Can Abstract Classes Have Constructors and What Are Their Benefits?

Can Abstract Classes Have Constructors?

An abstract class can indeed have a constructor. Contrary to popular belief, this feature allows for powerful class design and constraint enforcement.

Constructor Usage in Abstract Classes

Consider the following abstract class example:

abstract class Product {
    int multiplyBy;
    public Product(int multiplyBy) {
        this.multiplyBy = multiplyBy;
    }
    public int mutiply(int val) {
        return multiplyBy * val;
    }
}

In this example, the abstract class Product has a constructor that initializes the multiplyBy field. Concrete classes extending Product can then utilize this constructor to enforce class invariants or constrain field initialization.

Concrete Class Constructors

Concrete classes inheriting from Product must call the parent constructor explicitly since there's no default constructor in the abstract class. Here are some examples:

class TimesTwo extends Product {
    public TimesTwo() {
        super(2);
    }
}

class TimesWhat extends Product {
    public TimesWhat(int what) {
        super(what);
    }
}

The concrete class TimesTwo hardcodes the multiplyBy value to 2, while TimesWhat allows the caller to specify it.

Benefits of Abstract Class Constructors

Abstract class constructors provide several benefits:

• Enforcement of class constraints or invariants.
• Initialization of common fields shared across all subclasses.
• Initial setup of complex objects without sacrificing class structure.

Note: Explicit constructor invocation is required in subclasses, as abstract constructors do not have a default implementation.

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