Store 2D array (String) to file and retrieve it

Question

I have made a simple program that has a 2D String array storing lots of data. I have searched a lot of places for how to store and retrieve 2D arrays. I want to save the data in my array at the end of the program and have this data retrieved upon the start of the program. I have tried: ObjectOutputStream toFile = new ObjectOutputStream(new FileWriter("array.DATA")); toFile.writeObject(array); This came up with the error: incompatible types: FileWriter cannot be converted to OutputStream

I only know basic programming so try to be easy on me and explain everything you say thoroughly. If I'm honest I don't even know what 'ObjectOutputStream' is.

Many thanks in advance

Answer 1

In this example: This example is very simple(it's not a perfect solution but it's a good start to undestand how save and retrieve works).

Your 2d array is int a[][]= new int[12][12];

//or a is String a[][] = new String[12][12];

• Part 1: (Save)

  public void Save(){
  
   try {
      PrintWriter writer = new PrintWriter(new File("C:/Users/Alex.hp/Desktop/t.txt"));
  
      for(int i=0; i<a.length; i++){
          for(int j=0; j<a[i].length; j++){
  
            //use this if your array has (int,double..)
                // writer.write(String.valueOf(a[i][j])+" "); //Here you parse the int from array to String.
  
  
             //use this if your array has String
               writer.write(a[i][j]+" "); //Its String so you dont have to use String.valueOf(something(int,double,...)
          }
         writer.println(); //leave one line 
      }
  
      writer.flush();  //flush the writer
      writer.close();  //close the writer      
  
  
  
     } catch (FileNotFoundException e) {      
       e.printStackTrace();
     }
  

  }//end of method

Using a simple PrintWriter you save your array into a file exaclty how it is. Just you know change the path i have put to the file;

• Part 2: (Update or Retrieve on Open)

  public void UpdateOnOpen(){
  
   try {
      Scanner scan = new Scanner(new File("C:/Users/Alex.hp/Desktop/t.txt"));
  
  //Retrieve
          for(int i=0; i<a.length;  i++)
            for(int j=0; j<a[i].length; j++){
  
            //if you array use (int,double,...)
              //a[i][j]=Interger.parseInt(scan.next()) //for int
              //a[i][j]=Double.parseDouble(scan.next()) //for double
  
            //if your array use String
              a[i][j]=scan.next(); //Here you retrieve the array again from the file
  
  
            }//end of for(int j=0; j<a[i].length; j++)
  
  
  
       scan.close(); //close the resource file you opened
  
  //Print it to be sure all are right
            for(int i=0; i<a.length; i++){
                  for(int c=0; c<a[i].length; c++)
                        System.out.printf(""+a[i][c]+",");
  
              System.out.println();    
            }                         
  
    } catch (FileNotFoundException e){  e.printStackTrace(); }    
  
  
  }//end of method
  

  Using Scanner class you retrieve your array back from the file saved.

Also a better solution but you have to dig more here

Let me know it your job is done..

Answer 2

There is an additional question to resolve, which is how do you store the sizes of the arrays as well as the contents. I would recommend using conversion to JSON. If you get the Jackson library, you should be able to use the Jackson Object Mapper. I would recommend storing your arrays within another object.

e.g.

class MyClass {
    String[][] myArray;

    // add the getters and setters to this class for myArray too
}

Then for storing

MyClass myObject = new MyClass();

// set the arrays here to whatever you like

ObjectMapper mapper = new ObjectMapper();
mapper.write(new File("c:\somedir\somefile.txt"), myObject);

Then for loading

ObjectMapper mapper = new ObjectMapper();
MyClass loaded = mapper.readValue(new File("c:\somedir\somefile.txt", MyClass.class);

// then you can use loaded as the source of your data

Answer 3

You should be using FileOutputStream instead. FileWriter in character while ObjectOutputStream is binary output stream.

public static void main(String[] args) {
    try {
        ObjectOutputStream toFile = new ObjectOutputStream(new FileOutputStream("//home//user//array.DATA"));
        toFile.writeObject(args);
        toFile.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}