Modern Java Features

7 pages · ~5 min✓ Reviewed

JavaPage 1

var and Text Blocks

1. var

var = local type inference from the initializer
Type is static: still strongly typed, not dynamic
var list = new ArrayList<String>() is ArrayList, not List
Want the interface? Declare List<String>

Allowed	Fails
locals	fields
for-loop variables	method parameters
lambda parameters	return types
`var x = 1;`	`var x;`, `var x = null;`

2. Text Blocks

Delimiter """; line break after the opening one is required
Incidental indent stripped: least-indented line or closing """

java

String s = """
    Hi %s \
    there\s
    """.formatted("Ann");

joined line, kept trailing space, filled placeholder

Gotcha	Effect
`\` at line end	joins lines
`\s`	keeps trailing space
literal `"""`	must be escaped
`.formatted(args)`	fills `%s`

Mistake

var is not Object or dynamic: var x = 1; x = "a"; fails to compile.

JavaPage 2

Records

1. What a record is

record Point(int x, int y) {} = immutable data carrier
Generated: private final fields, canonical constructor
Accessors x() and y(): no get prefix
Also equals, hashCode, toString
Compact constructor: no parameter list, for validation or normalising

java

record Range(int lo, int hi) {
    Range {
        if (lo > hi)
            throw new IllegalArgumentException();
    }
}

2. Rules and comparison

Implicitly final; cannot extend a class
No instance fields beyond the components
Can implement interfaces; static members and extra methods are fine

JavaPage 3

Sealed Classes and Interfaces

1. A Closed Set of Subtypes

sealed + permits lists the only allowed subtypes
The compiler knows the closed set
Records are implicitly final: a natural fit as permitted subtypes

java

sealed interface Shape permits Circle, Square {}
record Circle(double r) implements Shape {}
record Square(double side) implements Shape {}

Shape can have no other implementors

2. Modifiers and Placement

Modifier	Effect
`final`	no further extension
`sealed`	continues the restriction

JavaPage 4

Switch Expressions and Pattern Matching

1. Switch Expressions

Switch expression returns a value, uses -> labels
No fall-through, no break
yield returns a value from a block
Must be exhaustive: all cases or default

java

int n = switch (day) {
  case MON, TUE -> 1;
  case WED -> { yield 2; }
  default -> 0;
};

2. Pattern Matching

o instanceof String s && s.length() > 3 binds s, no cast
Binding in scope only where the match is definitely true

JavaPage 5

Virtual Threads

1. What They Are

Virtual thread (final in Java 21): JVM-scheduled lightweight thread
Runs on a small pool of carrier platform threads
Blocks on I/O → unmounts; carrier runs another thread
So you can have millions of them

Mounton carrier
Blocks on I/Ounmount
Carrier freeruns another
I/O doneremount

JavaPage 6

Java 21 Cheat Sheet

1. var and Records

var: locals only, type inferred at compile time
Needs an initializer; never fields or parameters
record R(int a) {}: immutable, accessor a()
Compact constructor validates; record is final
Records can implement interfaces, not extend classes

2. Sealed Types

java

sealed interface Shape permits Circle, Square {}
record Circle(double r) implements Shape {}
record Square(double side) implements Shape {}

records are implicitly final

Subclass must be final, sealed or non-sealed
Closed set gives exhaustive switch, no default

JavaPage 7

Check yourself

Does var list = new ArrayList<String>(); list = new LinkedList<String>(); compile?

No: var infers the static type ArrayList<String>
Declare List<String> list to allow both

A record has a List<String> members component. Can t.members().add("z") change it?

Yes if the list is mutable: records are only shallowly immutable
Fix: members = List.copyOf(members); in the compact constructor

switch (o) has case String s and default. What happens when o is null?

NullPointerException: a pattern switch rejects null

	Record	Lombok @Value	Plain class
Fields	final	final	your choice
Extend a class	no	no (class is final)	yes
Collections	still mutable	still mutable	still mutable

Kind	Subclasses allowed
`final`	zero
`sealed`	a known list
open	anyone

Type	`case Circle c ->`
Null	`case null ->` (else NPE)
Guard	`case Circle c when c.r() > 10 ->`
Record	`case Point(int x, int y) -> x + y`

	Platform	Virtual
Maps to	1:1 OS thread	JVM-scheduled
Cost	~1 MB stack	cheap
Per task	pooled	one per task
Best for	CPU-bound	blocking I/O

Modern Java Features

var and Text Blocks

1. var

2. Text Blocks

Records

1. What a record is

2. Rules and comparison

Sealed Classes and Interfaces

1. A Closed Set of Subtypes

2. Modifiers and Placement

Switch Expressions and Pattern Matching

1. Switch Expressions

2. Pattern Matching

Virtual Threads

1. What They Are

Java 21 Cheat Sheet

1. var and Records

2. Sealed Types

Check yourself

3. Copy and fit

3. Compare and Mistakes

3. Exhaustive and Ordered

2. Platform vs Virtual

3. Mistakes

3. Switch Patterns

4. Text Blocks and Virtual Threads