Packages and imports

Two classes called Student

What are packages and imports?

Words you'll meet in this lesson

The kinds of import

Packages you'll use, and how to name your own

Imports, full names and java.lang

import java.time.LocalDate;
import java.util.*;
import static java.lang.Math.max;

public class Main {
    public static void main(String[] args) {
        LocalDate due = LocalDate.of(2025, 7, 15);           // single-type import
        List<String> fees = new ArrayList<>();            // from java.util.*
        fees.add("Tuition");
        java.math.BigDecimal gst = new java.math.BigDecimal("0.18");   // full name, no import
        String s = String.valueOf(max(3, 7));                // String: java.lang; max: static import
        System.out.println(due + " " + fees + " " + gst + " " + s);
    }
}

When two classes have the same simple name

// import java.util.*;  import java.sql.*;   then:  Date d;
error: reference to Date is ambiguous
  both class java.sql.Date in java.sql and class java.util.Date in java.util match

// import java.util.Date;  import java.sql.Date;
error: a type with the same simple name is already defined by the single-type-import of Date

Fix: import one of them and write the other's full name, e.g. java.sql.Date sqlDate = …;.

Building and running a two-package program

1. Put each class in the folder of its package

   src/in/myphatshala/fees/FeeCalc.java
   src/in/myphatshala/app/App.java

   Folder names follow the package names exactly, one folder per dot-separated part.

2. Declare the package and import what you need

   package in.myphatshala.fees;
   
   public class FeeCalc {
       public static int monthly(int annual) { return annual / 12; }
   }

   package in.myphatshala.app;
   
   import in.myphatshala.fees.FeeCalc;
   
   public class App {
       public static void main(String[] args) {
           System.out.println(FeeCalc.monthly(60000));
       }
   }

   FeeCalc and monthly must be public, because App is in a different package (Lesson 6.5).

3. Compile with -d so javac creates the package folders

   > javac -d out src/in/myphatshala/fees/FeeCalc.java src/in/myphatshala/app/App.java
   
   out/in/myphatshala/fees/FeeCalc.class
   out/in/myphatshala/app/App.class

   javac writes each class file under out/ in the folder path of its package.

4. Run with the classpath and the fully qualified name

   > java -cp out in.myphatshala.app.App
   5000
   > java -cp out App
   Error: Could not find or load main class App
   Caused by: java.lang.ClassNotFoundException: App

   -cp out tells the JVM where the package folders start. The class must be named in full: in.myphatshala.app.App. The class loader converts the dots to folders and finds out/in/myphatshala/app/App.class.

5. In real projects, a build tool does this

   Maven and Gradle use the standard layout src/main/java/<package folders>, compile into target/classes or build/classes, and package everything into a JAR. IDEs create the folders when you create a package.

How javac turns a short name into a class

Left: the names you write in your code. Right: how javac resolves each one to a fully qualified class, the order it checks, and what ends up in the class file.

Packages in real projects

In a real codebase, the package list is the map of the system. New team members read it first.

Watch it with an example: the class loader finds App by its address

You type java -cp out in.myphatshala.app.App. Watch the JVM turn the package name into folders, find each class file, and load FeeCalc from another package only when main first needs it.

Examples are simplified. Since Java 9, related packages can be grouped into a module (module-info.java) that states which packages it exports.

A fee reminder using four packages

The online editor runs single files, so this program uses classes from four JDK packages instead of its own package folders.

import java.time.LocalDate;
import java.time.temporal.ChronoUnit;
import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        LocalDate today = LocalDate.of(2025, 7, 1);
        LocalDate due = LocalDate.of(2025, 7, 15);
        long days = ChronoUnit.DAYS.between(today, due);

        List<String> reminders = new ArrayList<>();
        reminders.add("Tuition fee due in " + days + " days");
        reminders.add("Hostel fee due on " + due);

        java.math.BigDecimal fee = new java.math.BigDecimal("45000.50");
        System.out.println(reminders);
        System.out.println("Amount: " + fee);
    }
}

• java.time and java.time.temporalTwo separate packages, so two imports.
• import java.util.List;Single-type imports say exactly which List: never java.awt.List by accident.
• java.math.BigDecimalUsed once, written in full, no import needed.
• String, SystemFrom java.lang, available everywhere.

Classes from other packages at run time

At run time packages are just part of each class's full name. Watch objects of java.util.ArrayList and java.time.LocalDate appear on the heap like any other object.

Worked examples: real problems, solved and traced in memory

Three small programs that use classes from several JDK packages. Each one needs the right imports, and one shows how to handle two classes with the same name.

today 2025-03-01, exam 2025-03-20

Exam on 2025-03-20: 19 days left

0.10 + 0.20

BigDecimal: 0.30
double:     0.30000000000000004

DB date 2025-01-26

from DB: 2025-01-26
same instant as java.util.Date: true

Check yourself

Quick revision

Interview and viva questions

Practice problems

Fix missing and clashing imports, and practise using classes from several packages. The last three are business tasks.

items Tea, Vada

[Tea, Vada]

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> menu = new ArrayList<>();
        menu.add("Tea");
        menu.add("Vada");
        System.out.println(menu);
    }
}

2025-08-15

2025-08-15 is a FRIDAY

public class Main {
    public static void main(String[] args) {
        java.time.LocalDate d = java.time.LocalDate.of(2025, 8, 15);
        System.out.println(d + " is a " + d.getDayOfWeek());
    }
}

r = 7

154 sq m

import static java.lang.Math.PI;

public class Main {
    public static void main(String[] args) {
        double r = 7;
        double area = PI * r * r;
        System.out.println(Math.round(area) + " sq m");
    }
}

names Riya, Kabir

[Riya, Kabir]

import java.util.*;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Riya");
        names.add("Kabir");
        System.out.println(names);
    }
}

text "Asha 249.50", date 2025-02-14

2025-02-14 | Asha paid Rs 249.50

import java.math.BigDecimal;
import java.time.LocalDate;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner("Asha 249.50");
        String name = sc.next();
        BigDecimal amount = new BigDecimal(sc.next());
        LocalDate date = LocalDate.of(2025, 2, 14);
        System.out.println(date + " | " + name + " paid Rs " + amount);
    }
}

today 2025-04-10; due dates 2025-04-05, 2025-04-20

INV-101 due 2025-04-05: OVERDUE
INV-102 due 2025-04-20: OK

import java.time.LocalDate;

public class Main {
    public static void main(String[] args) {
        LocalDate today = LocalDate.of(2025, 4, 10);
        String[] ids = {"INV-101", "INV-102"};
        LocalDate[] due = {LocalDate.of(2025, 4, 5), LocalDate.of(2025, 4, 20)};
        for (int i = 0; i < ids.length; i++) {
            String status = due[i].isBefore(today) ? "OVERDUE" : "OK";
            System.out.println(ids[i] + " due " + due[i] + ": " + status);
        }
    }
}

two orders

ID length: 36
Different: true

import java.util.UUID;

public class Main {
    public static void main(String[] args) {
        String a = UUID.randomUUID().toString();
        String b = UUID.randomUUID().toString();
        System.out.println("ID length: " + a.length());
        System.out.println("Different: " + !a.equals(b));
    }
}

prices 1250, 125000, 12500000

Rs 1,250
Rs 125,000
Rs 12,500,000

import java.text.NumberFormat;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        NumberFormat f = NumberFormat.getNumberInstance(Locale.forLanguageTag("en-IN"));
        long[] prices = {1250, 125000, 12500000};
        for (long p : prices) {
            System.out.println("Rs " + f.format(p));
        }
    }
}

Real-world scenario questions (asked in company interviews)

Package questions in interviews are usually about build problems and name clashes. Think it through, then open the model answer.