I want to save the contents of my arraylist to a textfile. What I have so far is shown below, however instead of adding x.format("%s%s", "100", "control1"); to the textfile, I want to add objects from an arraylist, how do I go about this?
import java.util.*;
public class createfile
{
ArrayList<String> control = new ArrayList<String>();
private Formatter x;
public void openFile()
{
try {
x = new Formatter("ControlLog.txt");
} catch (Exception e) {
JOptionPane.showMessageDialog(null, "Error: Your file has not been created");
}
}
public void addRecords()
{
x.format("%s%s", "100", "control1");
}
public void closeFile()
{
x.close();
}
}
public class complete
{
public static void main(String[] args)
{
createfile g = new createfile();
g.openFile();
g.addRecords();
g.closeFile();
}
}
Both ArrayList and String implement Serializable. Since you have an ArrayList of string you can write it to the file like this:
FileOutputStream fos = new FileOutputStream("path/to/file");
ObjectOutputStream out = new ObjectOutputStream(fos);
out.writeObject(myArrayList); //Where my array list is the one you created
out.close();
Here is a really good tutorial that shows you how to write java objects to a file.
The written objects can be read back from the file in a similar way.
FileInputStream in = new FileInputStream("path/to/file");
ObjectInputStream is = new ObjectInputStream(in);
myArrayList = (ArrayList<String>) is.readObject(); //Note that you will get an unchecked warning here
is.close()
Here is a tutorial on how to read objects back from a file.
Related
I use ObjectInput/Output to initialize the hashmap named temp and it put all entry of the hashmap called map that is initialized to new and then use OutputStream to save it in file formatting is .ser
this work perfectly...
import java.io.*;
import java.util.HashMap;
public class PlayerInfo implements Serializable {
ObjectOutputStream out;
ObjectInputStream in;
File userData =new File("path.ser");
HashMap map ;
HashMap temp;
private Integer ID;
String name ;
boolean isItNull =false;
public static void main(String[] args) {
new PlayerInfo();
}
PlayerInfo(){
try {
initializeHashMap();
}catch (Exception e){
e.printStackTrace();
}
}
#SuppressWarnings("unchecked")
private void initializeHashMap(){
try {
//initialize ObjectInputStream in same method when I use it and close it then
in =new ObjectInputStream(new FileInputStream(userData));
if (isItNull){
temp =new HashMap<Integer,PlayerInfo>();
}else {
map =new HashMap<Integer,PlayerInfo>();
temp = (HashMap<Integer, PlayerInfo>) in.readObject();
in.close();
}
}catch (Exception e){
isItNull =true;
initializeHashMap();
}
}
private void getInfo(){
System.out.println("Ok we are in get info so write your ID:-");
int id = 10;
}
#SuppressWarnings("unchecked")
private void createInfo()throws IOException{
//same here initialize ObjectOutputStreamin same method when I use it and close it then
out =new ObjectOutputStream(new FileOutputStream(userData));
System.out.println("Ok we are in create info so write your ID:-");
ID =10;
String scnS ="Mohammed";
System.out.println("Write your name");
map.put(ID,new PlayerInfo(scnS));
temp.putAll(map);
System.out.println("Saving....");
out.writeObject(temp);
out.close();
}
public PlayerInfo(String name){
this.name =name;
}
}
but this throw EFOException
import java.io.*;
import java.util.HashMap;
public class PlayerInfo implements Serializable {
ObjectOutputStream out;
ObjectInputStream in;
File userData =new File("path.ser");
HashMap map ;
HashMap temp;
private Integer ID;
String name ;
boolean isItNull =false;
public static void main(String[] args) {
new PlayerInfo();
}
PlayerInfo(){
try {
openTheOutPutObjectStreamer();
openTheInPutObjectStreamer();
initializeHashMap();
}catch (Exception e){
e.printStackTrace();
}
}
//here I initialize it in separated method
private void openTheOutPutObjectStreamer()throws IOException{
out =new ObjectOutputStream(new FileOutputStream(userData));
}
//same here I initialize it in separated method
private void openTheInPutObjectStreamer()throws IOException{
in =new ObjectInputStream(new FileInputStream(userData));
}
#SuppressWarnings("unchecked")
private void initializeHashMap(){
try {
if (isItNull){
temp =new HashMap<Integer,PlayerInfo>();
}else {
map =new HashMap<Integer,PlayerInfo>();
temp = (HashMap<Integer, PlayerInfo>) in.readObject();
in.close();
}
}catch (Exception e){
isItNull =true;
initializeHashMap();
}
}
#SuppressWarnings("unchecked")
private void createInfo()throws IOException{
System.out.println("Ok we are in create info so write your ID:-");
ID =10;
String scnS ="Mohammed";
System.out.println("Write your name");
map.put(ID,new PlayerInfo(scnS));
temp.putAll(map);
System.out.println("Saving....");
out.writeObject(temp);
out.close();
}
public PlayerInfo(String name){
this.name =name;
}
}
if you see it the difference is only separate the Object Input/Output to a method and call them
and I am sorry I am a newbie in this website
I don't know a lot about IO but it seems like I cant separate it to methods and call it?
The problem is that in your first code you (correctly) open an input stream uses it and then closes it before doing anything else to the same file but in your second code version you also open the output stream on the same file before having read it and that output stream puts the marker (where to read or write) at the end of the file so when you use your input stream you get an End of file error.
Changing you code to this should work
openTheInPutObjectStreamer();
initializeHashMap();
openTheOutPutObjectStreamer();
I need to write a class that has two static methods: writeFile and readFile. However, after I do my readFile(), it returns nothing.
class writereadFile {
public static void writeFile(ArrayList<Object> list, File file){
try {
try (FileOutputStream fos = new FileOutputStream(file);ObjectOutputStream oos = new ObjectOutputStream(fos)) {
oos.writeObject(list);
oos.close();
}
}catch(IOException e){e.getMessage();}
}
public static ArrayList<Object> readFile(ArrayList<Object>list, File file){
try {
try (FileInputStream fis = new FileInputStream(file);ObjectInputStream ois = new ObjectInputStream(fis)) {
Object o = ois.readObject();
list = (ArrayList<Object>) o;
ois.close();
}
}catch(IOException | ClassNotFoundException e){e.getMessage();}
System.out.println(list);
return list;
}
}
EDIT:
This my class for testing. My object is an arraylist of custom objects if you need the custom object just comment.
class main {
public static void main(String[] args) {
Date date = new Date();
Book b1 = new Book("abc", "Phi", true, date, null);
Book b2 = new Book("cba", "Someone", true, date, null);
Books booklist = new Books();
booklist.add(b1);
booklist.add(b2);
File filetoDo = new File("book.txt");
//write arraylist into file
writereadFile.writeFile(booklist, filetoDo);
//clear the arraylist
booklist.clear();
//read book from file
writereadFile.readFile(booklist, filetoDo);
System.out.println(booklist);
}
}
Your test should read:
bookList = writereadFile.readFile(booklist, filetoDo);
and, by the way, you should really refactor your readFile method to simply:
public static ArrayList<Object> readFile(File file)
You can't modify the argument reference like that, since Java is always pass-by-value call semantics. (You could modify the list argument contents inside the function, but that's not what you are doing.)
If you are using Java 8 try using Streams:
public static readFile(String filePath) {
List<Object> list = new ArrayList<>();
try (Stream<String> stream = Files.lines(Paths.get(filePath))) {
stream.forEach(list::add);
} catch (IOException e) {
e.printStackTrace();
}
return list;
}
I'm playing around this topic a bit on my own, so below you can find some code snippets that might help you.
Examples are very short and simple, so I hope you will not just use e.printStackTrace() in your code :)
public class ExternalIO {
private ExternalIO() {
}
public static ObjectOutputStream objectOutputStream(String basePath, String pathToFile) throws IOException {
FileOutputStream fileOutputStream = new FileOutputStream(createFileIfDoesNotExist(absolutePath(basePath, pathToFile)));
return new ObjectOutputStream(fileOutputStream);
}
public static ObjectInputStream objectInputStream(String basePath, String pathToFile) throws IOException {
FileInputStream fileInputStream = new FileInputStream(absolutePath(basePath, pathToFile));
return new ObjectInputStream(fileInputStream);
}
private static File createFileIfDoesNotExist(String absolutePath) throws IOException {
File file = new File(absolutePath);
if (file.exists()) {
return file;
}
file.getParentFile().mkdirs();
file.createNewFile();
return file;
}
private static String absolutePath(String basePath, String pathToFile) {
return Paths.get(basePath, pathToFile).toAbsolutePath().toString();
}
}
output usage:
List<ItemType> input = null; //create your input list here
try (ObjectOutputStream objectOutputStream = ExternalIO.objectOutputStream(CONFIG, FILENAME)) {
objectOutputStream.writeObject(input);
} catch (Exception e) {
e.printStackTrace();
}
input usage:
try (ObjectInputStream objectInputStream = ExternalIO.objectInputStream(CONFIG, FILENAME)) {
return (List<ItemType>) objectInputStream.readObject();
} catch (Exception e) {
e.printStackTrace();
}
hope that helps ; )
I'm making a Java program that needs to read info from a text file and then store it in an array and pass it to another class when called. My issue is that I can't seem to call it due to the IOException needed in the file reader class.
This is the main class that is supposed to call the fileReader.
public class window {
public static void main(String[] args){
String[] people = readFromText.read("people.txt");
}
}
File Reader Class
public class readFromText{
public static String[] read(String textFile) throws IOException {
BufferedReader inputFile = new BufferedReader(new
FileReader(textFile));
String[] array = new String[10];
String line = inputFile.readLine().toString();
int cnt = 0;
while (line!=null){
array[cnt] = line;
line = inputFile.readLine().toString();
cnt++;
}
inputFile.close();
return array;
}
}
Is it possible to do this, this way?
Firstly your code is not correct. You can not return the String[] array for the function need String[][].
Secondly for problem about exception you just need to catch it in your main class.
try {
String[] people = readFromText.read("people.txt");
} catch (IOException e) {
e.printStackTrace();
}
I am writing a program in Java which displays a range of afterschool clubs (E.G. Football, Hockey - entered by user). The clubs are added into the following ArrayList:
private ArrayList<Club> clubs = new ArrayList<Club>();
By the followng Method:
public void addClub(String clubName) {
Club club = findClub(clubName);
if (club == null)
clubs.add(new Club(clubName));
}
'Club' is a class with a constructor - name:
public class Club {
private String name;
public Club(String name) {
this.name = name;
}
//There are more methods in my program but don't affect my query..
}
My program is working - it lets me add a new Club Object into my arraylist, i can view the arraylist, and i can delete any that i want etc.
However, I now want to save that arrayList (clubs) to a file, and then i want to be able to load the file up later and the same arraylist is there again.
I have two methods for this (see below), and have been trying to get it working but havent had anyluck, any help or advice would be appreciated.
Save Method (fileName is chosen by user)
public void save(String fileName) throws FileNotFoundException {
String tmp = clubs.toString();
PrintWriter pw = new PrintWriter(new FileOutputStream(fileName));
pw.write(tmp);
pw.close();
}
Load method (Current code wont run - File is a string but needs to be Club?
public void load(String fileName) throws FileNotFoundException {
FileInputStream fileIn = new FileInputStream(fileName);
Scanner scan = new Scanner(fileIn);
String loadedClubs = scan.next();
clubs.add(loadedClubs);
}
I am also using a GUI to run the application, and at the moment, i can click my Save button which then allows me to type a name and location and save it. The file appears and can be opened up in Notepad but displays as something like Club#c5d8jdj (for each Club in my list)
You should use Java's built in serialization mechanism.
To use it, you need to do the following:
Declare the Club class as implementing Serializable:
public class Club implements Serializable {
...
}
This tells the JVM that the class can be serialized to a stream. You don't have to implement any method, since this is a marker interface.
To write your list to a file do the following:
FileOutputStream fos = new FileOutputStream("t.tmp");
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(clubs);
oos.close();
To read the list from a file, do the following:
FileInputStream fis = new FileInputStream("t.tmp");
ObjectInputStream ois = new ObjectInputStream(fis);
List<Club> clubs = (List<Club>) ois.readObject();
ois.close();
As an exercise, I would suggest doing the following:
public void save(String fileName) throws FileNotFoundException {
PrintWriter pw = new PrintWriter(new FileOutputStream(fileName));
for (Club club : clubs)
pw.println(club.getName());
pw.close();
}
This will write the name of each club on a new line in your file.
Soccer
Chess
Football
Volleyball
...
I'll leave the loading to you. Hint: You wrote one line at a time, you can then read one line at a time.
Every class in Java extends the Object class. As such you can override its methods. In this case, you should be interested by the toString() method. In your Club class, you can override it to print some message about the class in any format you'd like.
public String toString() {
return "Club:" + name;
}
You could then change the above code to:
public void save(String fileName) throws FileNotFoundException {
PrintWriter pw = new PrintWriter(new FileOutputStream(fileName));
for (Club club : clubs)
pw.println(club); // call toString() on club, like club.toString()
pw.close();
}
In Java 8 you can use Files.write() method with two arguments: Path and List<String>, something like this:
List<String> clubNames = clubs.stream()
.map(Club::getName)
.collect(Collectors.toList())
try {
Files.write(Paths.get(fileName), clubNames);
} catch (IOException e) {
log.error("Unable to write out names", e);
}
This might work for you
public void save(String fileName) throws FileNotFoundException {
FileOutputStream fout= new FileOutputStream (fileName);
ObjectOutputStream oos = new ObjectOutputStream(fout);
oos.writeObject(clubs);
fout.close();
}
To read back you can have
public void read(String fileName) throws FileNotFoundException {
FileInputStream fin= new FileInputStream (fileName);
ObjectInputStream ois = new ObjectInputStream(fin);
clubs= (ArrayList<Clubs>)ois.readObject();
fin.close();
}
ObjectOutputStream.writeObject(clubs)
ObjectInputStream.readObject();
Also, you 'add' logic is logically equivalent to using a Set instead of a List. Lists can have duplicates and Sets cannot. You should consider using a set. After all, can you really have 2 chess clubs in the same school?
To save and load an arraylist of
public static ArrayList data = new ArrayList ();
I used (to write)...
static void saveDatabase() {
try {
FileOutputStream fos = new FileOutputStream("mydb.fil");
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(data);
oos.close();
databaseIsSaved = true;
}
catch (IOException e) {
e.printStackTrace();
}
} // End of saveDatabase
And used (to read) ...
static void loadDatabase() {
try {
FileInputStream fis = new FileInputStream("mydb.fil");
ObjectInputStream ois = new ObjectInputStream(fis);
data = (ArrayList<User>)ois.readObject();
ois.close();
}
catch (IOException e) {
System.out.println("***catch ERROR***");
e.printStackTrace();
}
catch (ClassNotFoundException e) {
System.out.println("***catch ERROR***");
e.printStackTrace();
}
} // End of loadDatabase
I am trying to print strings into a file. What have I done wrong and it always gives me a NullPointException? I believe my exceptions catch something or an argument is needed and I dont enter it. But where?
I have writen this code, that contains the main function.
EDIT: Getting error in the second line from the bottom some.items[0]="Testing One!";.
import java.io.*;
public class StringPrinter {
public String[] items;
public File file;
public StringPrinter(String fileName){
file = new File(fileName);}
public void toFile(){
try{
PrintWriter pw = new PrintWriter(new FileWriter(file, false));
for (String st:items){
pw.println(st);
}
}
catch(Exception exception){}
}
public static void main(String args[]){
StringPrinter some=new StringPrinter("Workyou.txt");
some.items[0]="Testing One!";
some.items[1]="Testing Two!";
some.toFile();
}
}
It seems you are getting Exception here
some.items[0]="Testing One!";
this is because you did not initialize
public String[] items;
initialize it something like this in your constructor
public StringPrinter(String fileName){
file = new File(fileName);
items = new String[SIZE];
}
first : As all said you have to initialize the array.
Second : Why not print data to file
Solution :
In method ToFile()
after the for loop printing the string[] value, you need to close the Printer Writer
PrintWriter pw = new PrintWriter(new FileWriter(file, false));
for (String st:items){
pw.println(st);
}
**pw.close()**
It will print your data to file.
You're trying to set the string Testing One! to the first position of the array items, but you did not initialize that string array
some.items[0]="Testing One!";
If you change this line.
public String[] items;
to this one
public String[] items = new String[2];
then it will work. Notice that you must predefine the size of the array. Notice that the array size is fixed. If you don't want the array size to be fixed, I suggest you use the wrapper class ArrayList, which size can be expanded.
import java.io.*;
import java.util.ArrayList;
public class StringPrinter {
public ArrayList<String> items = new ArrayList<String>();
public File file;
public StringPrinter(String filename) {
file = new File(filename);
}
public void toFile() {
try {
PrintWriter pw = new PrintWriter(new FileWriter(file, false));
for (String st : items) {
pw.println(st);
}
pw.close();
}
catch(Exception exception) { }
}
public static void main(String args[]) {
StringPrinter some = new StringPrinter("Workyou.txt");
some.items.add("Testing One!");
some.items.add("Testing Two!");
some.toFile();
}
}
The items array is not initialized. You got to initialize it before actually assigning some values. If you do not want to create a fixed size classical array then you can try to use ArrayList.
Try this, the complete solved code:
import java.io.*;
public class StringPrinter {
public String[] items;
public File file;
public StringPrinter(String filename) {
file = new File(filename);
}
public void toFile() {
try {
PrintWriter pw = new PrintWriter(new FileWriter(file, false));
for (String st : items){
pw.println(st);
}
pw.close(); //needed to close the writer stream to write data in file
}
catch(Exception exception) {}
}
public static void main(String args[]) {
StringPrinter some = new StringPrinter("Workyou.txt");
some.items = new String[2]; //needed to initialize array with size
some.items[0]="Testing One!";
some.items[1]="Testing Two!";
some.toFile();
}
}
Try with this , this should print the text in a file
public class StringPrinter {
public String[] items = new String [2];
public File file;
public StringPrinter(String fileName){
file = new File(fileName);}
public void toFile(){
try{
PrintWriter pw = new PrintWriter(new FileWriter(file, false));
for (String st:items){
pw.print(st);
pw.flush();
}
}
catch(Exception exception){}
}
public static void main(String args[]){
StringPrinter some=new StringPrinter("Workyou.txt");
some.items[0]="Testing One!";
some.items[1]="Testing Two!";
some.toFile();
}
}
Documentation for flush . Not required to close the writers compulsorily to see the content in a file , just calling the flush to the writer it will add the content to the files.