How to create a java class with dynamic number of variables

Define a Map<String, String> that you can access by the key,

but why?

at the end all those "variables" will be the same type... -> String...

the same principe is done in config or property files....

I think you need to do a bit more research on java classes. There is no java class that has a dynamic "number" of variables. But you can give a class attributes, but require only some are set, for example.

class DataFile {

    int data;
    String name;
    String addr;
    int age;
}

And then you can create setters and getters for each field.

public void setName(String name) {
    this.name = name;
}

public String getName() {
    return this.name;
}

That way you can instantiate a member of the class and set the data you have.

DataFile d = new DataFile();
d.setName("John");

Remember that any class methods like setName and getName have to be inside the { } that define the class to which they belong. They aren't here just to separate them visually.

All of that already exists. It is called "Properties" in Java, too.

So, simply read about them here!

In other words: you could/should use java.util.Properties; and there is full support for reading that information from files; or writing to.

The Java property file format does not match your current file format; but well, when you are doing "true" Java, then the most "off the shelf" solution are Java properties; so so you could consider to change your file format.

And to give the actual answer: Java does not support a dynamic number of fields. That is what Maps are used for; and the Property class is exactly that - some sort of Map with additional functionality.

How about something like this

public class Model {
  Map model;

  public Model(String json) {
    model = new Gson().fromJson(jsonModel, Map.class);
  }

  public Object getValue(String key) {
    return model.get(key);
  }
}

But you'd rather want your model to hold data for values, leaving the type inference of the fields to Java.