Question

So I’ve come across a bit of a snag in some code that I’m working with. Essentially I have the following three tidbits of code:

Abstract class:

public abstract class TestParent {
    int size;

    public TestParent(int i){
        size = i;
    }

}

Child Class:

     public class TestChild extends TestParent{
     public void mult(){
         System.out.println(this.size * 5);
     }

 }

Implementation:

public class TestTest {

   public static void main(String args[]) {
       TestChild Test = new TestChild(2);
       Test.mult();
   }
}

Answer 1

Consider the following case of abstract class and extends implementation.
https://stackoverflow.com/a/260755/1071979

abstract class Product { 
    int multiplyBy;
    public Product( int multiplyBy ) {
        this.multiplyBy = multiplyBy;
    }

    public int mutiply(int val) {
       return muliplyBy * 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 default constructor that just hardcodes the value 2. The concrete class TimesWhat has a constructor that allows the caller to specify the value.

NOTE: As there is no default (or no-arg) constructor in the parent abstract class the constructor used in subclasses must be specified.

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