---
title: Top-Level Definitions
layout: default
chapter: 9
---
# Top-Level Definitions
## Compilation Units
```ebnf
CompilationUnit ::= {‘package’ QualId semi} TopStatSeq
TopStatSeq ::= TopStat {semi TopStat}
TopStat ::= {Annotation} {Modifier} TmplDef
| Import
| Packaging
| PackageObject
|
QualId ::= id {‘.’ id}
```
A compilation unit consists of a sequence of packagings, import
clauses, and class and object definitions, which may be preceded by a
package clause.
A _compilation unit_
```scala
package $p_1$;
$\ldots$
package $p_n$;
$\mathit{stats}$
```
starting with one or more package
clauses is equivalent to a compilation unit consisting of the
packaging
```scala
package $p_1$ { $\ldots$
package $p_n$ {
$\mathit{stats}$
} $\ldots$
}
```
Every compilation unit implicitly imports the following packages, in the given order:
1. the package `java.lang`,
2. the package `scala`, and
3. the object [`scala.Predef`](12-the-scala-standard-library.html#the-predef-object), unless there is an explicit top-level import that references `scala.Predef`.
Members of a later import in that order hide members of an earlier import.
The exception to the implicit import of `scala.Predef` can be useful to hide, e.g., predefined implicit conversions.
## Packagings
```ebnf
Packaging ::= ‘package’ QualId [nl] ‘{’ TopStatSeq ‘}’
```
A _package_ is a special object which defines a set of member classes,
objects and packages. Unlike other objects, packages are not introduced
by a definition. Instead, the set of members of a package is determined by
packagings.
A packaging `package $p$ { $\mathit{ds}$ }` injects all
definitions in $\mathit{ds}$ as members into the package whose qualified name
is $p$. Members of a package are called _top-level_ definitions.
If a definition in $\mathit{ds}$ is labeled `private`, it is
visible only for other members in the package.
Inside the packaging, all members of package $p$ are visible under their
simple names. However this rule does not extend to members of enclosing
packages of $p$ that are designated by a prefix of the path $p$.
```scala
package org.net.prj {
...
}
```
all members of package `org.net.prj` are visible under their
simple names, but members of packages `org` or `org.net` require
explicit qualification or imports.
Selections $p$.$m$ from $p$ as well as imports from $p$
work as for objects. However, unlike other objects, packages may not
be used as values. It is illegal to have a package with the same fully
qualified name as a module or a class.
Top-level definitions outside a packaging are assumed to be injected
into a special empty package. That package cannot be named and
therefore cannot be imported. However, members of the empty package
are visible to each other without qualification.
## Package Objects
```ebnf
PackageObject ::= ‘package’ ‘object’ ObjectDef
```
A _package object_ `package object $p$ extends $t$` adds the
members of template $t$ to the package $p$. There can be only one
package object per package. The standard naming convention is to place
the definition above in a file named `package.scala` that's
located in the directory corresponding to package $p$.
The package object should not define a member with the same name as
one of the top-level objects or classes defined in package $p$. If
there is a name conflict, the behavior of the program is currently
undefined. It is expected that this restriction will be lifted in a
future version of Scala.
## Package References
```ebnf
QualId ::= id {‘.’ id}
```
A reference to a package takes the form of a qualified identifier.
Like all other references, package references are relative. That is,
a package reference starting in a name $p$ will be looked up in the
closest enclosing scope that defines a member named $p$.
The special predefined name `_root_` refers to the
outermost root package which contains all top-level packages.
###### Example
Consider the following program:
```scala
package b {
class B
}
package a.b {
class A {
val x = new _root_.b.B
}
}
```
Here, the reference `_root_.b.B` refers to class `B` in the
toplevel package `b`. If the `_root_` prefix had been
omitted, the name `b` would instead resolve to the package
`a.b`, and, provided that package does not also
contain a class `B`, a compiler-time error would result.
## Programs
A _program_ is a top-level object that has a member method
_main_ of type `(Array[String])Unit`. Programs can be
executed from a command shell. The program's command arguments are
passed to the `main` method as a parameter of type
`Array[String]`.
The `main` method of a program can be directly defined in the
object, or it can be inherited. The scala library defines a special class
`scala.App` whose body acts as a `main` method.
An objects $m$ inheriting from this class is thus a program,
which executes the initialization code of the object $m$.
###### Example
The following example will create a hello world program by defining
a method `main` in module `test.HelloWorld`.
```scala
package test
object HelloWorld {
def main(args: Array[String]) { println("Hello World") }
}
```
This program can be started by the command
```scala
scala test.HelloWorld
```
In a Java environment, the command
```scala
java test.HelloWorld
```
would work as well.
`HelloWorld` can also be defined without a `main` method
by inheriting from `App` instead:
```scala
package test
object HelloWorld extends App {
println("Hello World")
}
```
