Before you begin
This tutorial is part of the Introduction to Java™ programming series.
|101||Set up your Java development environment and learn basic object-oriented programming principles||Tutorial|
|102||Java language basics||Tutorial|
|103||Writing good Java code||Tutorial|
|201||Java constructs for real-world applications, Part 1||Tutorial|
|202||Java constructs for real-world applications, Part 2||Tutorial|
Although the concepts discussed in the individual tutorials are standalone in nature, the hands-on component builds as you progress through the series. I recommend that you review the prerequisites, setup, and series details before proceeding.
In this tutorial:
- Understand the function of each of the Java platform’s constituent components
- Learn how the Java language is structured
- Become familiar with navigating the Java API documentation
- Download and install the JDK and the Eclipse IDE
- Set up your Eclipse development environment
- Understand the main Eclipse components and how to use them for Java development
- Create a new Java project in Eclipse
- Grasp how the object-oriented paradigm differs from the structured-programming paradigm
- Know the key characteristics of an object
- Understand the benefits that stem from the defining principles of object-oriented programming (OOP)
Java platform overview
Java technology is used to develop applications for a wide range of environments, from consumer devices to heterogeneous enterprise systems. In this section, get a high-level view of the Java platform and its components.
The Java language
Like any programming language, the Java language has its own structure, syntax rules, and programming paradigm. The Java language’s programming paradigm is based on the concept of object-oriented programming (OOP), which the language’s features support.
The Java language is a C-language derivative, so its syntax rules look much like C’s. For example, code blocks are modularized into methods and delimited by braces (
}), and variables are declared before they are used.
Structurally, the Java language starts with packages. A package is the Java language’s namespace mechanism. Within packages are classes, and within classes are methods, variables, constants, and more. Learn more about the parts of the Java language in the “Java language basics” tutorial.
The Java compiler
When you program for the Java platform, you write source code in .java files and then compile them. The compiler checks your code against the language’s syntax rules, then writes out bytecode in .class files. Bytecode is a set of instructions targeted to run on a Java virtual machine (JVM). In adding this level of abstraction, the Java compiler differs from other language compilers, which write out assembly-language instructions suitable for the CPU chipset the program will run on.
At runtime, the JVM reads and interprets .class files and executes the program’s instructions on the native hardware platform for which the JVM was written. The JVM interprets the bytecode just as a CPU would interpret assembly-language instructions. The difference is that the JVM is a piece of software written specifically for a particular platform. The JVM is the heart of the Java language’s “write-once, run-anywhere” principle. Your code can run on any chipset for which a suitable JVM implementation is available. JVMs are available for major platforms like Linux™ and Windows®, and subsets of the Java language have been implemented in JVMs for mobile phones and hobbyist chips.
The garbage collector
Rather than forcing you to keep up with memory allocation (or use a third-party library to do so), the Java platform provides memory management out of the box. When your Java application creates an object instance at runtime, the JVM automatically allocates memory space for that object from the heap— a pool of memory set aside for your program to use. The Java garbage collector runs in the background, keeping track of which objects the application no longer needs and reclaiming memory from them. This approach to memory handling is called implicit memory management because it doesn’t require you to write any memory-handling code. Garbage collection is one of the essential features of Java platform performance.
The Java Development Kit
When you download a Java Development Kit (JDK), you get — in addition to the compiler and other tools — a complete class library of prebuilt utilities that help you accomplish most common application-development tasks. The best way to get an idea of the scope of the JDK packages and libraries is to check out the official online Java API documentation— also called the Javadoc. Watch the following quick demo to see how to get around in the Javadoc.
The Java Runtime Environment
The Java Runtime Environment (JRE; also known as the Java runtime) includes the JVM, code libraries, and components that are necessary for running programs that are written in the Java language. The JRE is available for multiple platforms. You can freely redistribute the JRE with your applications, according to the terms of the JRE license, to give the application’s users a platform on which to run your software. The JRE is included in the JDK.
Setting up your Java development environment
In this section, you’ll download and install the JDK and the current release of the Eclipse IDE, and you’ll set up your Eclipse development environment.
Your development environment
The JDK includes a set of command-line tools for compiling and running your Java code, including a complete copy of the JRE. Although you can use these tools to develop your applications, an IDE gives you additional functionality along with task management and a visual interface.
In this series, you use Eclipse, a popular open source IDE. Eclipse handles basic tasks, such as code compilation and debugging, so that you can focus on writing and testing code. In addition, you can use Eclipse to organize source code files into projects, compile and test those projects, and store project files in any number of source repositories. You need an installed JDK to use Eclipse for Java development.
Install the JDK
Follow these steps to download and install AdoptOpenJDK:
- Browse to the AdoptOpenJDK website.
- Choose OpenJDK 11 (LTS) as the version.
- Choose OpenJ9 as the JVM.
Click the Latest release download button to download the package.
Should you need to choose a different platform, select the Other plaftorms button, then navigate to your platform and download the associated package.
Follow the instructions for your platform (they will be slightly different for each platform).
You now have a Java environment on your computer. Next, you’ll install the Eclipse IDE and create a Java project in Eclipse.
Install and set up Eclipse
Follow along with this video demo to download and install Eclipse on your system, take a quick Eclipse tour, and create a Java project.
Recap: The Eclipse development environment
Eclipse is more than an IDE; it’s an entire development ecosystem. This section is a brief hands-on introduction to using Eclipse for Java development.
The Eclipse development environment has four main components:
The primary unit of organization in Eclipse is the workspace. A workspace contains all of your projects. A perspective is a way of looking at each project (hence the name), and within a perspective are one or more views.
The following figure shows the Java perspective, which is the default perspective for Eclipse. You see this perspective when you start Eclipse.
The Java perspective contains the tools that you need to begin writing Java applications. Each tabbed window shown in Figure 1 is a view for the Java perspective. Package Explorer and Outline are two particularly useful views.
The Eclipse environment is highly configurable. Each view is dockable, so you can move it around in the Java perspective and place it where you want it. For now, though, stick with the default perspective and view setup.
Now that you’ve created a new Eclipse Java project and source folder, your development environment is ready for action. However, an understanding of the OOP paradigm — covered in the next section — is essential before you start coding in Java.
Object-oriented programming concepts and principles
The Java language is (mostly) object-oriented. This section is an introduction to object-oriented programming (OOP) language concepts, using structured programming as a point of contrast.
What is an object?
Object-oriented languages follow a different programming pattern from structured programming languages like C and COBOL. The structured-programming paradigm is highly data oriented: You have data structures, and then program instructions act on that data. Object-oriented languages such as the Java language combine data and program instructions into objects.
An object is a self-contained entity that contains attributes and behavior, and nothing more. Instead of having a data structure with fields (attributes) and passing that structure around to all of the program logic that acts on it (behavior), in an object-oriented language, data and program logic are combined. This combination can occur at vastly different levels of granularity, from fine-grained objects such as a
Number, to coarse-grained objects, such as a
FundsTransfer service in a large banking application.
Parent and child objects
A parent object is one that serves as the structural basis for deriving more-complex child objects. A child object looks like its parent but is more specialized. With the object-oriented paradigm, you can reuse the common attributes and behavior of the parent object, adding to its child objects attributes and behavior that differ.
Object communication and coordination
Objects talk to other objects by sending messages (method calls, in Java parlance). Furthermore, in an object-oriented application, program code coordinates the activities among objects to perform tasks within the context of the specific application domain.
A well-written object:
- Has well-defined boundaries
- Performs a finite set of activities
- Knows only about its data and any other objects that it needs to accomplish its activities
In essence, an object is a discrete entity that has only the necessary dependencies on other objects to perform its tasks.
It’s time to see what a Java object looks like.
Example: A person object
My first example is based on a common application-development scenario: an individual being represented by a
You know from the definition of an object that an object has two primary elements: attributes and behavior. Here’s how these elements apply to the
As a rule of thumb, think of the attributes of an object as nouns and behavior as verbs.
What attributes can a person have? Some common ones include:
- Eye color
You can probably think of more (and you can always add more attributes later), but this list is a good start.
An actual person can do all sorts of things, but object behaviors usually relate to application context of some kind. In a business-application context, for instance, you might want to ask your
Person object, “What is your body mass index (BMI)?” In response,
Person would use the values of its height and weight attributes to calculate the BMI.
More-complex logic can be hidden inside of the
Person object, but for now, suppose that
Person has the following behavior:
- Calculate BMI
- Print all attributes
State and string
State is an important concept in OOP. An object’s state is represented at any moment in time by the values of its attributes.
In the case of
Person, its state is defined by attributes such as name, age, height, and weight. If you wanted to present a list of several of those attributes, you might do so by using a
String class, which you’ll learn more about later.
Using the concepts of state and string together, you can say to
Person, “Tell me all about you by giving me a listing (or
String) of your attributes.”
Principles of OOP
If you come from a structured-programming background, the OOP value proposition might not be clear yet. After all, the attributes of a person and any logic to retrieve (and convert) those values can be written in C or COBOL. The benefits of the OOP paradigm become clearer if you understand its defining principles: encapsulation, inheritance, and polymorphism.
Recall that an object is above all discrete, or self-contained. This characteristic is the principle of encapsulation at work. Hiding is another term that’s sometimes used to express the self-contained, protected nature of objects.
Regardless of terminology, what’s important is that the object maintains a boundary between its state and behavior and the outside world. Like objects in the real world, objects used in computer programming have various types of relationships with different categories of objects in the applications that use them.
On the Java platform, you can use access modifiers (which you’ll learn about later) to vary the nature of object relationships from public to private. Public access is wide open, whereas private access means the object’s attributes are accessible only within the object itself.
The public/private boundary enforces the object-oriented principle of encapsulation. On the Java platform, you can vary the strength of that boundary on an object-by-object basis. Encapsulation is a powerful feature of the Java language.
In structured programming, it’s common to copy a structure, give it a new name, and add or modify the attributes that make the new entity (such as an
Account record) different from its original source. Over time, this approach generates a great deal of duplicated code, which can create maintenance issues.
OOP introduces the concept of inheritance, whereby specialized classes — without additional code — can “copy” the attributes and behavior of the source classes that they specialize. If some of those attributes or behaviors need to change, you override them. The only source code you change is the code needed for creating specialized classes. The source object is called the parent, and the new specialization is called the child — terms that you’ve already been introduced to.
Suppose that you’re writing a human-resources application and want to use the
Person class as the basis (also called the super class) for a new class called
Employee. Being the child of
Employee would have all of the attributes of a
Person class, along with additional ones, such as:
- Taxpayer identification number
- Employee number
Inheritance makes it easy to create the new
Employee class without needing to copy all of the
Person code manually.
Polymorphism is a harder concept to grasp than encapsulation and inheritance. In essence, polymorphism means that objects that belong to the same branch of a hierarchy, when sent the same message (that is, when told to do the same thing), can manifest that behavior differently.
To understand how polymorphism applies to a business-application context, return to the
Person example. Remember telling
Person to format its attributes into a
String? Polymorphism makes it possible for
Person to represent its attributes in various ways depending on the type of
Person it is.
Polymorphism, one of the more complex concepts you’ll encounter in OOP on the Java platform, is beyond the scope of this introductory course. You’ll explore encapsulation and inheritance in more depth in subsequent tutorials.
Not a purely object-oriented language
Two qualities differentiate the Java language from purely object-oriented languages such as Smalltalk. First, the Java language is a mixture of objects and primitive types. Primitive types are the language’s most basic data types — the building blocks for data manipulation. Second, with Java, you can write code that exposes the inner workings of one object to any other object that uses it.
The Java language does give you the tools necessary to follow sound OOP principles and produce sound object-oriented code. Because Java is not purely object-oriented, you must exercise discipline in how you write code — the language doesn’t force you to do the right thing, so you must do it yourself. You can find tips in the tutorial “Writing good Java code.”
In this tutorial, you learned about object-oriented programming and familiarized yourself with an IDE that helps you control your development environment.
The next tutorial in this series focuses on the basics of the language, providing you with enough knowledge and practice to write simple programs.