The Java Spec: Program Structure

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1 Program Structure

The source code for an Java program consists of one or more compilation units. Each compilation unit can contain only the following (in addition to white space and comments):

a package statement (see Packages)
import statements (see Packages)
class declarations (see Classes)
interface declarations (see Interfaces)

Although each Java compilation unit can contain multiple classes or interfaces, at most one class or interface per compilation unit can be public (see Classes).

When Java source code is compiled, the result is Java bytecode. Java bytecode consists of machine-independent instructions that can be interpreted efficiently by the Java runtime system. The Java runtime system operates like a virtual machine, for information see The Java Virtual Machine Specification.

In the current Java implementation, each compilation unit is a file with a ".java" suffix.

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The Java Language Specification

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