javaLogic errorMedium

HashMap Key Lookup Fails Due to Incorrect hashCode/equals

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BugFind the bug and fix the code

What it should do

Add a book to a library represented by a HashMap. Example: addBookToLibrary(book) -> void

What happens instead

The same book added multiple times or not found when it should be.

import java.util.HashMap;

class Book {
    String title;
    String author;
    
    Book(String title, String author) {
        this.title = title;
        this.author = author;
    }
}

class Library {
    HashMap<Book, Integer> books;
    
    Library() {
        books = new HashMap<>();
    }
    
    void addBookToLibrary(Book book) {
        books.put(book, books.getOrDefault(book, 0) + 1);
    }
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the logic errors pattern

The wrong operator, an inverted condition, a formula the right way round but backwards.

Learn the logic errors pattern Step through your own code

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import java.util.HashMap; class Book { String title; String author; Book(String title, String author) { this.title = title; this.author = author; } } class Library { HashMap<Book, Integer> books; Library() { books = new HashMap<>(); } void addBookToLibrary(Book book) { books.put(book, books.getOrDefault(book, 0) + 1); } }

HashMap Key Lookup Fails Due to Incorrect hashCode/equals

What it should do

What happens instead

More logic errors to practise

HashMap Key Lookup Fails Due to Incorrect hashCode/equals

What it should do

What happens instead

More logic errors to practise