So, you want to program in Java? That's great, and you've come to the right place. The Java 101 series provides a self-guided introduction to Java programming, starting with the basics and covering all the core concepts you need to know to become a productive Java developer. This series is technical, with plenty of code examples to help you grasp the concepts as we go along. I will assume that you already have some programming experience, just not in Java.
This inaugural article introduces the Java platform and explains the difference between its three editions: Java SE, Java EE, and Java ME. You'll also learn about the role of the Java virtual machine (JVM) in deploying Java applications. I'll help you set up a Java Development Kit (JDK) on your system so that you can develop and run Java programs, and I'll get you started with the architecture of a typical Java application. Finally, you'll learn how to compile and run a simple Java app.
What is Java?
You can think of Java as a general-purpose, object-oriented language that looks a lot like C and C++, but which is easier to use and lets you create more robust programs. Unfortunately, this definition doesn't give you much insight into Java. A more detailed definition from Sun Microsystems is as relevant today as it was in 2000:
Let's consider each of these definitions separately:
- Java is a simple language. Java was initially modeled after C and C++, minus some potentially confusing features. Pointers, multiple implementation inheritance, and operator overloading are some C/C++ features that are not part of Java. A feature not mandated in C/C++, but essential to Java, is a garbage-collection facility that automatically reclaims objects and arrays.
- Java is an object-oriented language. Java's object-oriented focus lets developers work on adapting Java to solve a problem, rather than forcing us to manipulate the problem to meet language constraints. This is different from a structured language like C. For example, whereas Java lets you focus on savings account objects, C requires you to think separately about savings account state (such a balance) and behaviors (such as deposit and withdrawal).
- Java is a network-savvy language. Java's extensive network library makes it easy to cope with Transmission Control Protocol/Internet Protocol (TCP/IP) network protocols like HTTP (HyperText Transfer Protocol) and FTP (File Transfer Protocol), and simplifies the task of making network connections. Furthermore, Java programs can access objects across a TCP/IP network, via Uniform Resource Locators (URLs), with the same ease as you would have accessing them from the local file system.
- Java is an interpreted language. At runtime, a Java program indirectly executes on the underlying platform (like Windows or Linux) via a virtual machine (which is a software representation of a hypothetical platform) and the associated execution environment. The virtual machine translates the Java program's bytecodes (instructions and associated data) to platform-specific instructions through interpretation. Interpretation is the act of figuring out what a bytecode instruction means and then choosing equivalent "canned" platform-specific instructions to execute. The virtual machine then executes those platform-specific instructions.
Interpretation makes it easier to debug faulty Java programs because more compile-time information is available at runtime. Interpretation also makes it possible to delay the link step between the pieces of a Java program until runtime, which speeds up development.
- Java is a robust language. Java programs must be reliable because they are used in both consumer and mission-critical applications, ranging from Blu-ray players to vehicle-navigation or air-control systems. Language features that help make Java robust include declarations, duplicate type checking at compile time and runtime (to prevent version mismatch problems), true arrays with automatic bounds checking, and the omission of pointers. (We will discuss all of these features in detail later in this series.)
Another aspect of Java's robustness is that loops must be controlled by Boolean expressions instead of integer expressions where 0 is false and a nonzero value is true. For example, Java doesn't allow a C-style loop such as
while (x) x++;because the loop might not end where expected. Instead, you must explicitly provide a Boolean expression, such as
while (x != 10) x++;(which means the loop will run until
- Java is a secure language. Java programs are used in networked/distributed environments. Because Java programs can migrate to and execute on a network's various platforms, it's important to safeguard these platforms from malicious code that might spread viruses, steal credit card information, or perform other malicious acts. Java language features that support robustness (like the omission of pointers) work with security features such as the Java sandbox security model and public-key encryption. Together these features prevent viruses and other dangerous code from wreaking havoc on an unsuspecting platform.
In theory, Java is secure. In practice, various security vulnerabilities have been detected and exploited. As a result, Sun Microsystems then and Oracle now continue to release security updates.
- Java is an architecture-neutral language. Networks connect platforms with different architectures based on various microprocessors and operating systems. You cannot expect Java to generate platform-specific instructions and have these instructions "understood" by all kinds of platforms that are part of a network. Instead, Java generates platform-independent bytecode instructions that are easy for each platform to interpret (via its implementation of the JVM).
- Java is a portable language. Architecture neutrality contributes to portability. However, there is more to Java's portability than platform-independent bytecode instructions. Consider that integer type sizes must not vary. For example, the 32-bit integer type must always be signed and occupy 32 bits, regardless of where the 32-bit integer is processed (e.g., a platform with 16-bit registers, a platform with 32-bit registers, or a platform with 64-bit registers). Java's libraries also contribute to portability. Where necessary, they provide types that connect Java code with platform-specific capabilities in the most portable manner possible.
- Java is a high-performance language. Interpretation yields a level of performance that is usually more than adequate. For very high-performance application scenarios Java uses just-in-time compilation, which analyzes interpreted bytecode instruction sequences and compiles frequently interpreted instruction sequences to platform-specific instructions. Subsequent attempts to interpret these bytecode instruction sequences result in the execution of equivalent platform-specific instructions, resulting in a performance boost.
- Java is a multithreaded language. To improve the performance of programs that must accomplish several tasks at once, Java supports the concept of threaded execution. For example, a program that manages a Graphical User Interface (GUI) while waiting for input from a network connection uses another thread to perform the wait instead of using the default GUI thread for both tasks. This keeps the GUI responsive. Java's synchronization primitives allow threads to safely communicate data between themselves without corrupting the data. (See threaded programming in Java discussed elsewhere in the Java 101 series.)
- Java is a dynamic language. Because interconnections between program code and libraries happen dynamically at runtime, it isn't necessary to explicitly link them. As a result, when a program or one of its libraries evolves (for instance, for a bug fix or performance improvement), a developer only needs to distribute the updated program or library. Although dynamic behavior results in less code to distribute when a version change occurs, this distribution policy can also lead to version conflicts. For example, a developer removes a class type from a library, or renames it. When a company distributes the updated library, existing programs that depend on the class type will fail. To greatly reduce this problem, Java supports an interface type, which is like a contract between two parties. (See interfaces, types, and other object-oriented language features discussed elsewhere in the Java 101 series.)
Unpacking this definition teaches us a lot about Java. Most importantly, it reveals that Java is both a language and a platform. I'll have more to say about Java platform components -- namely the Java virtual machine and Java execution environment -- later in this article.
Three editions of Java
Sun Microsystems released the Java 1.0 software development kit (JDK) in 1995. The first JDK was used to develop desktop applications and applets, and Java subsequently evolved to encompass enterprise-server and mobile-device programming. Storing all of the necessary libraries in a single JDK would have made the JDK too large to distribute, especially because distribution in the 1990s was limited by small-size CDs and slow network speeds. Since most developers didn't need every last API (a desktop application developer would hardly need to access enterprise Java APIs), Sun solved the distribution issue by factoring Java into three main editions. These eventually became known as Java SE, Java EE, and Java ME:
- Java Platform, Standard Edition (Java SE) is the Java platform for developing client-side applications, which run on desktops, and applets, which run in web browsers.
- Java Platform, Enterprise Edition (Java EE) is the Java platform built on top of Java SE, which is used exclusively to develop enterprise-oriented server applications. Server-side applications include servlets, which are Java programs that are similar to applets but run on a server rather than a client. Servlets conform to the Java EE Servlet API.
- Java Platform, Micro Edition (Java ME) is also built on top of Java SE. It is the Java platform for developing MIDlets, which are Java programs that run on mobile information devices, and Xlets, which are Java programs that run on embedded devices.
Java SE is the foundation platform for Java and is the focus for this series. Code examples will be based on the most recent version of Java at the time of writing, which is currently Java SE 8 update 45.
Overview of the Java platform
Java is both a programming language and a platform for running compiled Java code. This platform consists mainly of the JVM, but also includes an execution environment that supports the JVM's execution on the underlying (native) platform. The JVM includes several components for loading, verifying, and executing Java code. Figure 1 shows how a Java program executes on this platform.
At the top of the diagram is a series of program class files, and one of these class files is denoted as the main class file. A Java program consists of at least the main class file, which is the first class file to be loaded, verified, and executed.
The JVM delegates class loading to its classloader component. Classloaders load class files from various sources, such as file systems, networks, and archive files. They insulate the JVM from the intricacies of class loading.
A loaded class file is stored in memory and represented as an object created from the
Class class. Once loaded, the bytecode verifier verifies the various bytecode instructions to ensure that they are valid and won't compromise security.
If the class file's bytecodes are not valid, the JVM terminates. Otherwise, its interpreter component interprets the bytecode one instruction at a time. Interpretation identifies bytecode instructions and executes equivalent native instructions.
Some bytecode instruction sequences execute more frequently than others. When the interpreter detects this situation, the JVM's just-in-time (JIT) compiler compiles the bytecode sequence to native code for faster execution.