Java Language and Platform: A Beginner’s Guide to Write Once, Run Anywhere

Java Language

Java can refer to both the language and the platform. The language is a high‑level, object‑oriented language. Source code is saved in plain‑text files with the .java extension and compiled by the javac compiler into .class files that contain bytecode, the instruction set of the Java Virtual Machine (JVM). The JVM then interprets or JIT‑compiles this bytecode to native code at runtime.

Figure 1 shows a source file MyProgram.java compiled by javac into MyProgram.class. Because the .class file contains only bytecode, it can be deployed on any operating system that provides a compatible JVM.

Since Windows, Solaris, Linux and macOS all have JVM implementations, the same .class file can run on any of these platforms (see Figure 2). Some JVMs, such as HotSpot, perform additional runtime optimizations that improve performance.

Java Platform

A platform is the combination of hardware and software on which a program runs. The Java platform is a pure software platform that sits on top of underlying hardware‑OS platforms. It consists of two main components: the Java Virtual Machine and the Java Application Programming Interface (API).

The JVM provides the execution environment for bytecode, while the API is a large collection of pre‑implemented software components organized into packages that offer useful functionality to developers.

Although some may think a platform‑independent environment runs slower than native code, advances in compilers and JVM technology have brought Java’s performance close to that of native applications.