Can an abstract class have a constructor?

Question

Can an abstract class have a constructor?

If so, how can it be used and for what purposes?

Answer 1

Yes, an abstract class can have a constructor. Consider this:

abstract class Product {      int multiplyBy;     public Product( int multiplyBy ) {         this.multiplyBy = multiplyBy;     }      public int mutiply(int val) {        return multiplyBy * val;     } }  class TimesTwo extends Product {     public TimesTwo() {         super(2);     } }  class TimesWhat extends Product {     public TimesWhat(int what) {         super(what);     } } 

The superclass Product is abstract and has a constructor. The concrete class TimesTwo has a constructor that just hardcodes the value 2. The concrete class TimesWhat has a constructor that allows the caller to specify the value.

Abstract constructors will frequently be used to enforce class constraints or invariants such as the minimum fields required to setup the class.

NOTE: As there is no default (or no-arg) constructor in the parent abstract class, the constructor used in subclass must explicitly call the parent constructor.

Answer 2

You would define a constructor in an abstract class if you are in one of these situations:

• you want to perform some initialization (to fields of the abstract class) before the instantiation of a subclass actually takes place
• you have defined final fields in the abstract class but you did not initialize them in the declaration itself; in this case, you MUST have a constructor to initialize these fields

Note that:

• you may define more than one constructor (with different arguments)
• you can (should?) define all your constructors protected (making them public is pointless anyway)
• your subclass constructor(s) can call one constructor of the abstract class; it may even have to call it (if there is no no-arg constructor in the abstract class)

In any case, don't forget that if you don't define a constructor, then the compiler will automatically generate one for you (this one is public, has no argument, and does nothing).