Why it is not a good idea to call Set method from constructor?

Question

Is it true only in Inheritance or most of the cases ?

public class MyClass {
   public int id;

     public MyClass() {
         // Some stuff 
         setI         


        

Answer 1

You should not call an overridable method from a constructor.

If you call a method that can be overridden by a subclass in a costructor, than the subclass might access variables of the superclass that have not been initialized yet.

For example the following code looks good so far

class MyClass {
    public int id;
    protected String someStr;

    public MyClass() {
        SetId(5);
        someStr = "test";
    }

    public void SetId(int Id) {
        id = Id;
    }
}

If you now subclass MyClass and override the SetId method you might access the superclass's someStr variable which has not been initialized yet and thus will cause a NullPointerException in this case.

class MySubClass extends MyClass {

    public void SetId(int Id) {
       id = Id;
       someStr.toString(); // will cause NullPointerException
    }
}

The cause of the NPE might be hard to see if there is a bigger inheritence hierarchy.