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 follow step-by-step instructions to compile and run a simple Java app. The Java 101: Foundations mini-series On March 31, 2000, Java World launched the original "Learn Java from the ground up," the first article in the new Java 101 column by Jacob Weintraub. After three articles, Jacob passed the torch to me and I contributed 30 articles. Java has evolved greatly since then, so I've decided to update some of the most popular articles in the series with code and examples that are current with the Java platform today. The Java 101: Foundations mini-series will start with the basics of Java's programming model, with updates to classic articles from the original Java 101 column. I will eventually cover all of the Java language features and key Java SE APIs through Java SE 8u45. I'll also introduce newer topics that were never addressed in the original Java 101 series, such as JavaFX, NIO, concurrency, and the Java Collections API. All topics will be introduced in a logical and easy-to-follow sequence, with plenty of code to bring you up to speed with Java programming today. There's a lot to learn about 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 Micro-systems 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 (Hyper Text 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 run-time, 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 byte-codes (instructions and associated data) to platform-specific instructions through interpretation. Interpretation is the act of figuring out what a byte-code 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 run-time. Interpretation also makes it possible to delay the link step between the pieces of a Java program until run-time, 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 run-time (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 x equals 10).
☆ 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 Micro-systems 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 byte-code 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 byte-code 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 byte-code instruction sequences and compiles frequently interpreted instruction sequences to platform-specific instructions. Subsequent attempts to interpret these byte-code instruction sequences result in the execution of equivalent platform-specific instructions, resulting in a performance boost.
☆ Java is a multi-threaded 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 run-time, 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.
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