Writing array data to a binary file in Java

Question

I'm trying to store the following arrays in binary, and read them back in after:

private int[10] number;
private String[10] name;
private int[10] age;

What is the best way to store them to a binary file, and read them back in afterwards? An example value for each array are:

number[0] = 1; name[0] = "Henry"; age[0] = 35;

So the data belongs sort of together.

Edit: Basically I want to store data from variables (arrays) using a binary text file, binary is a must, and then read the data back in. So in short I want to have a way to store data so when I close the application the data is not lost and can be retrieved each session.

Can you use like key value pairs but values being separated by comma, like below ? 0=1,Henry,35 — royalghost
– royalghost, Commented Mar 12, 2018 at 18:22
@BernardoRocha yes it has to be 3 different arrays, unless you could combine them in some sort of way as I think you try to suggest by storing an object? I want to use something like a FileWriter or OutputStreamWriter — Walt
– Walt, Commented Mar 12, 2018 at 18:37

Karol Dowbecki · Accepted Answer · 2018-03-12 20:22:25Z

2

You could write custom encoder and decoder e.g. by mapping String to byte[] with String.getBytes() and then writing each byte with OutputStream.write() method. The question is what do you mean by "the best way" because:

Do you need to support different locales? What is the String locale as getBytes() uses platform's default charset.
Do you need to compress the binary data to save space as String might be either very long or very short.
Do you plan to share the binary files with other applications? If so it's best to use a format supported in few languages e.g. Google Protobuf.

If the files are ever to be read outside of your application then Protobuf seems like a sane choice. It would however be an overkill for a Hello World application.

The simpliest approach would be to use ObjectOutputStream and ObjectInputStream but it's far from pretty:

String fileName = "data.set";

try (ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(fileName))) {
  for (int n : number) {
    out.writeInt(n);
  }
  for (String n : name) {
    out.writeUTF(n);
  }
  for (int a : age) {
    out.writeInt(a);
  }
}

try (ObjectInputStream in = new ObjectInputStream(new FileInputStream(fileName))) {
  for (int i = 0; i < number.length; i++) {
    number[i] = in.readInt();
  }
  for (int i = 0; i < name.length; i++) {
    name[i] = in.readUTF();
  }
  for (int i = 0; i < age.length; i++) {
    age[i] = in.readInt();
  }
}

edited Mar 12, 2018 at 20:22

answered Mar 12, 2018 at 19:02

Karol Dowbecki

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3 Comments

Walt Over a year ago

The binary files won't be shared with other applications. It's just used to save variables to a text file (binary is a must) and read back from them so when I close the application, the data is not lost.

Karol Dowbecki Over a year ago

I'd go with Protobuf if it's not a Hello World application. It's straightforward. Just wrap your data into a type: it looks like you can create new Person[10] with number, name and age being Person fields. Then create a .proto file, generate the Java classes from the .proto file and use it as show here. Alternatively you can use standard Java serialization using ObjectInputStream/ObjectOutputStream but you still need a wrapper class for your data.

Karol Dowbecki Over a year ago

@Walt added an example of Java serialization

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