If you don't want the ability to override existing attribute values, declare attributes as final:
Example
public class Main {
  final int x = 10;
  final double PI = 3.14;
  public static void main(String[] args) {
    Main myObj = new Main();
    myObj.x = 50; // will generate an error: cannot assign a value to a final variable
    myObj.PI = 25; // will generate an error: cannot assign a value to a final variable
    System.out.println(myObj.x);
  }
}
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