Java platform overview

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

Get to know the Java APIs

Most Java developers constantly reference the official online Java API documentatiON— also called the Javadoc. By default, you see three panes in the Javadoc. The top-left pane shows all of the packages in the API, and the bottom-left pane shows the classes in each package. The main pane (to the right) shows details for the currently selected package or class. For example, if you click the java.util package in the top-left pane and then click the ArrayList class listed below it, you see details about ArrayList in the right pane, including a description of what it does, how to use it, and its methods.
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 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 }), 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. You learn about the parts of the Java language in this 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 instructions suitable for the CPU chipset the program will run on.

The JVM

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 managementbecause it doesn't require you to write any memory-handling code. Garbage collection is one of the essential features of Java platform performance.

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