Question

So, here is a piece of my abstract class:

abstract public void insert(Object obj);

And here is the implementation in a class that extends it:

public void insert(User u){
    try{
        String sql = "INSERT INTO " + TABLE + " (username, assword) VALUES ("+u.getUsername()+", " +u.getPassword()+")";
        conn.execSQL(sql);
    }catch(Exception e){
        System.out.println(e.getMessage());
    }finally{
        conn.close();
    }
}

and Here is the error:

The type — must implement the inherited abstract method —.insert(Object) userDAO.java

Basically, it is saying to me that I didn’t implement any method that is called “insert” and receives an object as parameter. Doesn’t my model “User” count as an object? What am I doing wrong here?

Thanks for your attention.

Ps.: error message on Eclipse while developing an Android App (I don’t know if it changes anything).

Answer 1

Your method signature is incorrect, base and derived methods take different objects. Let me try and explain:

Assume you have a class Base which takes an instance of another class B1 in one of its methods (say m1). Also assume that you have a Derived class Derived1 which overrides the method (m1) in Base with the parameter D1 and similarly a derived class Derived2 which overrides the base method with parameter D2.

Both D1 and D2 implement/extend B1. This will be a problem because of the following scenario:

Base baseObj = new Derived1();
baseObj.m1(new D2()); // Should work if it is allowed to override a method in your way!

The above code snippet fails because the m1 method in Derived1 expects a parameter of type D1, but it got D2. Do you now see why this type of parameter casting is not permitted?

Note: It is a good practice to use @override annotation when overriding a method in base (from Java 1.5 specification)