Package: Naming, Type Imports

📖 Theory

Twenty-three files in one folder

BookBridge has grown: Book, Member, Librarian, FineReceipt, IssueRegister, OverdueException, Reminder... 23 files in one folder, and you just tried to add a second class called Report next to a teammate's Report.

A library solved this problem centuries ago: shelves with labels. Fiction here, reference there, same-titled books living peacefully on different shelves.

Java's labelled shelf is the package: a named group of related classes and interfaces.

📖 Theory

Declaring the shelf

One line at the top of the file puts a class on a shelf:

package circulation;

public class IssueRegister { ... }

Rules the exam checks:

• the package statement must be the first non-comment statement in the file, before any import or class
• the folder structure must mirror the name: circulation's classes live in a circulation/ folder; package com.bookbridge.util lives in com/bookbridge/util/
• convention: package names are lowercase, often a reversed domain (com.bookbridge) to stay globally unique

⌨️ Practical

Two files, two shelves

// file: circulation/IssueRegister.java
package circulation;              // first statement, folder matches

public class IssueRegister {
    public void note(String title) {
        System.out.println("Issued: " + title);
    }
}

// file: BookBridge.java (default package, for the demo)
import circulation.IssueRegister; // single-type import

public class BookBridge {
    public static void main(String[] args) {
        IssueRegister r = new IssueRegister();
        r.note("Let Us C");
    }
}

📖 Theory

Three ways to reach a shelved class

Once IssueRegister lives in circulation, other code has 3 access routes:

• Single-type import: import circulation.IssueRegister; then use the short name. Precise, self-documenting, preferred.
• On-demand import: import circulation.*; makes ALL of that package's classes available. Convenient, but hides where names come from.
• Fully qualified name, no import at all: circulation.IssueRegister r = new circulation.IssueRegister(); Verbose; useful when 2 packages both offer a Report and you need both.

🧩 Quiz

import java.util.*; is at the top of a file. Which statement is TRUE?

1. All classes of java.util AND its subpackages like java.util.regex are imported
2. Only the classes directly inside java.util are imported; subpackages are not
3. The entire class library is now imported, so the program compiles slower at run time
4. It is a compile error; the star form does not exist

💭 Think first

The import you never wrote

String, System, Math: you have used them since lesson 1 with no import statement anywhere. Before tapping: why do they work?

⚠️ Watch out

Where package files break

Misplaced package statement: any statement before package circulation; (even an import) is a compile error. Order forever: package, then imports, then the class.

Folder mismatch: declaring package circulation; while the file sits elsewhere compiles, then dies at run time with NoClassDefFoundError; the JVM looks for circulation/IssueRegister.class and finds nothing. Compile with javac -d . and the folder tree is built for you.

📖 Theory

You have been importing all semester

Nothing here is foreign: java.util, java.io and friends are just Sun's shelves, named by the reverse-domain convention with the domain shortened away. BookBridge now mirrors the pros: circulation for the desk classes, reports for month-end. The next lesson asks the natural follow-up: what does shelving do to ACCESS? That default no-modifier level from Unit 2, package-private, is about to finally show its purpose.

✅ Summary

Key takeaways

• A package is a named group of related classes; declare membership with package name; as the file's first statement.
• Folders mirror the dotted name: com.bookbridge.util lives in com/bookbridge/util/.
• Names are lowercase, conventionally reverse-domain for uniqueness.
• Three access routes: single-type import, on-demand import (package.*), or fully qualified names.
• The star imports one package's classes, never its subpackages.
• java.lang alone is auto-imported: String and System work import-free.
• Memory hook: package labels the shelf, import saves you reading the full address aloud.