presentation on inheritance in java

• Java Arrays
• Java Strings
• Java Collection
• Java 8 Tutorial
• Java Multithreading
• Java Exception Handling
• Java Programs
• Java Project
• Java Collections Interview
• Java Interview Questions
• Spring Boot
• Java Tutorial

Overview of Java

• Introduction to Java
• The Complete History of Java Programming Language
• C++ vs Java vs Python
• How to Download and Install Java for 64 bit machine?
• Setting up the environment in Java
• How to Download and Install Eclipse on Windows?
• JDK in Java
• How JVM Works - JVM Architecture?
• Differences between JDK, JRE and JVM
• Just In Time Compiler
• Difference between JIT and JVM in Java
• Difference between Byte Code and Machine Code
• How is Java platform independent?

Basics of Java

• Java Basic Syntax
• Java Hello World Program
• Java Data Types
• Primitive data type vs. Object data type in Java with Examples
• Java Identifiers

Operators in Java

• Java Variables
• Scope of Variables In Java

Wrapper Classes in Java

Input/output in java.

• How to Take Input From User in Java?
• Scanner Class in Java
• Java.io.BufferedReader Class in Java
• Difference Between Scanner and BufferedReader Class in Java
• Ways to read input from console in Java
• System.out.println in Java
• Difference between print() and println() in Java
• Formatted Output in Java using printf()
• Fast I/O in Java in Competitive Programming

Flow Control in Java

• Decision Making in Java (if, if-else, switch, break, continue, jump)
• Java if statement with Examples
• Java if-else
• Java if-else-if ladder with Examples
• Loops in Java
• For Loop in Java
• Java while loop with Examples
• Java do-while loop with Examples
• For-each loop in Java
• Continue Statement in Java
• Break statement in Java
• Usage of Break keyword in Java
• return keyword in Java
• Java Arithmetic Operators with Examples
• Java Unary Operator with Examples
• Java Assignment Operators with Examples
• Java Relational Operators with Examples
• Java Logical Operators with Examples
• Java Ternary Operator with Examples
• Bitwise Operators in Java
• Strings in Java
• String class in Java
• Java.lang.String class in Java | Set 2
• Why Java Strings are Immutable?
• StringBuffer class in Java
• StringBuilder Class in Java with Examples
• String vs StringBuilder vs StringBuffer in Java
• StringTokenizer Class in Java
• StringTokenizer Methods in Java with Examples | Set 2
• StringJoiner Class in Java
• Arrays in Java
• Arrays class in Java
• Multidimensional Arrays in Java
• Different Ways To Declare And Initialize 2-D Array in Java
• Jagged Array in Java
• Final Arrays in Java
• Reflection Array Class in Java
• util.Arrays vs reflect.Array in Java with Examples

OOPS in Java

• Object Oriented Programming (OOPs) Concept in Java
• Why Java is not a purely Object-Oriented Language?
• Classes and Objects in Java
• Naming Conventions in Java
• Java Methods

Access Modifiers in Java

• Java Constructors
• Four Main Object Oriented Programming Concepts of Java

Inheritance in Java

Abstraction in java, encapsulation in java, polymorphism in java, interfaces in java.

• 'this' reference in Java
• Inheritance and Constructors in Java
• Java and Multiple Inheritance
• Interfaces and Inheritance in Java
• Association, Composition and Aggregation in Java
• Comparison of Inheritance in C++ and Java
• abstract keyword in java
• Abstract Class in Java
• Difference between Abstract Class and Interface in Java
• Control Abstraction in Java with Examples
• Difference Between Data Hiding and Abstraction in Java
• Difference between Abstraction and Encapsulation in Java with Examples
• Difference between Inheritance and Polymorphism
• Dynamic Method Dispatch or Runtime Polymorphism in Java
• Difference between Compile-time and Run-time Polymorphism in Java

Constructors in Java

• Copy Constructor in Java
• Constructor Overloading in Java
• Constructor Chaining In Java with Examples
• Private Constructors and Singleton Classes in Java

Methods in Java

• Static methods vs Instance methods in Java
• Abstract Method in Java with Examples
• Overriding in Java
• Method Overloading in Java
• Difference Between Method Overloading and Method Overriding in Java
• Differences between Interface and Class in Java
• Functional Interfaces in Java
• Nested Interface in Java
• Marker interface in Java
• Comparator Interface in Java with Examples
• Need of Wrapper Classes in Java
• Different Ways to Create the Instances of Wrapper Classes in Java
• Character Class in Java
• Java.Lang.Byte class in Java
• Java.Lang.Short class in Java
• Java.lang.Integer class in Java
• Java.Lang.Long class in Java
• Java.Lang.Float class in Java
• Java.Lang.Double Class in Java
• Java.lang.Boolean Class in Java
• Autoboxing and Unboxing in Java
• Type conversion in Java with Examples

Keywords in Java

• Java Keywords
• Important Keywords in Java
• Super Keyword in Java
• final Keyword in Java
• static Keyword in Java
• enum in Java
• transient keyword in Java
• volatile Keyword in Java
• final, finally and finalize in Java
• Public vs Protected vs Package vs Private Access Modifier in Java
• Access and Non Access Modifiers in Java

Memory Allocation in Java

• Java Memory Management
• How are Java objects stored in memory?
• Stack vs Heap Memory Allocation
• How many types of memory areas are allocated by JVM?
• Garbage Collection in Java
• Types of JVM Garbage Collectors in Java with implementation details
• Memory leaks in Java
• Java Virtual Machine (JVM) Stack Area

Classes of Java

• Understanding Classes and Objects in Java
• Singleton Method Design Pattern in Java
• Object Class in Java
• Inner Class in Java
• Throwable Class in Java with Examples

Packages in Java

• Packages In Java
• How to Create a Package in Java?
• Java.util Package in Java
• Java.lang package in Java
• Java.io Package in Java
• Java Collection Tutorial

Exception Handling in Java

• Exceptions in Java
• Types of Exception in Java with Examples
• Checked vs Unchecked Exceptions in Java
• Java Try Catch Block
• Flow control in try catch finally in Java
• throw and throws in Java
• User-defined Custom Exception in Java
• Chained Exceptions in Java
• Null Pointer Exception In Java
• Exception Handling with Method Overriding in Java
• Multithreading in Java
• Lifecycle and States of a Thread in Java
• Java Thread Priority in Multithreading
• Main thread in Java
• Java.lang.Thread Class in Java
• Runnable interface in Java
• Naming a thread and fetching name of current thread in Java
• What does start() function do in multithreading in Java?
• Difference between Thread.start() and Thread.run() in Java
• Thread.sleep() Method in Java With Examples
• Synchronization in Java
• Importance of Thread Synchronization in Java
• Method and Block Synchronization in Java
• Lock framework vs Thread synchronization in Java
• Difference Between Atomic, Volatile and Synchronized in Java
• Deadlock in Java Multithreading
• Deadlock Prevention And Avoidance
• Difference Between Lock and Monitor in Java Concurrency
• Reentrant Lock in Java

File Handling in Java

• Java.io.File Class in Java
• Java Program to Create a New File
• Different ways of Reading a text file in Java
• Java Program to Write into a File
• Delete a File Using Java
• File Permissions in Java
• FileWriter Class in Java
• Java.io.FileDescriptor in Java
• Java.io.RandomAccessFile Class Method | Set 1
• Regular Expressions in Java
• Regex Tutorial - How to write Regular Expressions?
• Matcher pattern() method in Java with Examples
• Pattern pattern() method in Java with Examples
• Quantifiers in Java
• java.lang.Character class methods | Set 1
• Java IO : Input-output in Java with Examples
• Java.io.Reader class in Java
• Java.io.Writer Class in Java
• Java.io.FileInputStream Class in Java
• FileOutputStream in Java
• Java.io.BufferedOutputStream class in Java
• Java Networking
• TCP/IP Model
• User Datagram Protocol (UDP)
• Differences between IPv4 and IPv6
• Difference between Connection-oriented and Connection-less Services
• Socket Programming in Java
• java.net.ServerSocket Class in Java
• URL Class in Java with Examples

JDBC - Java Database Connectivity

• Introduction to JDBC (Java Database Connectivity)
• JDBC Drivers
• Establishing JDBC Connection in Java
• Types of Statements in JDBC
• JDBC Tutorial
• Java 8 Features - Complete Tutorial

Java, Inheritance is an important pillar of OOP(Object-Oriented Programming). It is the mechanism in Java by which one class is allowed to inherit the features(fields and methods) of another class. In Java, Inheritance means creating new classes based on existing ones. A class that inherits from another class can reuse the methods and fields of that class. In addition, you can add new fields and methods to your current class as well.  

Why Do We Need Java Inheritance?

• Code Reusability: The code written in the Superclass is common to all subclasses. Child classes can directly use the parent class code.
• Method Overriding: Method Overriding is achievable only through Inheritance. It is one of the ways by which Java achieves Run Time Polymorphism.
• Abstraction: The concept of abstract where we do not have to provide all details is achieved through inheritance. Abstraction only shows the functionality to the user.

Important Terminologies Used in Java Inheritance

• Class: Class is a set of objects which shares common characteristics/ behavior and common properties/ attributes. Class is not a real-world entity. It is just a template or blueprint or prototype from which objects are created.
• Super Class/Parent Class: The class whose features are inherited is known as a superclass(or a base class or a parent class).
• Sub Class/Child Class: The class that inherits the other class is known as a subclass(or a derived class, extended class, or child class). The subclass can add its own fields and methods in addition to the superclass fields and methods.
• Reusability: Inheritance supports the concept of “reusability”, i.e. when we want to create a new class and there is already a class that includes some of the code that we want, we can derive our new class from the existing class. By doing this, we are reusing the fields and methods of the existing class.

How to Use Inheritance in Java?

The extends keyword is used for inheritance in Java. Using the extends keyword indicates you are derived from an existing class. In other words, “extends” refers to increased functionality.

Inheritance in Java Example

Example: In the below example of inheritance, class Bicycle is a base class, class MountainBike is a derived class that extends the Bicycle class and class Test is a driver class to run the program. 

In the above program, when an object of MountainBike class is created, a copy of all methods and fields of the superclass acquires memory in this object. That is why by using the object of the subclass we can also access the members of a superclass. 

Please note that during inheritance only the object of the subclass is created, not the superclass. For more, refer to Java Object Creation of Inherited Class . 

Example 2: In the below example of inheritance, class Employee is a base class, class Engineer is a derived class that extends the Employee class and class Test is a driver class to run the program.

Illustrative image of the program: 

In practice, inheritance, and polymorphism are used together in Java to achieve fast performance and readability of code.

Java Inheritance Types

Below are the different types of inheritance which are supported by Java.

• Single Inheritance

Multilevel Inheritance

• Hierarchical Inheritance

Multiple Inheritance

Hybrid Inheritance

1. Single Inheritance

In single inheritance, subclasses inherit the features of one superclass. In the image below, class A serves as a base class for the derived class B.

Single inheritance

2. Multilevel Inheritance

In Multilevel Inheritance, a derived class will be inheriting a base class, and as well as the derived class also acts as the base class for other classes. In the below image, class A serves as a base class for the derived class B, which in turn serves as a base class for the derived class C. In Java, a class cannot directly access the grandparent’s members .

3. Hierarchical Inheritance

In Hierarchical Inheritance, one class serves as a superclass (base class) for more than one subclass. In the below image, class A serves as a base class for the derived classes B, C, and D.

4. Multiple Inheritance ( Through Interfaces)

In Multiple inheritances , one class can have more than one superclass and inherit features from all parent classes. Please note that Java does not support multiple inheritances with classes. In Java, we can achieve multiple inheritances only through Interfaces . In the image below, Class C is derived from interfaces A and B.

5. Hybrid Inheritance

It is a mix of two or more of the above types of inheritance. Since Java doesn’t support multiple inheritances with classes, hybrid inheritance involving multiple inheritance is also not possible with classes. In Java, we can achieve hybrid inheritance only through Interfaces if we want to involve multiple inheritance to implement Hybrid inheritance. However, it is important to note that Hybrid inheritance does not necessarily require the use of Multiple Inheritance exclusively. It can be achieved through a combination of Multilevel Inheritance and Hierarchical Inheritance with classes, Hierarchical and Single Inheritance with classes. Therefore, it is indeed possible to implement Hybrid inheritance using classes alone, without relying on multiple inheritance type. 

Java IS-A type of Relationship

IS-A is a way of saying: This object is a type of that object. Let us see how the extends keyword is used to achieve inheritance.

Now, based on the above example, in Object-Oriented terms, the following are true:-

• SolarSystem is the superclass of Earth class.
• SolarSystem is the superclass of Mars class.
• Earth and Mars are subclasses of SolarSystem class.
• Moon is the subclass of both Earth and SolarSystem classes.

What Can Be Done in a Subclass?

In sub-classes we can inherit members as is, replace them, hide them, or supplement them with new members: 

• The inherited fields can be used directly, just like any other fields.
• We can declare new fields in the subclass that are not in the superclass.
• The inherited methods can be used directly as they are.
• We can write a new instance method in the subclass that has the same signature as the one in the superclass, thus overriding it (as in the example above, toString() method is overridden).
• We can write a new static method in the subclass that has the same signature as the one in the superclass, thus hiding it.
• We can declare new methods in the subclass that are not in the superclass.
• We can write a subclass constructor that invokes the constructor of the superclass, either implicitly or by using the keyword super .

 Advantages Of Inheritance in Java:

• Code Reusability: Inheritance allows for code reuse and reduces the amount of code that needs to be written. The subclass can reuse the properties and methods of the superclass, reducing duplication of code.
• Abstraction: Inheritance allows for the creation of abstract classes that define a common interface for a group of related classes. This promotes abstraction and encapsulation, making the code easier to maintain and extend.
• Class Hierarchy: Inheritance allows for the creation of a class hierarchy, which can be used to model real-world objects and their relationships.
• Polymorphism: Inheritance allows for polymorphism, which is the ability of an object to take on multiple forms. Subclasses can override the methods of the superclass, which allows them to change their behavior in different ways.

Disadvantages of Inheritance in Java:

• Complexity: Inheritance can make the code more complex and harder to understand. This is especially true if the inheritance hierarchy is deep or if multiple inheritances is used.
• Tight Coupling: Inheritance creates a tight coupling between the superclass and subclass, making it difficult to make changes to the superclass without affecting the subclass.

Let us check some important points from the article are mentioned below:

• Default superclass : Except Object class, which has no superclass, every class has one and only one direct superclass (single inheritance). In the absence of any other explicit superclass, every class is implicitly a subclass of the Object class.
• Superclass can only be one: A superclass can have any number of subclasses. But a subclass can have only one superclass. This is because Java does not support multiple inheritances with classes. Although with interfaces, multiple inheritances are supported by Java.
• Inheriting Constructors: A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.
• Private member inheritance: A subclass does not inherit the private members of its parent class. However, if the superclass has public or protected methods(like getters and setters) for accessing its private fields, these can also be used by the subclass.

FAQs in Inheritance

1. what is inheritance java.

Inheritance is a concept of OOPs where one class inherits from another class that can reuse the methods and fields of the parent class.

2. What are the 4 types of inheritance in Java?

There are Single, Multiple, Multilevel, Hierarchical and Hybrid

3. What is the use of extend keyword?

Extend keyword is used for inheriting one class into another.

4. What is an example of inheritance in Java?

A real-world example of Inheritance in Java is mentioned below:

Consider a group of vehicles. You need to create classes for Bus, Car, and Truck. The methods fuelAmount(), capacity(), applyBrakes() will be the same for all three classes.

References Used: “Head First Java” by Kathy Sierra and Bert Bates “Java: A Beginner’s Guide” by Herbert Schildt “Java: The Complete Reference” by Herbert Schildt “Effective Java” by Joshua Bloch “Java: The Good Parts” by Jim Waldo.

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Java Inheritance Tutorial: explained with examples

• is-a relationship

In Java, inheritance is an is-a relationship. That is, we use inheritance only if there exists an is-a relationship between two classes. For example,

• Car is a Vehicle
• Orange is a Fruit
• Surgeon is a Doctor
• Dog is an Animal

Here, Car can inherit from Vehicle , Orange can inherit from Fruit , and so on.

Method Overriding in Java Inheritance

In Example 1 , we see the object of the subclass can access the method of the superclass.

However, if the same method is present in both the superclass and subclass, what will happen?

In this case, the method in the subclass overrides the method in the superclass. This concept is known as method overriding in Java.

Example 2: Method overriding in Java Inheritance

In the above example, the eat() method is present in both the superclass Animal and the subclass Dog .

Here, we have created an object labrador of Dog .

Now when we call eat() using the object labrador , the method inside Dog is called. This is because the method inside the derived class overrides the method inside the base class.

This is called method overriding. To learn more, visit Java Method Overriding .

Note : We have used the @Override annotation to tell the compiler that we are overriding a method. However, the annotation is not mandatory. To learn more, visit Java Annotations .

super Keyword in Java Inheritance

Previously we saw that the same method in the subclass overrides the method in superclass.

In such a situation, the super keyword is used to call the method of the parent class from the method of the child class.

Example 3: super Keyword in Inheritance

In the above example, the eat() method is present in both the base class Animal and the derived class Dog . Notice the statement,

Here, the super keyword is used to call the eat() method present in the superclass.

We can also use the super keyword to call the constructor of the superclass from the constructor of the subclass. To learn more, visit Java super keyword .

protected Members in Inheritance

In Java, if a class includes protected fields and methods, then these fields and methods are accessible from the subclass of the class.

Example 4: protected Members in Inheritance

In the above example, we have created a class named Animal. The class includes a protected field: name and a method: display() .

We have inherited the Dog class inherits Animal . Notice the statement,

Here, we are able to access the protected field and method of the superclass using the labrador object of the subclass.

• Why use inheritance?
• The most important use of inheritance in Java is code reusability. The code that is present in the parent class can be directly used by the child class.
• Method overriding is also known as runtime polymorphism. Hence, we can achieve Polymorphism in Java with the help of inheritance.
• Types of inheritance

There are five types of inheritance.

1. Single Inheritance

In single inheritance, a single subclass extends from a single superclass. For example,

2. Multilevel Inheritance

In multilevel inheritance, a subclass extends from a superclass and then the same subclass acts as a superclass for another class. For example,

3. Hierarchical Inheritance

In hierarchical inheritance, multiple subclasses extend from a single superclass. For example,

4. Multiple Inheritance

In multiple inheritance, a single subclass extends from multiple superclasses. For example,

Note : Java doesn't support multiple inheritance. However, we can achieve multiple inheritance using interfaces. To learn more, visit Java implements multiple inheritance .

5. Hybrid Inheritance

Hybrid inheritance is a combination of two or more types of inheritance. For example,

Here, we have combined hierarchical and multiple inheritance to form a hybrid inheritance.

Table of Contents

• Introduction
• Example: Java Inheritance
• Method Overriding Inheritance
• super Keyword Inheritance
• protected Members and Inheritance

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In Java, it is possible to inherit attributes and methods from one class to another. We group the "inheritance concept" into two categories:

• subclass (child) - the class that inherits from another class
• superclass (parent) - the class being inherited from

To inherit from a class, use the extends keyword.

In the example below, the Car class (subclass) inherits the attributes and methods from the Vehicle class (superclass):

Try it Yourself »

Did you notice the protected modifier in Vehicle?

We set the brand attribute in Vehicle to a protected access modifier . If it was set to private , the Car class would not be able to access it.

Why And When To Use "Inheritance"?

- It is useful for code reusability: reuse attributes and methods of an existing class when you create a new class.

Tip: Also take a look at the next chapter, Polymorphism , which uses inherited methods to perform different tasks.

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The final Keyword

If you don't want other classes to inherit from a class, use the final keyword:

If you try to access a final class, Java will generate an error:

The output will be something like this:

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Java Inheritance Uncovered: A Comprehensive Guide to Understanding, Implementing, and Leveraging Inheritance in Java

Inheritance is one of the four pillars of Object-Oriented Programming (OOP) and a powerful mechanism that allows one class to inherit the attributes and methods of another class. Inheritance promotes code reuse, modularity, and maintainability in Java applications. In this blog post, we will explore the concept of inheritance in Java, its syntax, types, and best practices.

Understanding Inheritance in Java

Inheritance enables a class (subclass) to inherit the properties and behaviors of another class (superclass). This relationship between the two classes creates a hierarchy, allowing the subclass to reuse code from the superclass and add or override its attributes and methods.

Terminology

• Superclass : The class that is being inherited by another class.
• Subclass : The class that inherits from another class.
• Inheritance : The process by which a subclass inherits the properties and behaviors of a superclass.

Implementing Inheritance in Java

To implement inheritance in Java, use the extends keyword followed by the name of the superclass.

In this example, the Car class inherits the attributes and methods of the Vehicle class.

Accessing Superclass Members

When a subclass inherits from a superclass, it can access the superclass's attributes and methods, subject to their access modifiers.

Accessing Superclass Attributes

A subclass can access the superclass's public and protected attributes directly. However, it cannot access the superclass's private attributes. To access private attributes, provide public getter and setter methods in the superclass.

Accessing Superclass Methods

A subclass can access the superclass's public and protected methods directly. However, it cannot access the superclass's private methods. To access private methods, provide public or protected methods in the superclass.

Method Overriding

Method overriding is the process of providing a new implementation for a method inherited from a superclass. The new implementation must have the same method signature (name, return type, and parameters) as the original method.

In this example, the Car class overrides the start() method inherited from the Vehicle class.

The super Keyword

The super keyword is used to refer to the superclass and can be used to access superclass members or call the superclass constructor.

Use the super keyword followed by the dot (.) operator to access the superclass's attributes and methods. This is particularly useful when you want to call the superclass's method from an overridden method in the subclass.

Calling the Superclass Constructor

The super keyword can also be used to call the superclass constructor from the subclass constructor. This is essential when the superclass has a parameterized constructor and requires specific initialization.

Types of Inheritance in Java

Java supports several types of inheritance, including:

Single Inheritance

In single inheritance, a class inherits from only one superclass. Java supports single inheritance through the extends keyword.

Multilevel Inheritance

In multilevel inheritance, a class inherits from a superclass, which in turn inherits from another superclass, forming a chain of inheritance. Java supports multilevel inheritance through the extends keyword.

Hierarchical Inheritance

In hierarchical inheritance, multiple classes inherit from a single superclass. Java supports hierarchical inheritance through the extends keyword.

Best Practices for Using Inheritance in Java

To make the most of inheritance in Java, follow these best practices:

Use Inheritance for "Is-A" Relationships

Inheritance should be used to model "is-a" relationships between classes. For example, a Car is a Vehicle , so it makes sense for the Car class to inherit from the Vehicle class.

Use Composition for "Has-A" Relationships

Instead of inheritance, use composition to model "has-a" relationships. Composition allows you to create more flexible and maintainable code by combining simpler objects to create more complex objects.

Favor Method Overriding Over Hiding

When redefining a method in a subclass, use method overriding instead of method hiding. Overriding provides a more consistent and maintainable approach to modifying inherited behavior.

Do Not Overuse Inheritance

Avoid overusing inheritance, as it can lead to code complexity and rigidity. Instead, focus on creating modular, maintainable code with composition and interfaces.

Inheritance is a powerful concept in Java that allows you to reuse code, create modular designs, and leverage existing functionality. By understanding the different types of inheritance and following best practices, you can create more maintainable and efficient Java applications.

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INHERITANCE IN JAVA.

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Inheritance in Java

Mar 17, 2019

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Inheritance in Java. CS 3331. Two Programming Languages Words . Overloading Overriding. Overloading of Methods and Constructors. public class Point { public Point() { /* … */ } public Point(int x, int y) { /* … */ } public double distance(Point other) { /* … */ }

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Inheritance in Java CS 3331

Two Programming Languages Words • Overloading • Overriding

Overloading of Methods and Constructors • public class Point { • public Point() { /* … */ } • public Point(int x, int y) { /* … */ } • public double distance(Point other) { /* … */ } • public double distance(int x, int y) { /* … */ } • public double distance() { /* … */ } • // … • }

Why use overloading?

Why use overloading? public class StringBuffer { public StringBuffer append(String str) { /* … */ } public StringBuffer append(boolean b) { /* … */ } public StringBuffer append(char c) { /* … */ } public StringBuffer append(int i) { /* … */ } public StringBuffer append(long l) { /* … */ } // … }

When to Overload? (Cont.) public class String { public String substring(int i, int j) { // base method: return substring from index i to j - 1. } public String substring(int i) { // provide default argument return substring(i, length()); } // … }

How does Java figure out which method to use? • new StringBuffer sb = append( …);

Overloading • 1: Point() Point() • 2: Point(int x,int y) Point(int,int) • 3: double distance(Point other) distance(Point) • 4: double distance(int x,int y) distance(int,int) • 5: double distance() distance() • Point p1 = new Point(); // which constructor? • Point p2 = new Point(10,20); • p2.distance(p1); // which method? • p2.distance(20,30); • p2.distance();

Overriding Methods • Def. Overriding • Consequences

Overriding Methods (Cont.) public class T { public void m() { … } } public class S extends T { public void m() { … } } T t = new T(); S s = new S(); t.m(); // invoke m of class T s.m(); // invoke m of class S

What value is returned? class Student { public int maxCredits() { return 15; } … } class GraduateStudent extends Student { public int maxCredits() { return 12; } … } Student s; // … s.getMaxCredits(); // which maxCredits method?

Implementation of Dynamic Binding • Storage structure of instance variables • Dynamic bindings of messages to methods

Dynamic Binding (Cont.) class Point { private int x, y; public Point(int x, int y) { this.x = x; this.y = y; } public int getX() { return x; } public int getY() { return y; } } class ColoredPoint extends Point { private Color color; public ColoredPoint(int x, int y, Color c) { super(x, y); color = c; } public Color getColor() { return color; } }

Object.class Point.class p: getX: getY: … super: vmt: … super: vmt: … super: vmt: … ColoredPoint.class class: x: 10 y: 20 color: getColor: … Color.RED Dynamic Binding (Cont.) ColoredPoint p = new ColoredPoint(10,20,Color.RED); p.getColor(); p.getX();

Overriding Methods (Cont.)

Hiding Fields and Class Methods • Def. Hiding

Example class Student { protected String description = “Student”; public String getDescription() { return description; } } class Undergraduate extends Student { protected String description = “Undergraduate”; } new Student().getDecription(); // what value is returned? new Undergraduate().getDescription(); // what value? Q. How to refer to hidden fields?

Hiding vs. overriding • Statically resolved (bound) at compile-time vs. dynamically dispatched at run-time

Another Example class Point { public String description = “Point”; } class CPoint extends Point { public String description = “CPoint”; } CPoint c1 = new CPoint(); Point c2 = c1; system.out.println (c1.description); system.out.println (c2.description);

Inheritance • Inheritance models the is-a relationship. • If class S extends class T, then all objects of S can act-like an object of T. • In Java, only single inheritance is allowed among classes. • All public and protected members of a superclass are accessible in the subclasses.* *All protected members are also accessible within the package.

Constructors of Subclasses • Can invoke a constructor of the direct superclass. • super(…) must be the first statement. • If the super constructor call is missing, by default the no-arg super() is invoked implicitly. • Can also invoke another constructor of the same class. • this(…) must be the first statement.

Example of “this” Calls public class Point { private int x, y; public Point(int x, int y) { this.x = x; this.y = y; } public Point() { // default constructor this.x = 0; this.y = 0; } }

Example of “super” Calls public class ColoredPoint extends Point { private Color color; public ColoredPoint(int x, int y, Color color) { this.x = x; this.y = y; this.color = color; } public ColoredPoint(int x, int y) { this(x, y, Color.BLACK); // point with default value } public ColoredPoint() { color = Color.BLACK; // what will be the values of x and y? } }

Default Constructor public ClassName() { super(); } Q. What would be the use of default constructor?

Execution Order of Constructors public class T { int x = 10; // first public T() { x = 20; // second } // ... } public class S extends T { int y = 30; // third public S() { super(); y = 40; // fourth } // ... }

Types • What is a “type”?

Subtyping • What’s subtyping?

Substitution Property • Def. Substitution property • A value of subtype can appear where a value of its supertype is expected, e.g., in arguments, results, receivers, and assignments.

Substitution Property (Cont.) • Rules of (polymorphic) assignment • The type of expression at the right-hand side of an assignment must be a subtype of the type of the variable at the left-hand side of the assignment. class Student { … } class Undergraduate extends Student { … } class Graduate extends Student { … } Student s1, s2; s1 = new Undergradute(); // polymorphic assignment s2 = new Graduate(); // polymorphic assignment Graduate s3 = s2; // is this OK? Graduate s3 = (Graduate) s2; // explicit casting

Widening and Narrowing • Def. widening and narrowing • The conversion of a subtype to one of its supertypes is called widening, and the conversion of a supertype to one of its subtype is called narrowing (or downcasting). // s is a stack of strings Stack s = new Stack(); s.push(“Hello”); … // Stack defines a method top, i.e., “public Object top()”. s.top().size(); // okay? ((String) s.top()).size(); // downcasting

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