Abstract classes - instantiation

[Arnb Sen]

Hi,

I read somewhere

A class that is abstract may not be instantiated (ie, you may not call its constructor)

Can someone please elaborate on this ?

This code works --

abstract class class8Base {
 
  public abstract void abc();
      public static void def() {
 System.out.println("in def");
      }
 
  }
 
  public class class8 extends class8Base {
      public static void main(String args[]) {
 System.out.println("in main ");
 class8 c8 = new class8();
 c8.abc();
  }
 
  public void abc() {
      System.out.println("in abc");
      class8Base.def();
  }
}

[Paul Santa Maria]

More to the point, please recompile with "javac -Xlint:unchecked" (or better, "javac -Xlint:all") and find out for sure exactly what the compiler disagrees with!

[Dave Kairys]

This has to do with polymorphism. All abstract methods must be implemented(or overridden). In order to override a method, you would need to have code in another method that would do this.

[Tony Morris]

It's not quite true. You can, and in fact, you must, call a constructor of an abstract class, from a subclass' constructor.

[Ernest Friedman-Hill]

Arnb --

You posted a couple of these today, questions that start with "I just read..." My advice to you (if these are all coming from the same source) is that you don't read this particular book, tutorial, or study guide any further. Wherever these quotes are coming from, you're getting a lot of bad information.

[Dave Kairys]

Tony,
What did you mean by

It's not quite true.

I am new to Java and I was referencing the Head First Java text when I provided some insight into the proposed question. Please elaborate on your comment.

Thanks!

[Tony Morris]

abstract class X
{
    X()
    {
 
    }
}
 
class Y extends X
{
    Y()
    {
        // Call the constructor of X, an abstract class.
        super();
    }
}

[Arnb Sen]

Hi Ernest,

I agree with you. I was actually looking for some certification book which explains or describes as the Java Objectives for SCJP.

Any book in which the chapters or the sections are kind of exact replica of the order in which the Objectives are mentioned in Sun's SCJP requirement ?

[Arnb Sen]

Any certification book which follows a similar organisation of the chapters and sections same as that of the SCJP Objective ?

[Arnb Sen]

Have heard that Kathy Sierra's book is very good and helpful. Does that follow similar SCJP Objective structure ?