Inheritance is the process by which the new child subclass automatically includes any public or protected primitives, objects, or methods defined in the parent class.
We refer to any class that inherits from another class as a child class, or a descendent of that class.
We refer to the class that the child inherits from as the parent class, or an ancestor of the class.
OCA Java SE 8 Exam Chapter 6 Exceptionsİbrahim Kürce
A program can fail for just about any reason. Here are just a few possibilities:
The code tries to connect to a website, but the Internet connection is down.
You made a coding mistake and tried to access an invalid index in an array.
One method calls another with a value that the method doesn't support.
Java abstract class & abstract methods,Abstract class in java
Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation. Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
OCA Java SE 8 Exam Chapter 6 Exceptionsİbrahim Kürce
A program can fail for just about any reason. Here are just a few possibilities:
The code tries to connect to a website, but the Internet connection is down.
You made a coding mistake and tried to access an invalid index in an array.
One method calls another with a value that the method doesn't support.
Java abstract class & abstract methods,Abstract class in java
Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation. Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
Java Threads Tutorial | Multithreading In Java Tutorial | Java Tutorial For B...Edureka!
(**** Java Certification Training: https://www.edureka.co/java-j2ee-soa-training ****)
This Edureka tutorial on “Java Threads” will talk about one of the core concepts of Java i.e Java Threads. It will give you a complete insight into how to create, work and synchronize with multiple threads. Through this tutorial you will learn the following topics:
What is a Java Thread?
Thread Lifecycle
Creating a Thread
Main Thread
Multi-Threading
Thread Pool
Check out our Java Tutorial blog series: https://goo.gl/osrGrS
Check out our complete Youtube playlist here: https://goo.gl/gMFLx3
Follow us to never miss an update in the future.
Instagram: https://www.instagram.com/edureka_learning/
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LinkedIn: https://www.linkedin.com/company/edureka
Superclasses, and Subclasses, Overriding and Hiding Methods, Polymorphism, Inheritance Hierarchies, Super keyword, Final Classes and Methods, Abstract,
Classes and Methods, Nested classes & Inner Classes,
finalization and garbage collection.
Java Strings Tutorial | String Manipulation in Java | Java Tutorial For Begin...Edureka!
**** Java Certification Training: https://www.edureka.co/java-j2ee-soa-training ****
This Edureka tutorial on “Java Strings” will talk about one of the most important classes of Java i.e String class. It will also talk about the various methods and interfaces implemented by the String class. Through this tutorial you will learn the following topics:
Java Strings
String Pool
Creating a String
String Methods
Check out our Java Tutorial blog series: https://goo.gl/osrGrS
Check out our complete Youtube playlist here: https://goo.gl/gMFLx3
Follow us to never miss an update in the future.
Instagram: https://www.instagram.com/edureka_learning/
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Chapter 02: Classes Objects and Methods Java by Tushar B KuteTushar B Kute
The lecture was condcuted by Tushar B Kute at YCMOU, Nashik through VLC orgnanized by MSBTE. The contents can be found in book "Core Java Programming - A Practical Approach' by Laxmi Publications.
Final keyword are used in java for three purpose;
1. Final keyword is used in java to make variable constant
2. Final keyword restrict method overriding
3. It used to restrict Inheritance
http://www.tutorial4us.com/java/java-final-keyword
Java Classes | Java Tutorial for Beginners | Java Classes and Objects | Java ...Edureka!
This Edureka 'Java Classes' tutorial will take you through in-depth details of Java classes and different key-components in Java. It is ideal for both beginners and professionals who want to learn or brush up the basics of Java. Below are the topics covered in this tutorial:
1. Classes in Java
2. Constructors
3. Structure of a Class
4. Overview of Objects
5. Working with Objects
6. Multiple classes
7. OOPS concepts: Inheritance, Encapsulation, Abstraction, Interface
In this you learn about Control Statements
1. Selection Statements
i. If
ii. If-else
iii. Nested-if
iv. If-Elseif ladder
2. Looping Statements
i. while loop
ii. do-while loop
iii. For loop
3. Jumping Statements
i. break
ii. continue
iii return
This presentation is continuation to “Software Craftsmanship - Code Smells - Object Orientation Abusers”. Here we will cover the fifth and the final category of code smells – “Change Preventers”.
Java Threads Tutorial | Multithreading In Java Tutorial | Java Tutorial For B...Edureka!
(**** Java Certification Training: https://www.edureka.co/java-j2ee-soa-training ****)
This Edureka tutorial on “Java Threads” will talk about one of the core concepts of Java i.e Java Threads. It will give you a complete insight into how to create, work and synchronize with multiple threads. Through this tutorial you will learn the following topics:
What is a Java Thread?
Thread Lifecycle
Creating a Thread
Main Thread
Multi-Threading
Thread Pool
Check out our Java Tutorial blog series: https://goo.gl/osrGrS
Check out our complete Youtube playlist here: https://goo.gl/gMFLx3
Follow us to never miss an update in the future.
Instagram: https://www.instagram.com/edureka_learning/
Facebook: https://www.facebook.com/edurekaIN/
Twitter: https://twitter.com/edurekain
LinkedIn: https://www.linkedin.com/company/edureka
Superclasses, and Subclasses, Overriding and Hiding Methods, Polymorphism, Inheritance Hierarchies, Super keyword, Final Classes and Methods, Abstract,
Classes and Methods, Nested classes & Inner Classes,
finalization and garbage collection.
Java Strings Tutorial | String Manipulation in Java | Java Tutorial For Begin...Edureka!
**** Java Certification Training: https://www.edureka.co/java-j2ee-soa-training ****
This Edureka tutorial on “Java Strings” will talk about one of the most important classes of Java i.e String class. It will also talk about the various methods and interfaces implemented by the String class. Through this tutorial you will learn the following topics:
Java Strings
String Pool
Creating a String
String Methods
Check out our Java Tutorial blog series: https://goo.gl/osrGrS
Check out our complete Youtube playlist here: https://goo.gl/gMFLx3
Follow us to never miss an update in the future.
Instagram: https://www.instagram.com/edureka_learning/
Facebook: https://www.facebook.com/edurekaIN/
Twitter: https://twitter.com/edurekain
LinkedIn: https://www.linkedin.com/company/edureka
Chapter 02: Classes Objects and Methods Java by Tushar B KuteTushar B Kute
The lecture was condcuted by Tushar B Kute at YCMOU, Nashik through VLC orgnanized by MSBTE. The contents can be found in book "Core Java Programming - A Practical Approach' by Laxmi Publications.
Final keyword are used in java for three purpose;
1. Final keyword is used in java to make variable constant
2. Final keyword restrict method overriding
3. It used to restrict Inheritance
http://www.tutorial4us.com/java/java-final-keyword
Java Classes | Java Tutorial for Beginners | Java Classes and Objects | Java ...Edureka!
This Edureka 'Java Classes' tutorial will take you through in-depth details of Java classes and different key-components in Java. It is ideal for both beginners and professionals who want to learn or brush up the basics of Java. Below are the topics covered in this tutorial:
1. Classes in Java
2. Constructors
3. Structure of a Class
4. Overview of Objects
5. Working with Objects
6. Multiple classes
7. OOPS concepts: Inheritance, Encapsulation, Abstraction, Interface
In this you learn about Control Statements
1. Selection Statements
i. If
ii. If-else
iii. Nested-if
iv. If-Elseif ladder
2. Looping Statements
i. while loop
ii. do-while loop
iii. For loop
3. Jumping Statements
i. break
ii. continue
iii return
This presentation is continuation to “Software Craftsmanship - Code Smells - Object Orientation Abusers”. Here we will cover the fifth and the final category of code smells – “Change Preventers”.
Access our sample Java8 certification questions.
1z0-808 certification questions ready to be used.
For each 1z0-808 certification question you can find the solution, the explanation and the exam objective of 1z0-808
This presentation provides an overview of oracle's associate and professional Java certifications - gives you ideas on how to prepare and crack the exam with ease.
Functional Thinking - Programming with Lambdas in Java 8Ganesh Samarthyam
Functional programming is on the rise. Almost all major and mainstream languages support functional programming features, including C++, Java, Swift, and Python, and Visual Basic. With Java 8’s lambda functions, Java now supports functional programming. Moving to functional programming can result in significantly better code and productivity gains. However, it requires a paradigm shift: you need to move away from imperative and object-oriented thinking to start thinking functionally. That’s what this workshop will help you achieve: it will help you make your shift towards functional programming. The workshop will introduce lambda functions in Java with examples from Java library itself. Presented in OSI Days 2015 workshop - http://osidays.com/osidays/shifting-to-functional-programming-lambdas-for-java-developers/
Variables declared in a Java interface is by default final. An abstract class may contain non-final variables.
Members of a Java interface are public by default. A member of an abstract class can either be private, protected or public.
An interface is absolutely abstract and cannot be instantiated. An abstract class also cannot be instantiated, but can be invoked if it contains a main method.
Similar to OCA Java SE 8 Exam Chapter 5 Class Design (20)
Effective Java - Madde 2: Birçok parametreli yapılandırıcıyla karşılaşırsan k...İbrahim Kürce
Statik fabrikalar ve yapılandırıcılar ortak bir kısıtı paylaşır: birçok isteğe bağlı parametreye karşı kendilerini iyi ölçekleyemezler. Paketli gıdaların üzerinde bulunan Beslenme Unsurları(Nutrition Facts) sınıfının gösterilmesi ele alalım. Bu sınıfın birkaç tane zorunlu alanı vardır: porsiyon boyutu(serving size), konteyner başına porsiyon(servings per container) ve porsiyon başına kalorileri(calories per serving). Ayrıca birçok isteğe bağlı alanı vardır: toplam yağ(total fat), doymuş yağ(saturated fat), kolesterol, sodyum vb. Birçok ürün için bu alanların değerleri sıfırdan farklıdır.
Effective Java - Madde 1: Yapılandırıcılar yerine statik fabrika(factory) met...İbrahim Kürce
Normal şekilde, sınıfı kullanan kişi, o sınıftan bir örnek oluşturabilme iznini public(herkese açık) yapılandırıcı sayesinde elde eder. Her programcının bilmesi gereken başka bir teknik daha vardır. Bir sınıf, kendi örneğini döndüren statik fabrika metodu ile de örneğini oluşturabilir. Boolean(basit boolean tipinin kutulanmış sınıf hali) sınıfından basit bir örnek.
Effective Java - Chapter 4: Classes and Interfacesİbrahim Kürce
Classes and interfaces lie at the heart of the Java programming language. They are its basic units of abstraction.
This chapter contains guidelines to help you make the best use of these elements so that your classes and interfaces are usable, robust, and flexible.
Effective Java - Chapter 3: Methods Common to All Objectsİbrahim Kürce
Although object is a concrete class, it is designed primarily for extension.
All of its nonfinal methods (equals, hashCode, toString, clone, and finalize) have explicit general contracts because they are designed to be overridden.
This chapter tells you when and how to override the nonfinal Object methods. The finalize method is omitted from this chapter because it was discussed in Item 7. While not an Object method, Comparable.compareTo is discussed in this chapter because it has a similar character.
Effective Java - Chapter 2: Creating and Destroying Objectsİbrahim Kürce
Creating and Destroying Objects: This chapter concerns creating and destroying objects: when and how to create them, when and how to avoid creating them, how to ensure they are destroyed in a timely manner, and how to manage any cleanup actions that must precede their destruction.
Launch Your Streaming Platforms in MinutesRoshan Dwivedi
The claim of launching a streaming platform in minutes might be a bit of an exaggeration, but there are services that can significantly streamline the process. Here's a breakdown:
Pros of Speedy Streaming Platform Launch Services:
No coding required: These services often use drag-and-drop interfaces or pre-built templates, eliminating the need for programming knowledge.
Faster setup: Compared to building from scratch, these platforms can get you up and running much quicker.
All-in-one solutions: Many services offer features like content management systems (CMS), video players, and monetization tools, reducing the need for multiple integrations.
Things to Consider:
Limited customization: These platforms may offer less flexibility in design and functionality compared to custom-built solutions.
Scalability: As your audience grows, you might need to upgrade to a more robust platform or encounter limitations with the "quick launch" option.
Features: Carefully evaluate which features are included and if they meet your specific needs (e.g., live streaming, subscription options).
Examples of Services for Launching Streaming Platforms:
Muvi [muvi com]
Uscreen [usencreen tv]
Alternatives to Consider:
Existing Streaming platforms: Platforms like YouTube or Twitch might be suitable for basic streaming needs, though monetization options might be limited.
Custom Development: While more time-consuming, custom development offers the most control and flexibility for your platform.
Overall, launching a streaming platform in minutes might not be entirely realistic, but these services can significantly speed up the process compared to building from scratch. Carefully consider your needs and budget when choosing the best option for you.
Innovating Inference - Remote Triggering of Large Language Models on HPC Clus...Globus
Large Language Models (LLMs) are currently the center of attention in the tech world, particularly for their potential to advance research. In this presentation, we'll explore a straightforward and effective method for quickly initiating inference runs on supercomputers using the vLLM tool with Globus Compute, specifically on the Polaris system at ALCF. We'll begin by briefly discussing the popularity and applications of LLMs in various fields. Following this, we will introduce the vLLM tool, and explain how it integrates with Globus Compute to efficiently manage LLM operations on Polaris. Attendees will learn the practical aspects of setting up and remotely triggering LLMs from local machines, focusing on ease of use and efficiency. This talk is ideal for researchers and practitioners looking to leverage the power of LLMs in their work, offering a clear guide to harnessing supercomputing resources for quick and effective LLM inference.
In the ever-evolving landscape of technology, enterprise software development is undergoing a significant transformation. Traditional coding methods are being challenged by innovative no-code solutions, which promise to streamline and democratize the software development process.
This shift is particularly impactful for enterprises, which require robust, scalable, and efficient software to manage their operations. In this article, we will explore the various facets of enterprise software development with no-code solutions, examining their benefits, challenges, and the future potential they hold.
Check out the webinar slides to learn more about how XfilesPro transforms Salesforce document management by leveraging its world-class applications. For more details, please connect with sales@xfilespro.com
If you want to watch the on-demand webinar, please click here: https://www.xfilespro.com/webinars/salesforce-document-management-2-0-smarter-faster-better/
In software engineering, the right architecture is essential for robust, scalable platforms. Wix has undergone a pivotal shift from event sourcing to a CRUD-based model for its microservices. This talk will chart the course of this pivotal journey.
Event sourcing, which records state changes as immutable events, provided robust auditing and "time travel" debugging for Wix Stores' microservices. Despite its benefits, the complexity it introduced in state management slowed development. Wix responded by adopting a simpler, unified CRUD model. This talk will explore the challenges of event sourcing and the advantages of Wix's new "CRUD on steroids" approach, which streamlines API integration and domain event management while preserving data integrity and system resilience.
Participants will gain valuable insights into Wix's strategies for ensuring atomicity in database updates and event production, as well as caching, materialization, and performance optimization techniques within a distributed system.
Join us to discover how Wix has mastered the art of balancing simplicity and extensibility, and learn how the re-adoption of the modest CRUD has turbocharged their development velocity, resilience, and scalability in a high-growth environment.
Essentials of Automations: The Art of Triggers and Actions in FMESafe Software
In this second installment of our Essentials of Automations webinar series, we’ll explore the landscape of triggers and actions, guiding you through the nuances of authoring and adapting workspaces for seamless automations. Gain an understanding of the full spectrum of triggers and actions available in FME, empowering you to enhance your workspaces for efficient automation.
We’ll kick things off by showcasing the most commonly used event-based triggers, introducing you to various automation workflows like manual triggers, schedules, directory watchers, and more. Plus, see how these elements play out in real scenarios.
Whether you’re tweaking your current setup or building from the ground up, this session will arm you with the tools and insights needed to transform your FME usage into a powerhouse of productivity. Join us to discover effective strategies that simplify complex processes, enhancing your productivity and transforming your data management practices with FME. Let’s turn complexity into clarity and make your workspaces work wonders!
Exploring Innovations in Data Repository Solutions - Insights from the U.S. G...Globus
The U.S. Geological Survey (USGS) has made substantial investments in meeting evolving scientific, technical, and policy driven demands on storing, managing, and delivering data. As these demands continue to grow in complexity and scale, the USGS must continue to explore innovative solutions to improve its management, curation, sharing, delivering, and preservation approaches for large-scale research data. Supporting these needs, the USGS has partnered with the University of Chicago-Globus to research and develop advanced repository components and workflows leveraging its current investment in Globus. The primary outcome of this partnership includes the development of a prototype enterprise repository, driven by USGS Data Release requirements, through exploration and implementation of the entire suite of the Globus platform offerings, including Globus Flow, Globus Auth, Globus Transfer, and Globus Search. This presentation will provide insights into this research partnership, introduce the unique requirements and challenges being addressed and provide relevant project progress.
AI Pilot Review: The World’s First Virtual Assistant Marketing SuiteGoogle
AI Pilot Review: The World’s First Virtual Assistant Marketing Suite
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https://sumonreview.com/ai-pilot-review/
AI Pilot Review: Key Features
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✅With one keyword, generate complete funnels, websites, landing pages, and more.
✅More than 85 AI features are included in the AI pilot.
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See My Other Reviews Article:
(1) TubeTrivia AI Review: https://sumonreview.com/tubetrivia-ai-review
(2) SocioWave Review: https://sumonreview.com/sociowave-review
(3) AI Partner & Profit Review: https://sumonreview.com/ai-partner-profit-review
(4) AI Ebook Suite Review: https://sumonreview.com/ai-ebook-suite-review
We describe the deployment and use of Globus Compute for remote computation. This content is aimed at researchers who wish to compute on remote resources using a unified programming interface, as well as system administrators who will deploy and operate Globus Compute services on their research computing infrastructure.
Code reviews are vital for ensuring good code quality. They serve as one of our last lines of defense against bugs and subpar code reaching production.
Yet, they often turn into annoying tasks riddled with frustration, hostility, unclear feedback and lack of standards. How can we improve this crucial process?
In this session we will cover:
- The Art of Effective Code Reviews
- Streamlining the Review Process
- Elevating Reviews with Automated Tools
By the end of this presentation, you'll have the knowledge on how to organize and improve your code review proces
First Steps with Globus Compute Multi-User EndpointsGlobus
In this presentation we will share our experiences around getting started with the Globus Compute multi-user endpoint. Working with the Pharmacology group at the University of Auckland, we have previously written an application using Globus Compute that can offload computationally expensive steps in the researcher's workflows, which they wish to manage from their familiar Windows environments, onto the NeSI (New Zealand eScience Infrastructure) cluster. Some of the challenges we have encountered were that each researcher had to set up and manage their own single-user globus compute endpoint and that the workloads had varying resource requirements (CPUs, memory and wall time) between different runs. We hope that the multi-user endpoint will help to address these challenges and share an update on our progress here.
Navigating the Metaverse: A Journey into Virtual Evolution"Donna Lenk
Join us for an exploration of the Metaverse's evolution, where innovation meets imagination. Discover new dimensions of virtual events, engage with thought-provoking discussions, and witness the transformative power of digital realms."
How to Position Your Globus Data Portal for Success Ten Good PracticesGlobus
Science gateways allow science and engineering communities to access shared data, software, computing services, and instruments. Science gateways have gained a lot of traction in the last twenty years, as evidenced by projects such as the Science Gateways Community Institute (SGCI) and the Center of Excellence on Science Gateways (SGX3) in the US, The Australian Research Data Commons (ARDC) and its platforms in Australia, and the projects around Virtual Research Environments in Europe. A few mature frameworks have evolved with their different strengths and foci and have been taken up by a larger community such as the Globus Data Portal, Hubzero, Tapis, and Galaxy. However, even when gateways are built on successful frameworks, they continue to face the challenges of ongoing maintenance costs and how to meet the ever-expanding needs of the community they serve with enhanced features. It is not uncommon that gateways with compelling use cases are nonetheless unable to get past the prototype phase and become a full production service, or if they do, they don't survive more than a couple of years. While there is no guaranteed pathway to success, it seems likely that for any gateway there is a need for a strong community and/or solid funding streams to create and sustain its success. With over twenty years of examples to draw from, this presentation goes into detail for ten factors common to successful and enduring gateways that effectively serve as best practices for any new or developing gateway.
Listen to the keynote address and hear about the latest developments from Rachana Ananthakrishnan and Ian Foster who review the updates to the Globus Platform and Service, and the relevance of Globus to the scientific community as an automation platform to accelerate scientific discovery.
Prosigns: Transforming Business with Tailored Technology SolutionsProsigns
Unlocking Business Potential: Tailored Technology Solutions by Prosigns
Discover how Prosigns, a leading technology solutions provider, partners with businesses to drive innovation and success. Our presentation showcases our comprehensive range of services, including custom software development, web and mobile app development, AI & ML solutions, blockchain integration, DevOps services, and Microsoft Dynamics 365 support.
Custom Software Development: Prosigns specializes in creating bespoke software solutions that cater to your unique business needs. Our team of experts works closely with you to understand your requirements and deliver tailor-made software that enhances efficiency and drives growth.
Web and Mobile App Development: From responsive websites to intuitive mobile applications, Prosigns develops cutting-edge solutions that engage users and deliver seamless experiences across devices.
AI & ML Solutions: Harnessing the power of Artificial Intelligence and Machine Learning, Prosigns provides smart solutions that automate processes, provide valuable insights, and drive informed decision-making.
Blockchain Integration: Prosigns offers comprehensive blockchain solutions, including development, integration, and consulting services, enabling businesses to leverage blockchain technology for enhanced security, transparency, and efficiency.
DevOps Services: Prosigns' DevOps services streamline development and operations processes, ensuring faster and more reliable software delivery through automation and continuous integration.
Microsoft Dynamics 365 Support: Prosigns provides comprehensive support and maintenance services for Microsoft Dynamics 365, ensuring your system is always up-to-date, secure, and running smoothly.
Learn how our collaborative approach and dedication to excellence help businesses achieve their goals and stay ahead in today's digital landscape. From concept to deployment, Prosigns is your trusted partner for transforming ideas into reality and unlocking the full potential of your business.
Join us on a journey of innovation and growth. Let's partner for success with Prosigns.
1. OCA EXAM 8
CHAPTER 5 – Class Design
By İbrahim Kürce
Brief of OCA: Oracle Certified Associate Java SE 8 Programmer I Study Guide: Exam 1Z0-808
Jeanne Boyarsky, Scott Selikoff
2. Introducing Class Inheritance
Inheritance is the process by which the new child subclass automatically includes
any public or protected primitives, objects, or methods defined in the parent class.
We refer to any class that inherits from another class as a child class, or a
descendent of that class.
We refer to the class that the child inherits from as the parent class, or an ancestor
of the class.
Java supports single inheritance, by which a class may inherit from only one direct
parent class.
Java does allow one exception to the single inheritance rule: classes may
implement multiple interfaces.
3. Class Inheritance
Single inheritance vs multiple inheritance
It is possible in Java to prevent a class from being extended by marking the class
with the final modifier.
4. Extending a Class
In Java, you can extend a class by adding the parent class name in the definition
using the extends keyword.
6. Applying Class Access Modifiers
You can apply access modifiers (public, private, protected, default) to both class
methods and variables.
Note: For the OCA exam, you should only be familiar with public and default
package-level class access modifiers. The protected and private modifiers can only
be applied to inner classes, which are classes that are defined within other classes,
but this is well out of scope for the OCA exam.
A Java file can have many classes but at most one public class. In fact, it may have
no public class at all.
The rules for applying class access modifiers are identical for interfaces. There can
be at most one public class or interface in a Java file.
7. Creating Java Objects
In Java, all classes inherit from a single class, java.lang.Object.
The key is that when Java sees you define a class that doesn't extend another class,
it immediately adds the syntax extends java.lang.Object to the class definition.
If you define a new class that extends an existing class, Java doesn't add this syntax.
8. Defining Constructors
Every class has at least one constructor.
In the case that no constructor is declared, the compiler will automatically insert a
default no-argument constructor.
The first statement of every constructor is either a call to another constructor within
the class, using this(), or a call to a constructor in the direct parent class, using
super().
9. Defining Constructors
The super() command may only be used as the first statement of the constructor.
If the parent class has more than one constructor, the child class may use any valid
parent constructor in its definition.
10. Understanding Compiler Enhancements
Up to now, we defined numerous classes that did not explicitly call the parent
constructor via the super() keyword, so why did the code compile? The answer is
that the Java compiler automatically inserts a call to the no-argument constructor
super() if the first statement is not a call to the parent constructor.
The following three class and constructor definitions are equivalent.
11. Constructors
What happens if the parent class doesn't have a no-argument constructor?
We must explicitly define at least one constructor, as in the following code:
12. Constructors
This code still doesn't compile, though, because the compiler tries to insert the no-
argument super() as the first statement of the constructor in the Elephant class, and
there is no such constructor in the parent class.
constructor. Note that the class Elephant now has a no-argument constructor even
though its parent class Mammal doesn't. Subclasses may define no-argument
constructors even if their parent classes do not.
13. Reviewing Constructor Rules
The first statement of every constructor is a call to another constructor within the
class using this(), or a call to a constructor in the direct parent class using super().
The super() call may not be used after the first statement of the constructor.
If no super() call is declared in a constructor, Java will insert a no-argument super()
as the first statement of the constructor.
If the parent doesn't have a no-argument constructor and the child doesn't define
any constructors, the compiler will throw an error and try to insert a default no-
argument constructor into the child class.
If the parent doesn't have a no-argument constructor, the compiler requires an
explicit call to a parent constructor in each child constructor.
14. Calling Constructors
In Java, the parent constructor is always executed before the child constructor.
Result: Primate Ape
15. Calling Inherited Class Members
If the parent class and child class are part of the same package, the child class may
also use any default members defined in the parent class. Finally, a child class may
never access a private member of the parent class, at least not through any direct
reference.
16. Calling Inherited Class Members
Consider the following alternative definition to the displaySharkDetails() method in
the previous example:
In Java, you can explicitly reference a member of the parent class by using the
super keyword.
17. Calling Inherited Class Members
If the child class overrides a member of the parent class, this and super could have
very different effects when applied to a class member.
super() vs. super
super() and super are quite different but may be used in the same methods on the
exam.
The first, super(), is a statement that explicitly calls a parent constructor and may
only be used in the first line of a constructor of a child class. The second, super, is a
keyword used to reference a member defined in a parent class and may be used
throughout the child class.
18. Inheriting Methods
Inheriting a class grants us access to the public and protected members of the
parent class, but also sets the stage for collisions between methods defined in both
the parent class and the subclass.
Overriding a Method
What if there is a method defined in both the parent and child class?
You may want to define a new version of an existing method in a child class that
makes use of the definition in the parent class.
In this case, you can override a method a method by declaring a new method with
the signature and return type as the method in the parent class.
19. Inheriting Methods
In this manner, the keywords this and super allow you to select between the
current and parent version of a method.
What would the following code output if we removed the super keyword in the
getAverageWeight() method of the Wolf class?
20. Overriding a Method
Overriding a method is not without limitations,
The method in the child class must have the same signature as the method in the parent
class.
The method in the child class must be at least as accessible or more accessible than the
method in the parent class.
The method in the child class may not throw a checked exception that is new or broader
than the class of any exception thrown in the parent class method.
If the method returns a value, it must be the same or a subclass of the method in the
parent class, known as covariant return types.
21. Overloading vs. Overriding
Overloading a method and overriding a method are similar in that they both
involve redefining a method using the same name.
They differ in that an overloaded method will use a different signature than an
overridden method.
22. Overloading vs. Overriding
The first method, fly(), is overloaded in the subclass Eagle, since the signature
changes from a no-argument constructor to a constructor with one int argument.
Because the method is being overloaded and not overridden, the return type can
be changed from void to int without issue.
The second method, eat(), is overridden in the subclass Eagle, since the signature is
the same as it is in the parent class Bird— they both take a single argument int.
Because the method is being overridden, the return type of the method in Eagle
must be a subclass of the return type of the method in Bird. In this example, the
return type void is not a subclass of int; therefore, the compiler will throw an
exception on this method definition.
23. Overriding
First, it violates the second rule of overriding methods: the child method must be at
least as accessible as the parent. In the example, the parent method uses the
protected modifier, but the child method uses the private modifier.
The return type of the parent method and child method must be covariant. In this
example, the return type of the parent method is String, whereas the return type of
the child method is int.
24. Overriding
The first method, hasLegs(), throws an exception InsufficientDataException in the
parent class but doesn't throw an exception in the child class. This does not violate
the third rule of overriding methods, though, as no new exception is defined.
The second method, getWeight(), throws Exception in the parent class and
InsufficientDataException in the child class. This is also permitted, as
InsufficientDataException is a subclass of Exception by construction.
25. Overriding
The getHeight() method in the parent class throws an InsufficientDataException,
whereas the method in the child class throws an Exception. Since Exception is not a
subclass of InsufficientDataException
Next, the getLength() method doesn't throw an exception in the parent class, but it
does throw an exception.
26. Redeclaring private Methods
In Java, it is not possible to override a private method in a parent class since the
parent method is not accessible from the child class.
Just because a child class doesn't have access to the parent method, doesn't mean
the child class can't define its own version of the method. It just means, strictly
speaking, that the new method is not an overridden version of the parent class's
method.
Java permits you to redeclare a new method in the child class with the same or
modified signature as the method in the parent class.
27. Hiding Static Methods
A hidden method occurs when a child class defines a static method with the same
name and signature as a static method defined in a parent class.
The four previous rules for overriding a method must be followed when a method
is hidden. In addition, a new rule is added for hiding a method, namely that the
usage of the static keyword must be the same between parent and child classes.
The following list summarizes the five rules for hiding a method:
1. The method in the child class must have the same signature as the method in the
parent class.
2. The method in the child class must be at least as accessible or more accessible than
the method in the parent class.
28. Hiding Static Methods
3. The method in the child class may not throw a checked exception that is new or
broader than the class of any exception thrown in the parent class method.
4. If the method returns a value, it must be the same or a subclass of the method in
the parent class, known as covariant return types.
5. The method defined in the child class must be marked as static if it is marked as
static in the parent class (method hiding). Likewise, the method must not be
marked as static in the child class if it is not marked as static in the parent class
(method overriding).
Note that the first four are the same as the rules for overriding a method.
29. Hiding Static Methods
Some examples of the new rule:
The code compiles and runs without issue. The eat() method in the child class hides the
eat() method in the parent class.
Because they are both marked as static, this is not considered an overridden method.
30. Hiding Static Methods
Let's contrast this with examples that violate the fifth rule:
In this example, sneeze() is marked as static in the parent class but not in the child class. The
compiler detects that you're trying to override a method that should be hidden and generates a
compiler error. In the second method, hibernate() is an instance member in the parent class but a
static method in the child class.
31. Overriding vs. Hiding Methods
At runtime the child version of an overridden method is always executed for an
instance regardless of whether the method call is defined in a parent or child class
method.
At runtime the parent version of a hidden method is always executed if the call to
the method is defined in the parent class. // False True
32. Overriding vs. Hiding Methods
Contrast this first example with the following example, which uses an overridden
version of isBiped() instead of a hidden version:
Marsupial walks on two legs: true Kangaroo hops on two legs: true
33. Overriding vs. Hiding Methods
In this example, the isBiped() method is overridden, not hidden, in the child class.
Therefore, it is replaced at runtime in the parent class with the call to the child
class's method.
Make sure you understand these examples as they show how hidden and
overridden methods are fundamentally different. This example makes uses of
polymorphism.
34. Creating final methods
final methods cannot be overridden.
This rule is in place both when you override a method and when you hide a
method. In other words, you cannot hide a static method in a parent class if it is
marked as final.
36. Hiding Variables
When you hide a variable, you define a variable with the same name as a variable in
a parent class.
This creates two copies of the variable within an instance of the child class: one
instance defined for the parent reference and another defined for the child
reference.
As when hiding a static method, you can't override a variable; you can only hide it.
If you're referencing the variable from within the parent class, the variable defined
in the parent class is used. Alternatively, if you're referencing the variable from
within a child class, the variable defined in the child class is used.
37. Hiding Variables
You can reference the parent value of the variable with an explicit use of the super
keyword.
This code compiles without issue and outputs the following when executed:
[parentTail = 4] [tail = 8, parentTail = 4]
38. Hiding Variables
Notice that the instance of Mouse contains two copies of the tailLength variables:
one defined in the parent and one defined in the child.
The important thing to remember is that there is no notion of overriding a member
variable. For example, there is no code change that could be made to cause Java to
override the value of tailLength, making it the same in both parent and child.
These rules are the same regardless of whether the variable is an instance variable
or a static variable.
39. Don't Hide Variables in Practice – Real
World Scenario
Although Java allows you to hide a variable defined in a parent class with one
defined in a child class, it is considered an extremely poor coding practice.
Hiding variables makes the code very confusing and difficult to read, especially if
you start modifying the value of the variable in both the parent and child methods,
since it may not be clear which variable you're updating.
40. Creating Abstract Classes
An abstract class is a class that is marked with the abstract keyword and cannot be
instantiated.
An abstract method is a method marked with the abstract keyword defined in an
abstract class, for which no implementation is provided in the class in which it is
declared.
41. Defining an Abstract Class
an abstract class may include nonabstract methods and variables, as you saw with
the variable age and the method eat().
In fact, an abstract class is not required to include any abstract methods.
42. Defining an Abstract Class
The following code snippet will not compile:
a method may not be marked as both abstract and private.
43. Defining an Abstract Class
If we changed the access modified from private to protected in the parent class
Whale,
In this modified example, the code will still not compile but for a completely
different reason. If you remember the rules earlier in this chapter for overriding a
method, the subclass cannot reduce the visibility of the parent method, sing().
44. Creating a Concrete Class
Abstract classes cannot be instantiated and therefore do not do much other than
define static variables and methods.
An abstract class becomes useful when it is extended by a concrete subclass. A
concrete class is the first nonabstract subclass that extends an abstract class and is
required to implement all inherited abstract methods.
45. Extending an Abstract Class
Extending an abstract class with another abstract class
Abstract classes can extend other abstract classes and are not required to provide
implementations for any of the abstract methods.
46. Extending an Abstract Class
The following concrete class Lion must implement two methods, getName() and
roar():
47. Extending an Abstract Class
There is one exception to the rule for abstract methods and concrete classes: a
concrete subclass is not required to provide an implementation for an abstract
method if an intermediate abstract class provides the implementation.
48. Abstract Class Definition Rules:
Abstract classes cannot be instantiated directly.
Abstract classes may be defined with any number, including zero, of abstract and
nonabstract methods.
Abstract classes may not be marked as private or final.
An abstract class that extends another abstract class inherits all of its abstract
methods as its own abstract methods.
The first concrete class that extends an abstract class must provide an
implementation for all of the inherited abstract methods.
49. Abstract Method Definition Rules:
Abstract methods may only be defined in abstract classes.
Abstract methods may not be declared private or final.
Abstract methods must not provide a method body/ implementation in the
abstract class for which is it declared.
Implementing an abstract method in a subclass follows the same rules for
overriding a method. For example, the name and signature must be the same, and
the visibility of the method in the subclass must be at least as accessible as the
method in the parent class.
50. Implementing Interfaces
Although Java doesn't allow multiple inheritance, it does allow classes to
implement any number of interfaces. An interface is an abstract data type that
defines a list of abstract public methods that any class implementing the interface
must provide.
An interface can also include a list of constant variables and default methods.
52. Implementing Interfaces
abstract and public, are assumed. In other words, whether or not you provide them,
the compiler will automatically insert them as part of the method definition.
A class may implement multiple interfaces, each separated by a comma,
53. Defining an Interface
The following is a list of rules for creating an interface,
1. Interfaces cannot be instantiated directly.
2. An interface is not required to have any methods.
3. An interface may not be marked as final.
4. All top-level interfaces are assumed to have public or default access, and they must
include the abstract modifier in their definition. Therefore, marking an interface as
private, protected, or final will trigger a compiler error, since this is incompatible with
these assumptions.
5. All nondefault methods in an interface are assumed to have the modifiers abstract and
public in their definition. Therefore, marking a method as private, protected, or final will
trigger compiler errors as these are incompatible with the abstract and public keywords.
54. Defining an Interface
The fourth rule doesn't apply to inner interfaces, although inner classes and
interfaces are not in scope for the OCA exam.
as WalksOnTwoLegs is an interface and cannot be instantiated directly.
Interfaces may not be marked as final
55. Defining an Interface
The compiler will insert them automatically if you do not. Following two interface
definitions are equivalent,
The abstract keyword is first automatically added to the interface definition. Then,
each method is prepended with abstract and public keywords.
56. Defining an Interface
The first line doesn't compile for two reasons. First, it is marked as final, which
cannot be applied to an interface since it conflicts with the assumed abstract
keyword. Next, it is marked as private, which conflicts with the public or default
required access for interfaces. The second and third line do not compile because all
interface methods are assumed to be public and marking them as private or
protected throws a compiler error. Finally, the last line doesn't compile because the
method is marked as final and since interface methods are assumed to be abstract,
the compiler throws an exception for using both abstract and final keywords on a
method.
57. Inheriting an Interface
There are two inheritance rules you should keep in mind when extending an
interface:
1. An interface that extends another interface, as well as an abstract class that
implements an interface, inherits all of the abstract methods as its own abstract
methods.
2. The first concrete class that implements an interface, or extends an abstract class
that implements an interface, must provide an implementation for all of the
inherited abstract methods.
An interface may be extended using the extend keyword. In this manner, the new
child interface inherits all the abstract methods of the parent interface.
58. Inheriting an Interface
Any class that implements the Seal interface must provide an implementation for
all methods in the parent interfaces— in this case, getTailLength() and
getNumberOfWhiskers().
The abstract class is treated in the same way as an interface extending another
interface.
59. Classes, Interfaces, and Keywords
Although a class can implement an interface, a class cannot extend an interface.
Likewise, whereas an interface can extend another interface, an interface cannot
implement another interface.
60. Abstract Methods and Multiple Inheritance
What will happen if you define a class that inherits from two interfaces that contain the
same abstract method:
The signatures for the two interface methods eatPlants() are compatible, so you can
define a class that fulfills both interfaces simultaneously:
61. Multiple Inheritance
What happens if the two methods have different signatures? If the method name is
the same but the input parameters are different, there is no conflict because this is
considered a method overload.
62. Multiple Inheritance
If the method name and input parameters are the same but the return types are
different between the two methods, the class or interface attempting to inherit
both interfaces will not compile.
63. Interface Variables
Like interface methods, interface variables are assumed to be public. Interface
variables are also assumed to be static and final.
Two interface variables rules:
1. Interface variables are assumed to be public, static, and final. Therefore, marking a
variable as private or protected will trigger a compiler error, as will marking any
variable as abstract.
2. The value of an interface variable must be set when it is declared since it is marked
as final.
64. Default Interface Methods
With the release of Java 8, the authors of Java have introduced a new type of
method to an interface, referred to as a default method.
A default method is a method defined within an interface with the default keyword
in which a method body is provided.
The purpose of adding default methods to the Java language was in part to help
with code development and backward compatibility.
By providing a default implementation of the method, though, the interface
becomes backward compatible with the existing codebase, while still providing
those individuals who do want to use the new method with the option to override
it.
65. Default Interface Methods
This example defines two interface methods, one is a normal abstract method and
the other a default method.
The following are the default interface method rules you need to be familiar with:
1. A default method may only be declared within an interface and not within a class
or abstract class.
2. A default method must be marked with the default keyword. If a method is marked
as default, it must provide a method body.
66. Default Interface Methods
3. A default method is not assumed to be static, final, or abstract, as it may be used or
overridden by a class that implements the interface.
4. Like all methods in an interface, a default method is assumed to be public and will
not compile if marked as private or protected.
When an interface extends another interface that contains a default method, it may
choose to ignore the default method
The interface may redeclare the method as abstract
67. Default Methods and Multiple Inheritance
If a class implements two interfaces that have default methods with the same name
and signature, the compiler will throw an error.
If the subclass overrides the duplicate default methods, the code will compile
without issue
68. Static Interface Methods
Java 8 also now includes support for static methods within interfaces.
These methods are defined explicitly with the static keyword and function nearly
identically to static methods defined in classes.
A static method defined in an interface is not inherited in any classes that
implement the interface.
1. Like all methods in an interface, a static method is assumed to be public and will
not compile if marked as private or protected.
2. To reference the static method, a reference to the name of the interface must be
used.
69. Static Interface Methods
The method getJumpHeight() works just like a static method as defined in a class.
The static interface methods are not inherited by a class
71. Understanding Polymorphism
This code compiles and executes without issue and yields the following output: 10
false true
The most important thing to note about this example is that only one object,
Lemur, is created and referenced.
Once the object has been assigned a new reference type, only the methods and
variables available to that reference type are callable on the object without an
explicit cast.
72. Object vs. Reference
All objects are accessed by reference.
Since all objects inherit java.lang.Object, they can all be reassigned to java.lang.Object.
Without an explicit cast back to Lemur, as you'll see in the next section, we no longer
have access to the Lemur properties of the object.
We can summarize this principle with the following two rules:
1. The type of the object determines which properties exist within the object in memory.
2. The type of the reference to the object determines which methods and variables are
accessible to the Java program.
73. Casting Objects
Once we changed the reference type, we lost access to more specific methods
defined in the subclass that still exist within the object.
Some basic rules to keep in mind when casting variables:
1. Casting an object from a subclass to a superclass doesn't require an explicit cast.
2. Casting an object from a superclass to a subclass requires an explicit cast.
3. The compiler will not allow casts to unrelated types.
74. Casting Objects
Casting is not without its limitations.
Although this code will compile without issue, it will throw a ClassCastException at
runtime since the object being referenced is not an instance of the Capybara class.
Note:Topic is out of scope for the OCA exam, keep in mind that the instanceof
operator can be used to check whether an object belongs to a particular class
75. Virtual Methods
The most important feature of polymorphism is to support virtual methods. A
virtual method is a method in which the specific implementation is not determined
until runtime.
In fact, all non-final, non-static, and non-private Java methods are considered
virtual methods, since any of them can be overridden at runtime.
What makes a virtual method special in Java is that if you call a method on an
object that overrides a method, you get the overridden method, even if the call to
the method is on a parent reference or within the parent class.
76. Virtual Methods
We'll illustrate this principle with the following example:
The bird name is: Peacock
77. Polymorphic Parameters
One of the most useful applications of polymorphism is the ability to pass
instances of a subclass or interface to a method. For example, you can define a
method that takes an instance of an interface as a parameter.
This property is referred to as polymorphic parameters of a method.
79. Polymorphism and Method Overriding
Let's conclude this chapter by returning to the last three rules for method
overriding to demonstrate how polymorphism requires them to be included as part
of the Java specification.
The first rule is that an overridden method must be at least as accessible as the
method it is overriding.
80. Polymorphism and Method Overriding
The Java compiler disallows overriding methods with new or broader exceptions.
Finally, overridden methods must use covariant return types for the same kinds of
reasons as just discussed.
81. Exam Essentials
Be able to write code that extends other classes. A Java class that extends another
class inherits all of its public and protected methods and variables. The first line of
every constructor is a call to another constructor within the class using this() or a
call to a constructor of the parent class using the super() call. If the parent class
doesn't contain a no-argument constructor, an explicit call to the parent
constructor must be provided. Parent methods and objects can be accessed
explicitly using the super keyword. Finally, all classes in Java extend java.lang.Object
either directly or from a superclass.
Understand the rules for method overriding. The Java compiler allows methods to
be overridden in subclasses if certain rules are followed: a method must have the
same signature, be at least as accessible as the parent method, must not declare
any new or broader exceptions, and must use covariant return types.
82. Exam Essentials
Understand the rules for hiding methods and variables. When a static method is re-
created in a subclass, it is referred to as method hiding. Likewise, variable hiding is when
a variable name is reused in a subclass. In both situations, the original method or
variable still exists and is used in methods that reference the object in the parent class.
For method hiding, the use of static in the method declaration must be the same
between the parent and child class. Finally, variable and method hiding should generally
be avoided since it leads to confusing and difficult-to-follow code.
Recognize the difference between method overriding and method overloading. Both
method overloading and overriding involve creating a new method with the same name
as an existing method. When the method signature is the same, it is referred to as
method overriding and must follow a specific set of override rules to compile. When the
method signature is different, with the method taking different inputs, it is referred to as
method overloading and none of the override rules are required.
83. Exam Essentials
Be able to write code that creates and extends abstract classes. In Java, classes and
methods can be declared as abstract. Abstract classes cannot be instantiated and
require a concrete subclass to be accessed. Abstract classes can include any number,
including zero, of abstract and nonabstract methods. Abstract methods follow all the
method override rules and may only be defined within abstract classes. The first
concrete subclass of an abstract class must implement all the inherited methods.
Abstract classes and methods may not be marked as final or private.
Be able to write code that creates, extends, and implements interfaces. Interfaces are
similar to a specialized abstract class in which only abstract methods and constant static
final variables are allowed. New to Java 8, an interface can also define default and static
methods with method bodies. All members of an interface are assumed to be public.
Methods are assumed to be abstract if not explicitly marked as default or static. An
interface that extends another interface inherits all its abstract methods. An interface
cannot extend a class, nor can a class extend an interface. Finally, classes may
implement any number of interfaces.
84. Exam Essentials
Be able to write code that uses default and static interface methods. A default
method allows a developer to add a new method to an interface used in existing
implementations, without forcing other developers using the interface to recompile
their code. A developer using the interface may override the default method or use
the provided one. A static method in an interface follows the same rules for a static
method in a class.
Understand polymorphism. An object in Java may take on a variety of forms, in part
depending on the reference used to access the object. Methods that are
overridden will be replaced everywhere they are used, whereas methods and
variables that are hidden will only be replaced in the classes and subclasses that
they are defined. It is common to rely on polymorphic parameters— the ability of
methods to be automatically passed as a superclass or interface reference— when
creating method definitions.
85. Exam Essentials
Recognize valid reference casting. An instance can be automatically cast to a
superclass or interface reference without an explicit cast. Alternatively, an explicit
cast is required if the reference is being narrowed to a subclass of the object. The
Java compiler doesn't permit casting to unrelated types. You should be able to
discern between compiler-time casting errors and those that will not occur until
runtime and that throw a CastClassException.
89. Answer
B, It fails to compile because the type of the exception the method throws is a
subclass of the type of exception the parent method throws.