Using Constructors in Java: Solution, Explanation & Practice

Problem summary

Create a Book class with a constructor that takes title and author.

Starter code

// Create Book class with constructor(title, author)
// Add getInfo() method

public class Main {
    public static void main(String[] args) {
        // Create book: "1984" by "George Orwell"
        // Print book info
    }
}

Expected output and test cases

• Book info

  1984 by George Orwell

Hints

1. Constructor has same name as class
2. No return type for constructors
3. Use 'this' to refer to instance variables

Validated solution

class Book {
    private final String title;
    private final String author;

    Book(String title, String author) {
        if (title.isBlank() || author.isBlank()) throw new IllegalArgumentException("book details required");
        this.title = title;
        this.author = author;
    }
    String description() { return title + " by " + author; }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(new Book("1984", "George Orwell").description());
    }
}

How to approach the problem

A constructor establishes the values an object needs before it can be used. Private final fields make the Book immutable after construction, while one descriptive method controls its presentation.

Approach

1. Choose the fields required for a valid Book.
2. Validate arguments at construction.
3. Assign through this.field to distinguish fields from parameters.

Time and space complexity

Time: O(1). Space: O(1).

Edge cases to test

• Blank titles and authors are rejected.
• Null has to be rejected separately if it is allowed to reach this constructor.

Common mistakes

• Leaving fields uninitialized and creating a half-valid object.
• Using static fields for data that belongs to each book.

Follow-up challenge

Add an ISBN value object and define equality for two editions.

Related OOP Basics exercises

• Practice Create a Simple Class in Java
• Practice Getters and Setters in Java
• Practice Static Members in Java

Practice all OOP Basics exercises · Run this idea in the Java compiler