CHAPTER 7: Packages

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7.3 Compilation Units

CompilationUnit is the goal symbol (S2.1) for the syntactic grammar (S2.3) of Java programs. It is defined by the following productions:

CompilationUnit:

	PackageDeclarationopt ImportDeclarationsopt TypeDeclarationsopt

ImportDeclarations:

	ImportDeclaration

	ImportDeclarations ImportDeclaration

TypeDeclarations:

	TypeDeclaration

	TypeDeclarations TypeDeclaration

Types declared in different compilation units can depend on each other, circularly. A Java compiler must arrange to compile all such types at the same time.

A compilation unit consists of three parts, each of which is optional:

• A package declaration (S7.4), giving the fully qualified name (S6.7) of the package to which the compilation unit belongs
• import declarations (S7.5) that allow types from other packages to be referred to using their simple names
• Type declarations (S7.6) of class and interface types

Every compilation unit automatically and implicitly imports every public type name declared in the predefined package java.lang , so that the names of all those types are available as simple names, as described in S7.5.3.