Packages and the Standard Library

1A package names the folder the class lives in

A package is a namespace that matches a folder path. package cinema.desk; means the file sits under cinema/desk/ and the type's full name is cinema.desk.Ticket.

import cinema.desk.Ticket; lets another file write Ticket instead of the full name. Two classes named Ticket in two packages do not clash.

Figure. cinema.desk.Ticket is a path, not a decoration. The package line and the folder must match.

How it works

1. A package names the folder the class lives inpackage is the first line. The folder path must match the dots.

First line of Ticket.java

package cinema.desk;

public class Ticket {
}

2The classpath is where Java looks

The classpath is the list of directories and jar files the machine searches for compiled classes. If cinema.desk.Ticket is on disk but not on that list, run fails with a class-not-found error.

javac and java both need to see the same tree. Adding a library is adding its jar to that list — the JDBC lesson will do exactly that with Connector/J.

No diagram — the idea is carried by the prose or the Java listing.

3java.lang is already imported

java.lang holds Object, String, Math, System and the wrappers. You never write import java.lang.Object;.

Object is the parent every class already has. This lesson only needs that fact: the language package is the one you are already using.

No diagram — the idea is carried by the prose or the Java listing.

4Math is a bag of static functions

Math lives in java.lang and offers static methods: abs, max, min, pow, sqrt, round. There is no Math object to construct for ordinary desk work.

Meera's surcharge is Math.max(price, 150) so a floor fare still holds. The methods take primitives and return primitives.

Figure. Math.max(120, 150) returns 150. The methods are static on Math — there is no Math object to construct for ordinary desk work.

Static calls, no new Math()

int fare = Math.max(price, 150);
int cut = Math.min(price / 2, 80);
long rounded = Math.round(199.6);

5Wrappers box a primitive; autoboxing writes the box

A wrapper is an object that holds one primitive: Integer for int, Double for double, Boolean for boolean. Collections and generic types need objects, not primitives.

Autoboxing converts int to Integer when a method wants an object. Unboxing goes the other way. A null Integer unboxed to int throws — the primitive has no null.

No diagram — the idea is carried by the prose or the Java listing.

200 boxes to Integer; null cannot unbox

Integer boxed = 200;
int raw = boxed;
Integer missing = null;
// int crash = missing;  throws at run time

6Formatter writes text; Random draws numbers

java.util.Formatter (and String.format) fills a pattern: seat %s costs %d. The desk uses it for stubs, not for math.

java.util.Random draws numbers. nextInt(10) is 0..9. A seed makes the sequence repeatable in a test; without one, each run differs.

Figure. String.format fills %s %d with seat and price. Random.nextInt(10) is 0 inclusive to 10 exclusive. A seed makes that sequence repeatable; without one, each run differs.

How it works

1. Formatter writes text; Random draws numbersFormatter/String.format fill a pattern. Random.nextInt(n) is 0 inclusive to n exclusive.

format a stub; draw 0..9

import java.util.Random;
String stub = String.format("%s %d", seat, price);
Random r = new Random();
int lucky = r.nextInt(10);

7Instant is a moment; format it for humans

java.time.Instant is a point on the UTC timeline — when Meera bought the ticket, not a wall-clock in one city.

DateTimeFormatter turns an Instant (via a zone) into text a stub can print. Do not store that text as the source of truth; store the Instant and format at the edge.

Figure. Instant is the UTC moment Meera bought the ticket. DateTimeFormatter plus ZoneId.of("Asia/Kolkata") is the printed clock. Store the Instant; format at the edge.

How it works

1. Instant is a moment; format it for humansInstant is the moment. A formatter plus a zone is the printed clock time.

Store Instant; format with a zone

import java.time.Instant;
import java.time.ZoneId;
import java.time.format.DateTimeFormatter;

Instant bought = Instant.now();
DateTimeFormatter fmt = DateTimeFormatter
    .ofPattern("yyyy-MM-dd HH:mm")
    .withZone(ZoneId.of("Asia/Kolkata"));
System.out.println(fmt.format(bought));