Checkpoint: Modern Java

This checkpoint consolidates the modern Java unit — var , text blocks, switch expressions, pattern matching, sealed types, record patterns, method references, and functional interfaces — into one build challenge you complete end to end.

Learn Checkpoint: Modern Java in our free Java course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Java course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

How This Checkpoint Works

First skim the recap, then tackle the multi-step build challenge on your own. A full, compiled solution is hidden under "Show solution" — try before you peek. Finish with the written checkpoint quiz and the timed quiz to confirm it all stuck.

Unit Recap — Eight Features

Quick Recap in Code

A single program touching several of the features at once: var , a Predicate + method reference, a switch expression, a text block, and an instanceof pattern.

🏗️ Build Challenge — A Shape Report

Model a small sealed hierarchy of shape records , then process it with a pattern switch that deconstructs each record — wiring in var , a text block, a guard, method references, and streams. Follow the five steps in the starter, aim for the exact expected output, and only then open the solution.

✅ Full Solution

Attempt the challenge first. When you're ready, expand the compiled, runnable solution below.

Notice the design: a sealed Shape of three records; describe and area are both exhaustive pattern switches with no default ; the when w == h guard splits squares from rectangles; and main uses var , a text block, Main::describe as a method reference, and a stream sum .

📝 Checkpoint Quiz

Answer: Shape is sealed and permits exactly Circle , Rectangle , and Triangle . With an arm for each, the compiler proves the switch is exhaustive, so a default is unnecessary — and omitting it means adding a fourth shape would turn the switch into a compile error until you handle it.

Answer: It is a record pattern . It matches a Circle and immediately deconstructs it, binding the component to r — so you use r directly instead of calling c.radius() .

Answer: A switch tries arms top to bottom and takes the first match. The plain case Rectangle(double w, double h) matches every rectangle, so the when w == h arm must precede it — otherwise the guarded arm would be unreachable, which is a compile error.

Answer: var shapes = List.of(...) is fine — a local with an initializer. var could not appear on the record components, the method parameters ( Shape s ), or the return types of area / describe , because those are part of the API contract.

Answer: Type::staticMethod — describe is a static method of Main . In .map(Main::describe) it stands in for s -> Main.describe(s) .

Answer: "Circle: area=%.2f".formatted(value) is the instance form of String.format ; %.2f inserts the area rounded to two decimals. There is no string interpolation in Java, so placeholders plus .formatted are the idiom.

Watch Out For

• ❌ Putting the plain Rectangle arm before the guarded one. Unreachable code. Fix: guard first.
• ❌ Adding a default to the sealed switch. Hides the safety net. Fix: rely on exhaustiveness.
• ❌ Using var on a record component or parameter. Illegal. Fix: declare the type.
• ❌ Wrong record-pattern arity. Each component needs a sub-pattern. Fix: Rectangle(double w, double h) .
• ❌ Expecting $ { area } interpolation in the text. None exists. Fix: %.2f + .formatted .
• ❌ Compiling on Java 20 or earlier. Record patterns need 21. Fix: use Java 21+.

Carry These Forward

• 💡 sealed + records + switch is the modern recipe for modelling and processing a closed set of cases safely.
• 💡 Let the compiler be your checklist — skip default on closed hierarchies so new cases surface as errors.
• 💡 Favour small functional pieces — predicates, mappers, and method references compose into readable pipelines.
• 💡 Reach for var and text blocks to cut boilerplate, but keep explicit types where they document intent.

📋 Quick Reference — Modern Java at a Glance

Feature

Syntax

Since

var

var x = expr;

Java 10

Text block

"""\n...\n"""

Java 15

Switch expression

var v = switch (k) {...} ;

Java 14

yield

case X -> { yield v; }

instanceof pattern

o instanceof String s

Java 16

sealed / permits

sealed ... permits A, B

Java 17

switch patterns

case Integer i -> ...

Java 21

Guard

case X x when cond -> ...

Record pattern

case Point(int x, int y)

Method reference

Type::method, Type::new

Java 8

Functional interface

Function, Predicate, ...

.formatted()

"%s".formatted(x)

📚 Resources / Keep Learning

• 📘 Oracle: Java Language Changes — every feature, version by version.
• 📗 dev.java: Learn Java — the official, up-to-date tutorial hub.
• 📙 Oracle: Pattern Matching — instanceof, switch, and record patterns in depth.

❓ Frequently Asked Questions

🎉 Checkpoint Cleared!

You combined var , text blocks, switch expressions, pattern matching, sealed types, record patterns, method references, and functional interfaces into a single working program — the heart of modern, data-oriented Java.

Next up: Stream Collectors Deep Dive — grouping, partitioning, joining, and summarising with the Collectors toolkit.

Practice quiz

Continue this course

• Previous: Built-in Functional Interfaces
• Next: Stream Collectors Deep Dive