Package

Canonicalization

What we’d really like is the ability to provide a canonical name for everything.

• Canonical representation: A unique representation for a thing.
• Not-canonical: License plate number (can be reused, can change).
• Canonical: The VIN number JYA3AWC04VA071286 (refers to a specific motorcycle).

To address the fact that classes might share names: In Java, we (attempt to) provide canonicity through by giving every a class a “package name”.

• A package is a namespace that organizes classes and interfaces.
• Naming convention: Package name starts with website address (backwards).

Creation

Two steps:

• At the top of every file in the package, put the package name.
• Make sure that the file is stored in a folder with the appropriate folder name.
  For a package with name ug.joshh.animal, use folder ug/joshh/animal.

Use

1. If used from the outside, use entire canonical name.
2. By using an import statement, we can use the simple name instead.
3. If used from another class in same package, can just use simple name.

The Default Package

Any Java class without a package name at the top are part of the “default” package.

You should avoid the default package except for very small example programs.

Note: We cannot import code from the default package!

PS: About import

JAR Files

Usage

Sharing dozens of .class files in special directories is annoy, we can instead share a single .jar file that contains all of .class files.
In fact, JAR files are zip file with some extra information added.

Note:

• They do not keep our code safe!
• Easy to unzip and transform back into .java files.