With this example we are going to demonstrate the polymorphism of a class and the constructors behaviour. In short, to see how constructors are used in a class and the changes that a statement can cause to a class we have performed the following steps:
- We have created an
abstractclassA, with anabstractmethodfunc(), that it uses in its constructor. - We have also created a class
Bthat extendsAand has anintfield. In its constructor it initializes its int value to a given one. It also overrides thefunc()method ofA. - We create a new instance of
Bwith a given int value. - The constructor of
Ais first called. - The
func()method is called, and since it is overriden byB, the overridenfunc()is called, that prints the int field ofB, that is equal to 0, since it is not initialized yet. Then inB‘s constructor the int value ofBbecomes equal to the given value. - Note that if we make the
abstract func()methodfinal, then we cannot modify it inB.
Let’s take a look at the code snippet that follows:
package com.javacodegeeks.snippets.core;
abstract class A {
// Make the method func final and run again the example
abstract void func();
A() {
System.out.println("A() before func()");
func();
System.out.println("A() after func()");
}
}
class B extends A {
private int value = 1;
B(int val) {
value = val;
System.out.println("B.B(), value = " + value);
}
@Override
void func() {
System.out.println("B.func(), value = " + value);
}
}
public class Polymorphism {
public static void main(String[] args) {
new B(5);
}
}
Output:
A() before func()
B.func(), value = 0
A() after func()
B.B(), value = 5
This was an example of polymorphism and constructors in a class in Java.
