78+ Java Coding Challenges for Practice

Combine repeated day entries and return the totals in sorted date order using TreeMap.

First Unique Customer with LinkedHashMap

Find the first customer name that appears exactly once by using LinkedHashMap.

Top Three Scores with PriorityQueue

Keep only the three highest scores by using a PriorityQueue.

Locker Lookup with HashMap

Build a student-to-locker lookup using HashMap and return the target locker number.

Duplicate Coupon Check with HashSet

Detect whether any coupon code appears more than once using HashSet.

Guest Queue with ArrayList

Append walk-in guests after the confirmed guest list using ArrayList.

Reverse a String

Write a Java method that takes a string and returns its reverse.

Java OOP 6

Practice object-oriented programming in Java through small class-based challenges that focus on constructors, fields, instance methods, and clean class design for real-world style tasks.

Alert Channel with Interface

Use an interface to send the same message through different alert channel implementations.

Pay Slip with Abstract Class

Use an abstract class to model an employee and calculate a pay slip from a concrete subclass.

Student Route Card with Inheritance

Use inheritance to build a student route card label from a base card class.

Snack Order Summary

Create an order item object and return a summary line that includes the item name, quantity, and total price.

Library Book Status

Create a Book object and return a status message based on whether the book is checked out.

Student Badge Label

Create a Student object and return the badge label text built from its data.

Stream API 12

Embrace the modern functional programming style in Java. Master the declaration of data processing pipelines using filter, map, and reduce to write highly readable, maintainable, and parallelizable code for large collections.

Catalog Price Merge with toMap()

Build a price lookup map from catalog entries using Java Stream API toMap() with a merge rule.

Room Schedule Merge with flatMap()

Flatten multiple room schedules into one cleaned session list using Java Stream API flatMap().

Club Signup Summary with groupingBy()

Group club sign-up slips into a summary map using Java Stream API groupingBy().

Welcome Board Line with joining()

Build one welcome-board string from many names using Java Stream API joining().

Passing Score Count with count()

Count how many scores reach the passing mark using Java Stream API count().

Name Tag Cleanup with map()

Clean and uppercase each name tag using Java Stream API map().

Handle Nullable Values Safely

Handle absent values without null checks

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Group Words by First Letter

Organize data into groups using collectors

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Transform Strings Using References

Simplify code with direct method references