2);
string bssid = namesArr.get(i - 1);
string rssid = namesArr.get(i);
records.add(new Record(ssid, bssid, rssid));
}
class Record{
string ssid;
string bssid;
string rssid;
// Constructor...
// Getter and setter to be implemented...
}

ok from what i understand you want to divide the arraylist each 3 elements thats how you do it with streams and it will return an a collection of arraylists each one has 3 elements
final int chunkSize = 3;
final AtomicInteger counter = new AtomicInteger();
//arrayList here us your array list
final Collection<List<String>> result = arrayList.stream()
.collect(Collectors.groupingBy(it -> counter.getAndIncrement() / chunkSize))
.values();
and mentioning supermar10 answer you code make a class to map the strings to it like that
class Record{
string ssid;
string bssid;
string rssid;
Record(String ssid,String bssid,String rssid){
this.ssid=ssid;
this.bssid=bssid;
this.rssid=rssid;
}
}
now you have a class to map to now save the records in a list of Record
create a a list in the main class
static List<Record> lists=new ArrayList<>();
then map the data like that
result.stream().forEach(nowList -> saveRecord(nowList));
and thats the save method
static void saveRecord(List<String> list){
lists.add(new Record(list.get(0),list.get(1),list.get(2)));
}

I have simplified it to one loop and also modified insertValues so that it takes 3 more parameters. This
int size = arrayList2.size();
for (int j = 0; j < size; j += 3) {
if (size - j < 3 ) {
break;
}
String ssid = arrayList2.get(j);
String bssid = arrayList2.get(j + 1);
String rssid = arrayList2.get(j + 2);
insertValues(this, ssid, bssid, rssid);
}
if one the other hand ssid and so on are class variables the inside of the loop can be changed to
ssid = arrayList2.get(j);
bssid = arrayList2.get(j + 1);
rssid = arrayList2.get(j + 2);
insertValues();

Storing numbers with nested Lists java

I have a .txt file with the next format:
I need to storage that values in this way, at index 0 i need to have
[[154.5,0.0],[13.3333333333333, 102.0],[6,272.0],[10.3333333333333,1860.0]...
(there will be more pairs cause the second column in txt is not ordered, and there will be zeros again later)]. I hope that u understand my question, i think i need arraylist of arraylist of arraylist, i just need an idea, thanks.
This is the code:
String working_directory = System.getProperty("user.dir");
File file = new File(working_directory + "\\src\\" + filename);
BufferedReader br = new BufferedReader(new FileReader(file));
ArrayList<ArrayList<Double>> array_A = new ArrayList<ArrayList<Double>>();
ArrayList<Double> aux = new ArrayList<Double>();
aux.add((double)0);
aux.add((double)0);
for ( int i = 0 ; i < matrixSize; i ++ ) {
array_A.add(aux);
}
String[] elements_in_line = new String[3];
String line;
while ((line = br.readLine()) != null) {
ArrayList<Double> value_col_index = new ArrayList<Double>();
ArrayList<Double> pair = new ArrayList<Double>();
elements_in_line = line.split(", ");
double value = Double.parseDouble(elements_in_line[0]);
int line_index = Integer.parseInt(elements_in_line[1]);
int column_index = Integer.parseInt(elements_in_line[2]);
boolean already_exists = false;
if (array_A.get(line_index).size() > 0) {
pair = array_A.get(line_index);
for (int i = 0; i < pair.size(); i++) {
if (column_index == pair.get(1)) {
already_exists = true;
break;
}
}
}
if (already_exists) {
pair.set(0, pair.get(0) + value);
} else {
if (limit_line != 0) {
if (array_A.get(line_index).size() == limit_line) {
System.out.println("Dimension of matrix exceeded!");
System.exit(1);
}
}
array_A.set(line_index,new ArrayList<Double>(Arrays.asList(value,(double)column_index)));
}
}
return array_A;
}

How do I create an Array or ArrayList with a looped, column of strings that contain integers?

I have tried so hard to find a solution to this problem! Here is my code:
import java.io.*;
import java.lang.reflect.Array;
import java.util.ArrayList;
public class Weather {
public static void main(String[] args) throws IOException {
//Getting the file.
String fileName = "weather2013.txt";
//Lines!
String line;
//Creating arrayList object
ArrayList aList = new ArrayList();
try {
BufferedReader input = new BufferedReader(new FileReader(fileName));
while ((line = input.readLine()) != null) {
aList.add(line);
}
//Close the file
input.close();
} catch (FileNotFoundException ex) {
System.out.println("File not found!");
}
//Station ID Number:
String firstLine = aList.get(1).toString();
String stationIDStr = firstLine.substring(0, 6);
int StationID = Integer.parseInt(stationIDStr);
//System.out.println(StationID);
//WBAN ID Number:
String wbanIDstr = firstLine.substring(7, 12);
int wbanID = Integer.parseInt(wbanIDstr);
//System.out.println(wbanID);
//Year!
String yearStr = firstLine.substring(12, 18).trim();
int year = Integer.parseInt(yearStr);
//System.out.println(year);
//Remove line of text (not needed)
aList.remove(0);
//Fog days:
int fogDays = 0;
for (int i = 0; i < aList.size(); i++) {
String listString = aList.get(i).toString(); //iterate each readLINE -> String
String lastDigits = listString.substring(132, 137); //Each entry from 132-137 only
char fogIndicator = lastDigits.charAt(0);
if (fogIndicator == '1') {
fogDays++;
}
}
//System.out.println(fogDays);
//Maximum and minimum average temps
for (int i = 0; i < aList.size(); i++) {
String listString = aList.get(i).toString();
String averageTempDigits = listString.substring(24, 30).trim();
}
}
}
The specific part of the code where I am having trouble is the VERY last for loop.
Here's what's being outputted:
47.9
41.8
.
.
.
.
41.8
67.0
66.5
I would like to know how to get this column above into an Array or ArrayList?

Array throws NullPointerException in Java

I have the following little problem... I have this code that uses the method OpenFile() of one class ReadData to read a .txt file and also I have another class ArraysTZones used to create an object that stores 3 arrays (data1,data2,data3) and 3 integers (total1,total2,total3) returned by the method OpenFile(). The problem is that when I try to display each array (data1,data2,data3) using the method getArray() of ArrayTZones it stops and displays the error NullPointerException. Anyone knows how could I fix this?
public static void main (String args[]) throws IOException {
String fileName = ".//data.txt";
int[] def = new int[180];
try {
ReadData file = new ReadData(fileName);
ArraysTZones summaryatz = new ArraysTZones();
summaryatz = file.OpenFile();
for (int i = 0; i < 180; i++)
System.out.print (summaryatz.getArray1()[i] + " ");
System.out.println ("");
System.out.println (summaryatz.getTotal1());
for (int i = 0; i < 180; i++)
System.out.print (summaryatz.getArray2()[i] + " ");
System.out.println ("");
System.out.println (summaryatz.getTotal2());
for (int i = 0; i < 180; i++)
System.out.print (summaryatz.getArray3()[i] + " ");
System.out.println ("");
System.out.println (summaryatz.getTotal3());
}
catch (IOException e) {
System.out.println(e.getMessage());
}
}
Heres OpenFile()
public ArraysTZones OpenFile() throws IOException {
FileReader reader = new FileReader(path);
BufferedReader textReader = new BufferedReader(reader);
int numberOfTimeZones = 3;
int[] data1 = new int[180];
int[] data2 = new int[180];
int[] data3 = new int[180];
int total1 = 0;
int total2 = 0;
int total3 = 0;
ArraysTZones atz = new ArraysTZones();
for (int i = 0; i < numberOfTimeZones; i++){
if (i == 0) {
String firstTimeZone = textReader.readLine();
String[] val = firstTimeZone.split ("\\s+");
for (int u = 0; u < val.length; u++)
{
int stats = (int)(Math.ceil(Math.abs(Double.parseDouble(val[u]))));
total1 += stats;
data1[u] = stats;
}
total1= total1/180;
atz.setTotal1(total1);
atz.setArray1(data1);
}
else
if (i == 1) {
String secondTimeZone = textReader.readLine();
String[] val = secondTimeZone.split ("\\s+");
for (int u = 0; u < val.length; u++)
{
int stats = (int)(Math.ceil(Math.abs(Double.parseDouble(val[u]))));
total2 += stats;
data2[u] = stats;
}
total2= total2/180;
atz.setTotal2(total2);
atz.setArray2(data2);
}
else {
String thirdTimeZone = textReader.readLine();
String[] val = thirdTimeZone.split ("\\s+");
for (int u = 0; u < val.length; u++)
{
int stats = (int)(Math.ceil(Math.abs(Double.parseDouble(val[u]))));
total3 += stats;
data3[u] = stats;
}
total3= total3/180;
atz.setTotal3(total3);
atz.setArray3(data3);
}
}
textReader.close();
return atz;
}
The getArray()
public int[] getArray1 () {
return data1;
}
And setArray()
public void setArray1 (int[] farray) {
int[] data1 = new int[180];
//int[] farray = new int[180];
data1 = farray;
}

The problem seems to be here
public void setArray1 (int[] farray)
{
int[] data1 = new int[180];
//int[] farray = new int[180];
data1 = farray;
}
You're declaring a new variable called data1 and storing the content of farray to it.
After that method is done, that variable will be removed, due to his scope.
Remove int[] from the line int[] data1 = new int[180]; (or just remove the whole line .. it is unnecessary) and your data will be stored in the correct variable that was declared for the class.
public void setArray1 (int[] farray) {
data1 = farray;
}

You have to initialize ArraysTZones

Java Serialization issues

I have this code that writes an object:
FileOutputStream fileOut = new FileOutputStream("Model.ser");
ObjectOutputStream out = new ObjectOutputStream(fileOut);
out.writeObject(this);
out.close();
fileOut.close();
And this code that loads the object:
Model m = null;
try {
FileInputStream fileIn = new FileInputStream("Model.ser");
ObjectInputStream in = new ObjectInputStream(fileIn);
m = (Model) in.readObject();
in.close();
fileIn.close();
} catch (Exception e) {}
setStudentList(m.getStudentList());
setModuleList(m.getModuleList());
I'm pretty sure that saving works, as when I opened the file in notepad++ I saw most of the data that I had saved, but when I load there is no data in module list.
Full source code:
import java.io.*;
import java.util.*;
public class Model implements java.io.Serializable {
private Student[] studentList = new Student[0];
private Module[] moduleList = new Module[0];
public void menu() {
while (true) {
System.out.println ("MENU");
System.out.println ("");
System.out.println (" 1 - Run Tests");
System.out.println (" 2 - Add Student");
System.out.println (" 3 - Add Module");
System.out.println (" 4 - Add Student To Module");
System.out.println (" 5 - Save System (Text file)");
System.out.println (" 6 - Load System (Text file)");
System.out.println (" 7 - Save System (Serialized)");
System.out.println (" 8 - Load System (Serialized)");
System.out.println (" 9 - Print Report");
System.out.println ("");
System.out.print ("Enter choice: ");
String input = keyboard.readString();
switch (input) {
case "1" :
runTests();
break;
case "2" :
System.out.print("First Name : ");
String fN = keyboard.readString();
System.out.print("Surname : ");
String sN = keyboard.readString();
System.out.print("Course Code : ");
String c = keyboard.readString();
System.out.print("User ID : ");
String iD = keyboard.readString();
AddStudent(iD, sN, fN, c);
break;
case "3" :
System.out.print("Module Code : ");
String code = keyboard.readString();
String[] temp = new String[0];
AddModule(code,temp);
break;
case "4" :
System.out.print("Module Code : ");
code = keyboard.readString();
Module m = findAModule(code);
if (m != null) {
System.out.print("User ID : ");
iD = keyboard.readString();
Student s = findAStudent(iD);
if (s != null) {
m.addThisStudent(s);
} else {
System.out.println("Student not found");
}
} else {
System.out.println("Module not found");
}
break;
case "5" :
saveToTextFiles();
break;
case "6" :
loadFromTextFiles();
break;
case "7" :
saveSerialized();
break;
case "8" :
break;
case "9" :
printReport();
break;
}
}
}
public void runTests() {
loadFromTextFiles();
saveSerialized();
printReport();
}
public void loadFromTextFiles() {
studentList = new Student[0];
moduleList = new Module[0];
try {
Scanner fileReader = new Scanner(new InputStreamReader(new FileInputStream("students.txt")));
int num = fileReader.nextInt(); fileReader.nextLine();
for (int i = 0; i < num; i++) {
String u = fileReader.nextLine();
String sn = fileReader.nextLine();
String fn = fileReader.nextLine();
String c = fileReader.nextLine();
AddStudent(u, sn, fn, c);
}
fileReader.close();
fileReader = new Scanner(new InputStreamReader(new FileInputStream("modules.txt")));
num = fileReader.nextInt(); fileReader.nextLine();
for (int i = 0; i < num; i++) {
String code = fileReader.nextLine();
int numOfStudents = fileReader.nextInt(); fileReader.nextLine();
String[] students = new String[numOfStudents];
for (int j = 0; j < numOfStudents; j++) {
students[j] = fileReader.nextLine();
}
AddModule(code, students);
}
fileReader.close();
} catch (IOException e) {}
}
public void saveToTextFiles () {
try {
PrintWriter outfile = new PrintWriter(new OutputStreamWriter (new FileOutputStream("students.txt")));
outfile.println(studentList.length);
for (int i = 0; i < studentList.length; i++) {
outfile.println(studentList[i].getUID());
outfile.println(studentList[i].getSN());
outfile.println(studentList[i].getFN());
outfile.println(studentList[i].getDegree());
}
outfile.close();
outfile = new PrintWriter(new OutputStreamWriter (new FileOutputStream("modules.txt")));
outfile.println(moduleList.length);
for (int i = 0; i < moduleList.length; i++) {
outfile.println(moduleList[i].getCode());
outfile.println(moduleList[i].getStudents().length);
for (int j = 0; j < moduleList[i].getStudents().length; j++) {
outfile.println(moduleList[i].getStudents()[j]);
}
}
outfile.close();
} catch (IOException e) {}
}
public void saveSerialized() {
try {
FileOutputStream fileOut = new FileOutputStream("Model.ser");
ObjectOutputStream out = new ObjectOutputStream(fileOut);
out.writeObject(this);
out.close();
fileOut.close();
FileOutputStream fileOut2 = new FileOutputStream("Module.ser");
ObjectOutputStream out2 = new ObjectOutputStream(fileOut2);
out2.writeObject(studentList);
out2.close();
fileOut2.close();
FileOutputStream fileOut3 = new FileOutputStream("Student.ser");
ObjectOutputStream out3 = new ObjectOutputStream(fileOut3);
out3.writeObject(moduleList);
out3.close();
fileOut3.close();
} catch (IOException e) {}
}
public void loadSerialized() {
Model m = null;
try {
FileInputStream fileIn = new FileInputStream("Model.ser");
ObjectInputStream in = new ObjectInputStream(fileIn);
m = (Model) in.readObject();
in.close();
fileIn.close();
} catch (Exception e) {}
setStudentList(m.getStudentList());
setModuleList(m.getModuleList());
}
private Module[] getModuleList() {
return moduleList;
}
private Student[] getStudentList() {
return studentList;
}
private void setModuleList(Module[] m) {
moduleList = m.clone();
}
private void setStudentList(Student[] s) {
studentList = s.clone();
}
private void AddModule(String code, String[] students) {
int length = moduleList.length;
Module NewArray[] = new Module[length + 1];
for (int i = 0; i < length + 1; i++) {
if (i < length) {
NewArray[i] = new Module(moduleList[i]);
}
}
NewArray[length] = new Module(code, students);
moduleList = NewArray.clone();
}
private void AddStudent(String u, String sn, String fn, String c) {
int length = studentList.length;
Student NewArray[] = new Student[length + 1];
for(int i = 0; i < length + 1; i++) {
if (i < length) {
NewArray[i] = new Student(studentList[i]);
}
}
NewArray[length] = new Student(u, sn, fn, c);
studentList = NewArray.clone();
}
public void printReport() {
for (int i = 0; i < moduleList.length; i++) {
System.out.println(moduleList[i].toString(this));
}
}
public Student findAStudent(String uid) {
for (int i = 0; i < studentList.length; i++) {
if (studentList[i].getUID().compareTo(uid) == 0) {
return studentList[i];
}
}
return null;
}
public Module findAModule(String code) {
for (int i = 0; i < moduleList.length; i++) {
if (moduleList[i].getCode().compareTo(code) == 0) {
return moduleList[i];
}
}
return null;
}
}

Look at your code sample, in particular the switch statement:
switch (input) {
...
case "8" :
break;
...
}
I'd assume the method loadSerialized should be called there but it's missing and is not called anywhere else in the code.
Once you actually call the method that does the loading, the code will work, assuming you have declared a serialVersionUID for both Student and Module.
Edit: why using serialization to persist objects is a bad idea
In simple terms, using serialization to persist objects is brittle.
An object's serialized form is tied to its class. Classes tend to change over time, meaning the serialized instance of the old cannot be loaded into the new.
While you can work round this by setting a serialVersionUID doing so introduces a reverse of the problem where, in the case new fields are introduced, the new cannot be read into objects of the old class, which can be a problem if you do rolling updates to the deployed system.
There's a host of other reasons - it's not easily readable (meaning you can't update it like you would a database or XML/JSON doc), it's inefficient, etc.