Day 2: Intermediate Java · Learn Java 25

More Java Ideas

Mon, 01 Jan 0001 00:00:00 +0000

Converting types double d = 42; // widening: automatic int y = (int) 3.99; // narrowing: cast, truncates → 3 int r = (int) Math.round(3.99); // 4 int small = (int) bigLong; // may overflow silently Text ↔ number: Integer.parseInt("42") Double.parseDouble("3.14") Boolean.parseBoolean("true") String.valueOf(42) Integer.toString(42) Integer.parseInt("abc") // NumberFormatException Integer.parseInt("ff", 16) // 255 Integer.toBinaryString(10) // "1010" Integer.toHexString(255) // "ff" 0b1010 0xFF // literals Strings, part 2 "a;b;c".split(";") String.join("|", parts) "line1\nline2".lines().toList() "abc".compareTo("abd") // negative "Hello".toCharArray() "%-10s|".formatted("left") // "left |" "%,.2f".formatted(1234567.891) // "1,234,567.89" "%08.3f".formatted(3.14159) // "0003.142" String.format(Locale.US, "%,.2f", 1999.5); Enums A type with a fixed set of values:

Lambdas & Streams

Mon, 01 Jan 0001 00:00:00 +0000

Lambdas A lambda is an unnamed function. It fits anywhere a functional interface — an interface with exactly one method — is expected. Runnable task = () -> IO.println("running"); Predicate isLong = s -> s.length() > 5; BinaryOperator add = (a, b) -> a + b; Function pretty = s -> { var t = s.strip().toLowerCase(); return t.substring(0, 1).toUpperCase() + t.substring(1); }; Interface Shape Used by Predicate T → boolean filter Function T → R map Consumer T → void forEach Supplier () → T lazy values UnaryOperator T → T replaceAll Comparator (T,T) → int sorted Method references s -> s.toUpperCase() → String::toUpperCase s -> IO.println(s) → IO::println s -> Integer.parseInt(s) → Integer::parseInt () -> new ArrayList<>() → ArrayList::new x -> obj.handle(x) → obj::handle Streams Source → any number of intermediate steps → exactly one terminal operation.

Object Oriented Java

Mon, 01 Jan 0001 00:00:00 +0000

Classes public class Account { private final String owner; // state private long cents; public Account(String owner, long opening) { // constructor this.owner = owner; this.cents = opening; } public void deposit(long amount) { // behaviour if (amount <= 0) throw new IllegalArgumentException("must be positive"); cents += amount; } public long balance() { return cents; } } var account = new Account("Ann", 10_000); account.deposit(5_000); Modifier Visible in private the same class (none) the same package protected package + subclasses public everywhere Fields private, methods as narrow as possible.

Exceptions

Mon, 01 Jan 0001 00:00:00 +0000

Throwable ├── Error ← JVM problems, do not catch │ ├── OutOfMemoryError │ └── StackOverflowError └── Exception ├── RuntimeException ← unchecked │ ├── NullPointerException │ ├── IllegalArgumentException │ ├── IllegalStateException │ └── NumberFormatException └── IOException ← checked Checked exceptions describe expected trouble from the outside world; the caller must handle or declare them. Unchecked ones usually mean a bug. try { var text = Files.readString(path); } catch (IOException e) { IO.println("cannot read: " + e.getMessage()); } String read(Path p) throws IOException { // or pass it up return Files.readString(p); } try / catch / finally try { risky(); } catch (NumberFormatException e) { // specific first … } catch (RuntimeException e) { // general later … } finally { // always runs } e.getMessage() e.getCause() e.getStackTrace() e.printStackTrace() // debugging only try-with-resources Anything AutoCloseable is closed for you, in reverse order, even on failure:

Packages, Modules & Libraries

Mon, 01 Jan 0001 00:00:00 +0000

Packages A package is a namespace and a directory, named after your reversed domain: src/com/example/shop/Cart.java src/com/example/shop/pricing/Discount.java package com.example.shop; import com.example.shop.pricing.Discount; import java.util.List; import java.util.List; // one class import java.util.*; // whole package (rare in projects) import static java.lang.Math.PI; // static member java.lang (with String, Math, IO) is always imported.

Command Line Tools

Mon, 01 Jan 0001 00:00:00 +0000

Reading input void main() { var name = IO.readln("Your name? "); IO.println("Hello, " + name + "!"); } IO.readln returns null at end of input (Ctrl+D, or redirected files) — check for it. The classic way: var scanner = new Scanner(System.in); var n = scanner.nextInt(); scanner.nextLine(); // consume the rest of the line! var text = scanner.nextLine(); Warning nextInt() leaves the newline behind, so the next nextLine() returns empty. Safer: read whole lines and parse them yourself.

Virtual Threads & Concurrency

Mon, 01 Jan 0001 00:00:00 +0000

A classic platform thread maps to an OS thread: ~1 MB of stack, expensive to create, so they were pooled and rationed. var t = new Thread(() -> IO.println("in parallel")); t.start(); t.join(); Virtual threads Managed by the JVM, not the OS. They cost almost nothing, and when one blocks it releases its carrier thread.

Tests & Web Frameworks

Mon, 01 Jan 0001 00:00:00 +0000

A test is code that checks other code. The payoff is not in writing it — it is in changing code later without guessing what you broke. JUnit 5 org.junit.jupiter junit-jupiter 5.11.4 test Tests live in src/test/java, mirroring the package structure. import org.junit.jupiter.api.Test; import static org.junit.jupiter.api.Assertions.*; class DiscountTest { @Test void tenPercentOffOneHundred() { var discount = new Discount(0.10); // arrange var result = discount.applyTo(10_000); // act assertEquals(9_000, result); // assert } @Test void negativeDiscountIsRejected() { assertThrows(IllegalArgumentException.class, () -> new Discount(-0.1)); } } mvn test One behaviour per test, and the method name states the rule — not the method being called.