Pattern matching is a language feature that has been in use since the 1960s. This session explores how pattern matching is being added to Java

1. The Art of Java
Type Patterns
Simon Ritter, Deputy CTO | Azul

2. 2
Modern Java
• The six-month release cadence for Java has been really good
• Lots of new features added much faster than we've ever seen before
• Significant language changes are initially developed under project Amber
o "... explore and incubate smaller, productivity-oriented Java language features..."
o Most features go through at least two rounds of preview
• Many of the new features work both separately and combined with others

3. 3
Pattern Matching In Java
• java.util.regex
• This is not what we're here to talk about
Pattern p = Pattern.compile("a*b");
Matcher m = p.matcher("aaaaab");
boolean b = m.matches();
boolean b = Pattern.matches("a*b", "aaaaab");

4. 4
Pattern Matching Fundamentals
• A well used technique, been in use since the 1960s (used in Haskell, AWK, etc.)
match predicate pattern variables
Determines whether the pattern
matches a target
A pattern
Conditionally extracted if the
pattern matches the target

5. 5
Pattern Types
• Constant
o Match on a constant (already in use in a switch statement)
• Type
o Match on a type
• Deconstruction
o Match and extract
• var
o Uses type inference to map to a type pattern (effecively matches anything)
• Any (_)
o Matches anything but binds to nothing (an unused pattern variable). See JEP 302

6. 6
Before we get into to Pattern Matching

7. 7
Switch Expressions
• Switch construct was a statement
o No concept of generating a result that could be assigned
• Rather clunky syntax
o Every case statement needs to be separated
o Must remember break (default is to fall through)
o Scope of local variables is not intuitive

8. 8
Switch Statement
int numberOfLetters;
switch (day) {
case MONDAY:
case FRIDAY:
case SUNDAY:
numberOfLetters = 6;
break;
case TUESDAY:
numberOfLetters = 7;
break;
case THURSDAY:
case SATURDAY:
numberOfLetters = 8;
break;
case WEDNESDAY:
numberOfLetters = 9;
break;
default:
throw new IllegalStateException("Huh?: " + day); };

9. 9
Switch Expression (JDK 12)
• Switch expressions must be complete (exhaustive)
o We'll come back to this later
int numberOfLetters = switch (day) {
case MONDAY, FRIDAY, SUNDAY -> 6;
case TUESDAY -> 7;
case THURSDAY, SATURDAY -> 8;
case WEDNESDAY -> 9;
default -> throw new IllegalStateException("Huh?: " + day);
};

10. 10
Algebraic Data Types in Java

11. 11
Simple Java Data Class
class Point {
private final double x;
private final double y;
public Point(double x, double y) {
this.x = x;
this.y = y;
}
public double x() {
return x;
}
public double y() {
return y;
}
}

12. 12
Records (JDK 14)
record Point(double x, double y) { }
record Anything<T>(T t) { } // Generic Record
public record Circle(double radius) {
private static final double PI = 3.142; // Static instance fields are allowed
public double area() {
return PI * radius * radius;
}
}

13. 13
Records Additional Details
• The base class of all records is java.lang.Record
o Records cannot sub-class (but may implement interfaces)
• Object methods equals(), hashCode() and toString() can be overridden
• Records are implicitly final (although you may add the modifier)
• Records do not follow the Java bean pattern
o x() not getX() in Point example
o record Point(getX, getY) // If you must

14. 14
Java Inheritance
• A class (or interface) in Java can be sub-classed by any class
o Unless it is marked as final
Shape
Triangle Square Pentagon

15. 15
Sealed Classes (JEP 360)
public sealed class Shape permits Triangle, Square, Pentagon { ... }
Shape
Triangle Square Pentagon Circle
X

16. 16
Sealed Classes (JEP 360)
• All sub-classes must have inheritance capabilities explicitly specified
// Restrict sub-classes to defined set
public sealed class Triangle permits Equilateral, Isosoles extends Shape { ... }
// Prevent any further sub-classing
public final class Square extends Shape { ... }
// Allow any classes to sub-class this one (open)
public non-sealed class Pentagon extends Shape { ... }

17. 17
Current Pattern Matching in Java

18. 18
Using The instanceof Operator
if (obj instanceof String) {
String s = (String)obj;
System.out.println(s.length());
}
We must always perform an
explicit cast with an assignment

19. 19
Pattern Matching For instanceof (JDK 14)
if (obj instanceof String s)
System.out.println(s.length());
else
// Use of s not allowed here
if (obj instanceof String s && s.length() > 0)
System.out.println(s.length());
// Compiler error
if (obj instanceof String s || s.length() > 0)
System.out.println(s.length());

20. 20
Pattern Matching For instanceof
public void doSomething(Object o) {
if (!(o instanceof String s))
return;
System.out.println(s.length()); // Scope of s valid
// Several hundred lines of code
System.out.println(s.length()); // Still in scope
}

21. 22
Flow Scoping For Binding Variables
• Scope of local variable runs from its declaration until the end of the block in which it is declared
o Locals are subject to definite assigment
• Binding variables are also subject to definite assignment
o The scope of a binding variable is the set of places in the program where it would be definitely assigned
o This is flow scoping
• However, scope is not the same as local variables
if (o instanceof Integer num) { ... }
else if (o instanceof Float num) { ... }
else if (o instanceof Long num) { ... }
Need flow scoping to be able
to reuse num as variable name

22. 23
Pattern Matching For instanceof Puzzler
• Will this work?
Object s = new Object();
if (s instanceof String s)
System.out.println("String of length " + s.length());
else
System.out.println("No string");

23. 25
Pattern Matching For switch
• Switch is limited on what types you can use (Integral values, Strings, enumerations)
• Expanded to allow type patterns to be matched
void typeTester(Object o) {
switch (o) {
case null -> System.out.println("Null type");
case String s -> System.out.println("String: " + s);
case Color c -> System.out.println("Color with RGB: " + c.getRGB());
case int[] ia -> System.out.println("Array of ints, length" + ia.length);
default -> System.out.println(o.toString());
}
}

24. 26
Pattern Matching For switch
• Null is special (and complicated)
• case null can be used in all switch statements and expressions
o If not included, it will be added by compiler at start (throwing NullPointerException)
void typeTester(Object o) {
switch (o) {
case String s -> System.out.println("String: " + s);
case Color c -> System.out.println("Color with RGB: " + c.getRGB());
case int[] ia -> System.out.println("Array of ints, length" + ia.length);
default -> System.out.println("Bad input!");
}
}

25. 27
Pattern Matching For switch
• Null is special (and complicated)
• case null can be used in all switch statements and expressions
o If not included, it will be added by compiler at start (throwing NullPointerException)
void typeTester(Object o) {
switch (o) {
case null -> throw new NullPointerException(); // Added by compiler
case String s -> System.out.println("String: " + s);
case Color c -> System.out.println("Color with RGB: " + c.getRGB());
case int[] ia -> System.out.println("Array of ints, length" + ia.length);
default -> System.out.println("Bad input!");
}
}

26. 28
Pattern Matching For switch
• Null is special (and complicated)
• case null can be used in all switch statements and expressions
o If not included, it will be added by compiler at start (throwing NullPointerException)
void typeTester(Object o) {
switch (o) {
case String s -> System.out.println("String: " + s);
case Color c -> System.out.println("Color with RGB: " + c.getRGB());
case int[] ia -> System.out.println("Array of ints, length" + ia.length);
null, default -> System.out.println("Bad input!"); // No NullPointerException
}
}

27. 29
Pattern Matching for switch (Completeness)
• Pattern switch statements (and all switch expressions) must be exhaustive
o All possible values must be handled
void typeTester(Object o) {
switch (o) {
case String s -> System.out.println("String: " + s);
case Integer i -> System.out.println("Integer with value " + i.getInteger());
}
}
void typeTester(Object o) {
switch (o) {
case String s -> System.out.println("String: " + s);
case Integer i -> System.out.println("Integer with value " + i.getInteger());
default -> System.out.println("Some other type");
}
}

28. 30
Pattern Matching for switch (Completeness)
void typeTester(Shape shape) { // Using previous sealed class example
switch (shape) {
case Triangle t -> System.out.println("It's a triangle");
case Square s -> System.out.println("It's a square");
case Pentagon p -> System.out.println("It's a pentagon");
case Shape s -> System.out.println("It's a shape");
}
}

29. 31
Guarded Patterns
void shapeTester(Shape shape) { // Using previous sealed class example
switch (shape) {
case Triangle t && t.area() > 25 -> System.out.println("It's a big triangle");
case Triangle t -> System.out.println("It's a small triangle");
case Square s -> System.out.println("It's a square");
case Pentagon p -> System.out.println("It's a pentagon");
case Shape s -> System.out.println("It's a shape");
}
}
GuardedPattern:
PrimaryPattern && ConditionalAndExpression

30. 32
Pattern Dominance
• Less specific cases must not hide more specific cases
void typeTester(Shape shape) {
switch (shape) {
case Shape s -> System.out.println("It's a shape");
case Triangle t -> System.out.println("It's a triangle");
case Square s -> System.out.println("It's a square");
case Pentagon p -> System.out.println("It's a pentagon");
}
}
Shape will always match first
Triangle, Square, Pentagon cases
are unreachable

31. 33
Pattern Dominance
void shapeTester(Shape shape) {
switch (shape) {
case Triangle t -> System.out.println("It's a small triangle");
case Triangle t && t.area() > 25 -> System.out.println("It's a big triangle");
case Square s -> System.out.println("It's a square");
case Pentagon p -> System.out.println("It's a pentagon");
case Shape s -> System.out.println("It's a shape");
}
}
Again, Triangle will match before
Triangle with a guard

32. 34
Pattern Matching in Future Java

33. 35
Pattern Matching instanceof And Records
• What we can do now
• This is good but we can do better
record Point(double x, double y) { }
public void pythagoras(Object o) {
if (o instanceof Point p) {
double x = p.x();
double y = p.y();
System.out.println("Hypotonuse = " + Math.sqrt((x*x) + (y*y));
}
}

34. 36
Pattern Matching For Records (JEP 405)
• Use of a record pattern (which is a deconstruction pattern)
• Since a Record is just a special form of class, this will work with normal classes as well
public void pythagoras(Object o) {
if (o instanceof Point(double x, double y))
System.out.println("Hypotonuse = " + Math.sqrt((x*x) + (y*y));
}

35. 37
Patterns Are Composable
record Point(double x, double y) {};
enum Colour { RED, GREEN, BLUE };
record ColourPoint(Point p, Colour c) {};
record ColourRectangle(ColourPoint topLeft,
ColourPoint bottomRight) implements Shape { ... };
public void printColour(Shape s) {
if (s instanceof ColourRectangle(ColourPoint topleft, ColourPoint bottomRight))
System.out.println(topLeft.c());
}
But this is a record as well

36. 38
Patterns Are Composable
public void printColour(Shape s) {
if (s instanceof ColourRectangle(ColourPoint(Point p, Colour c), ColourPoint br))
System.out.println(c);
}
public void printTopLeftX(Shape s) {
if (s instanceof
ColourRectangle(ColourPoint(Point(double x, double y), Colour c), ColourPoint br)
System.out.println(x);
}
Yet another record

37. 39
Patterns And Local Variable Type Inference
• Use var, introduced in JDK 10
• An example of the any pattern matching type
public void printTopLeftX(Shape s) {
if (s instanceof
ColourRectangle(ColourPoint(Point(var x, var y), var c), var br)
System.out.println(x);
}

38. 40
Using The Any Pattern Match
• Not yet part of any proposed JEP
• Related to JEP 302, Lambda Leftovers
• This is not a proposed change, at this time (i.e. I'm speculating this as a feature)
public void printTopLeftX(Shape s) {
if (s instanceof
ColourRectangle(ColourPoint(Point(var x, _), _), _)
System.out.println(x);
}
We have no interest in these
variables so let's ignore them

39. 41
Pattern Matching For Arrays
• Why not use a decompose pattern for arrays?
• This was part of JEP 405 but has been dropped for now
static void printFirstTwoStrings(Object o) {
if (o instanceof String[] sa && sa.length >= 2) {
String s1 = sa[0];
String s2 = sa[1];
System.out.println(s1 + s2);
}
}
static void printFirstTwoStrings(Object o) {
if (o instanceof String[] { String s1, String s2, ... })
System.out.println(s1 + s2);
}

40. Summary

41. 43
Conclusions
• Pattern matching is a powerful set of language constructs
• Simplifies certain tasks
o Less boilerplate code
o More declarative
• Also has the potential for better optimisation of code and enhanced performance
• Some features already included
• More to come
• Yet more may be added later

42. 44
Azul Zulu Builds of OpenJDK
• Enhanced build of OpenJDK source code
o Fully TCK tested
o JDK 7, 8, 11, 13, 15 and 17
o JDK 6 available for commercial customers
• Wide platform support:
o Intel 64-bit Windows, Mac, Linux
o Intel 32-bit Windows and Linux
• Free community edition, optional commercial support (Azul Platform Core)

43. Thank you!
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