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The Java Interview Handbook: 300+ Interview Questions

<div class="card">
 # Notes on Inheritance vs Composition
<div class="ans">
<ol>
<li> Class (not interface) inheritance may be inappropriate across package boundaries since the subclass will need to evolve alongside the superclass and is prone to breaking if the superclass undergoes changes. The resulting software is fragile and less maintainable.</li>

<li>Composition is an alternative to inheritance. Composition works by passing an instance of a class we intended to extend from, to an intended derived class which stores the reference in a private field. The new class offers wrapper methods which invoke corresponding methods on the contained class's object. The class whose object we pass in to the new class becomes a component of the new class and is why the approach is called composition.</li>

<li>Inheritance violates encapsulation. Inheritance is appropriate only in circumstances where the subclass truly is a subtype of the superclass i.e. a definite "is-a" relationship exists between the super and derived classes.</li>
<li>Providing implementation documentation in comments and <code class="clazz">protected</code> methods as hooks make it easier for other developers to extend your class.</li>

<li>Overrideable methods should never be invoked in the super class's constructors as the superclass constructor runs before the subclass constructor, so the overriding method in the subclass will get invoked before the subclass constructor has run. If the overriding method depends on any initialization performed by the subclass constructor, the method will not behave as expected. Below is an example, where the constructor for class <code class="clazz">A</code> invokes an overrideable method in class <code class="clazz">B</code> which makes use of a field that gets initialized in the constructor of class <code class="clazz">B</code>. The constructor of class <code class="clazz">B</code> is invoked after the super class's constructor and results in broken logic.
</li>
</ol>
</div>

<div>
 # Notes on Inheritance vs Composition
<div>
<ol>
<li> Class (not interface) inheritance may be inappropriate across package boundaries since the subclass will need to evolve alongside the superclass and is prone to breaking if the superclass undergoes changes. The resulting software is fragile and less maintainable.</li>

<li>Composition is an alternative to inheritance. Composition works by passing an instance of a class we intended to extend from, to an intended derived class which stores the reference in a private field. The new class offers wrapper methods which invoke corresponding methods on the contained class's object. The class whose object we pass in to the new class becomes a component of the new class and is why the approach is called composition.</li>

<li>Inheritance violates encapsulation. Inheritance is appropriate only in circumstances where the subclass truly is a subtype of the superclass i.e. a definite "is-a" relationship exists between the super and derived classes.</li>
<li>Providing implementation documentation in comments and <code>protected</code> methods as hooks make it easier for other developers to extend your class.</li>

<li>Overrideable methods should never be invoked in the super class's constructors as the superclass constructor runs before the subclass constructor, so the overriding method in the subclass will get invoked before the subclass constructor has run. If the overriding method depends on any initialization performed by the subclass constructor, the method will not behave as expected. Below is an example, where the constructor for class <code>A</code> invokes an overrideable method in class <code>B</code> which makes use of a field that gets initialized in the constructor of class <code>B</code>. The constructor of class <code>B</code> is invoked after the super class's constructor and results in broken logic.
</li>
</ol>
</div>

This lesson discusses the reasons why composition may be a better choice than inheritance.

Java Ecosystem

Methods

Classes

Interfaces

Inheritance

Lambda Expressions

Generics

Multi-Threading

Memory Management

Collections

Exceptions

Reflection

Serialization

Miscellaneous Topics

Java in Practice

Patterns

The End

Inheritance vs Composition

Notes on Inheritance vs Composition