Creating packages in Java - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Java package usage

What is the output of the following Java code when run?

Java

package com.example.utils;

public class Helper {
    public static String greet() {
        return "Hello from Helper!";
    }
}

// In another file:
package com.example.app;

import com.example.utils.Helper;

public class Main {
    public static void main(String[] args) {
        System.out.println(Helper.greet());
    }
}

A Hello from Helper!

B Compilation error: cannot find symbol Helper

C Runtime error: NoClassDefFoundError

D Hello from Main!

💡 Hint

Check how the import statement allows access to classes in other packages.

The class Helper is in package com.example.utils and is imported correctly in com.example.app.Main. Calling Helper.greet() prints the returned string.

❓ Predict Output

intermediate

Effect of missing package declaration

What happens if you remove the package declaration from a Java file that is supposed to be in a package?

Java

package mypkg;

public class Test {
    public static void main(String[] args) {
        System.out.println("In package mypkg");
    }
}

// Now remove the line 'package mypkg;' and compile and run.

A The program runs and prints 'In package mypkg'

B Compilation error: class not found in package

C Runtime error: ClassNotFoundException

D The program runs but prints nothing

💡 Hint

Think about what happens when no package is declared in a Java file.

If no package declaration is present, the class is in the default package and runs normally.

🔧 Debug

advanced

Identify the cause of compilation error with packages

Why does the following code cause a compilation error?

Java

package com.example;

public class A {
    public static void main(String[] args) {
        B b = new B();
        b.sayHi();
    }
}

// In file B.java:
package com.example.other;

public class B {
    public void sayHi() {
        System.out.println("Hi from B");
    }
}

A Class B's method sayHi is static but called on instance

B Class B is not public

C Class A does not import com.example.other.B

D Class A and B must be in the same package

💡 Hint

Check if class A can see class B without import.

Class A is in package com.example and tries to use class B from com.example.other without importing it, causing a compilation error.

📝 Syntax

advanced

Correct package declaration syntax

Which of the following is the correct way to declare a package in a Java file?

A package com.example.utils

B package com.example.utils;

C package com.example.utils()

D package com.example.utils {}

💡 Hint

Remember the package declaration ends with a semicolon.

The package declaration must end with a semicolon and not have parentheses or braces.

🚀 Application

expert

Number of classes accessible without import

Given these two packages and classes, how many classes can class Main access directly without import statements?

Java

package alpha;
public class A {}
public class B {}

package beta;
public class C {}

package alpha;
public class Main {
    public static void main(String[] args) {
        // How many of A, B, C can be used here without import?
    }
}

A 0 classes

B 3 classes (A, B, and C)

C 1 class (A only)

D 2 classes (A and B)

💡 Hint

Classes in the same package can be accessed without import.

Classes A and B are in the same package alpha as Main, so they can be accessed directly. Class C is in beta and requires import.

Practice

1. What is the main purpose of using packages in Java?

easy

A. To group related classes and organize code

B. To execute Java programs faster

C. To create graphical user interfaces

D. To store variables globally

Solution

1. Step 1: Understand what packages do

   Packages help organize classes by grouping related ones together.

2. Step 2: Identify the correct purpose

   Grouping related classes improves code organization and readability.

3. Final Answer:

   To group related classes and organize code -> Option A

4. Quick Check:

   Packages organize code = D [OK]

Hint: Packages group classes logically for better organization [OK]

Common Mistakes:

• Thinking packages speed up program execution
• Confusing packages with GUI creation
• Assuming packages store variables globally

2. Which of the following is the correct way to declare a package named com.example.app in a Java file?

easy

A. package com_example_app;

B. Package com.example.app;

C. package com.example.app;

D. import com.example.app;

Solution

1. Step 1: Check package declaration syntax

   The package statement must be lowercase and end with a semicolon.

2. Step 2: Match the correct syntax

   package com.example.app; uses correct lowercase 'package' keyword and dot notation with semicolon.

3. Final Answer:

   package com.example.app; -> Option C

4. Quick Check:

   Correct package syntax = C [OK]

Hint: Use lowercase 'package' and end with semicolon [OK]

Common Mistakes:

• Capitalizing 'Package' keyword
• Using underscores instead of dots
• Using 'import' instead of 'package'

3. Given the following Java file content, what will be the package of the class MyClass?

package org.test.project;

public class MyClass {
    public void display() {
        System.out.println("Hello from MyClass");
    }
}

medium

A. org.test.project

B. org.test

C. project

D. default package

Solution

1. Step 1: Read the package declaration

   The first line declares the package as 'org.test.project'.

2. Step 2: Identify the package of MyClass

   Since MyClass is declared after the package statement, it belongs to 'org.test.project'.

3. Final Answer:

   org.test.project -> Option A

4. Quick Check:

   Package line matches class package = B [OK]

Hint: Class package matches the declared package line [OK]

Common Mistakes:

• Assuming partial package names
• Thinking class is in default package without declaration
• Confusing package with folder names

4. Identify the error in the following Java file related to package declaration:

package MyApp.Utilities;

public class Helper {
    // class code
}

medium

A. Missing semicolon after package statement

B. Package name should be lowercase

C. Class name should be lowercase

D. Package keyword should be capitalized

Solution

1. Step 1: Check package naming conventions

   Package names should be all lowercase letters separated by dots.

2. Step 2: Identify the error

   'MyApp.Utilities' uses uppercase letters, which is against convention.

3. Final Answer:

   Package name should be lowercase -> Option B

4. Quick Check:

   Package names lowercase = A [OK]

Hint: Package names must be lowercase letters only [OK]

Common Mistakes:

• Using uppercase letters in package names
• Forgetting semicolon after package
• Capitalizing 'package' keyword

5. You want to create a package structure for a company named "TechCorp" with a project "Inventory" and a module "Reports". Which is the best package declaration for a Java class in the Reports module?

hard

A. package techcorp_inventory_reports;

B. package TechCorp.Inventory.Reports;

C. package tech_corp.inventory.reports;

D. package techcorp.inventory.reports;

Solution

1. Step 1: Follow Java package naming conventions

   Package names should be lowercase and use dots to separate levels.

2. Step 2: Evaluate options

   package techcorp.inventory.reports; uses all lowercase and dot notation correctly. Others use uppercase or underscores incorrectly.

3. Final Answer:

   package techcorp.inventory.reports; -> Option D

4. Quick Check:

   Lowercase with dots = A [OK]

Hint: Use lowercase names separated by dots for packages [OK]

Common Mistakes:

• Using uppercase letters in package names
• Using underscores instead of dots
• Combining all names without dots