Introduction to Pattern Matching in Java [Free Meetup]

Unlocking The Power of Pattern Matching
Pattern Matching in Java
Haim Michael https://lifemichael.com

What is Pattern Matching?
• Pattern matching simplifies conditional logic by allowing developers to
test and extract data from objects more concisely. Using Pattern
Matching we can eliminate boilerplate code (especially when using
instanceof checks and switch expressions).

The instanceof Operator
public class InstanceOfDemo {
public static void main(String[] args) {
var ob = new Dog();
//the old way
if(ob instanceof Dog) {
((Dog)ob).hau();
}
//the new way
if(ob instanceof Dog dog) {
dog.hau();
}
}
}
The dog variable is a pattern variable
assigned with the value of ob if instanceof
returns true.
com.lifemichael.meetups.java.patternmatching.InstanceOfDemo

The switch Expression
public class SwitchPatternMatching {
var ob = new Dog();
hello(ob);
}
public static void hello(Object object) {
switch(object) {
case Dog dog -> dog.hau();
case Tiger tiger -> tiger.miaurrr();
case Cat cat -> cat.miau();
case null, default -> System.out.println("not supported");
}
}
}
Make sure you know the difference between
Switch Expression and Switch Statement.
com.lifemichael.meetups.java.patternmatching.SwitchPatternMatching
com.lifemichael.meetups.java.patternmatching.SwitchExpressionDemo

The Completeness Requirement
• When using pattern matching together with Switch Statement (or Switch
Expressions), all possible values must be handled. Usually, we will
achieve that using the default case, and using a sealed class.

The Completeness Requirement
public class SealedProgram {
var ob = new Dog();
hello(ob);
}
public static void hello(Animal ob) {
switch(ob) {
case Dog dog -> dog.hau();
case Tiger tiger -> tiger.miaurrr();
case Cat cat -> cat.miau();
case Human human -> human.hello();
}
}
}
com.lifemichael.meetups.java.patternmatching.sealed.demo.SealedProgram

Guarded Patterns
public class GuardedProgram {
test(new Rectangle(3,5));
}
public static void test(Shape o) {
switch(o) {
case Rectangle rec when rec.area()>20 -> System.out.println("big rectangle");
case Rectangle rec when rec.area()<=20 -> System.out.println("small rectangle");
case Circle circle -> System.out.println("nice circle");
case Shape shape -> System.out.println("it is impossible to calculate area");
}
}
}
The when keyword works when comparing
objects only.
com.lifemichael.meetups.java.patternmatching.guarded

Record Deconstruction Patterns
• We can easily use Pattern Matching with classes that were defined as
record classes in order to extract values from the object we compare.

Record Deconstruction Patterns
record Rectangle (double width, double height) {}
public class Program {
Rectangle rec = new Rectangle(3,4);
describe(rec);
}
public static void describe(Object ob) {
if(ob instanceof Rectangle(double w, double h)) {
System.out.println("width="+w);
System.out.println("height="+h);
System.out.println("area="+(w*h));
System.out.println("perimeter="+(2*(w+h)));
}
}
}
com.lifemichael.meetups.java.patternmatching.simplerecord

Composable Patterns Deconstruction
• We can compose new patterns from patterns that already exist.

interface Line {}
record Point(double x, double y) {}
enum Color {BLUE, YELLOW, RED, GREEN}
interface Rectangle {}
record ColourLine(Point p1, Point p2, Color color) {}
record ColourRectangle(Point topLeft, Point bottomRight, Color color) implements Rectangle {}
com.lifemichael.meetups.java.patternmatching.composable.SimpleDemo

public class SimpleDemo {
var ob = new ColourRectangle(new Point(3,4),new Point(10,2),Color.BLUE);
printDetails(ob);
}
public static void printDetails(Rectangle rec) {
if(rec instanceof ColourRectangle(Point topLeft, Point bottomRight, Color color)) {
System.out.println("color="+color);
System.out.println("top left="+topLeft);
System.out.println("bottom right="+bottomRight);
}
}
}
com.lifemichael.meetups.java.patternmatching.composable.SimpleDemo

Unnamed Variables and Patterns
• We can improve the readability of our code by using the _ character when
dealing with pattern variables we don’t use.

interface Line {}
record Point(double x, double y) {}
enum Color {BLUE, YELLOW, RED, GREEN}
interface Rectangle {}
record ColourLine(Point p1, Point p2, Color color) {}
record ColourRectangle(Point topLeft, Point bottomRight, Color color) implements Rectangle {}
com.lifemichael.meetups.java.patternmatching.composable.unnamed

var ob = new ColourRectangle(new Point(3,4),new Point(10,2),Color.BLUE);
printDetails(ob);
}
public static void printDetails(Rectangle rec) {
if(rec instanceof ColourRectangle(Point topLeft, Point bottomRight, _)) {
System.out.println("top left="+topLeft);
System.out.println("bottom right="+bottomRight);
}
}
}
com.lifemichael.meetups.java.patternmatching.composable.unnamed

• There are various cases in which we can use the unnamed pattern
variable.

com.lifemichael.meetups.java.patternmatching.composable.unnamed.switchdemo
var data = 12;
testType(data);
}
public static void testType(Object data) {
switch(data) {
case String _ -> System.out.println("String");
case Integer _ -> System.out.println("Integer");
case Double _ -> System.out.println("Double");
default -> System.out.println("Un Known");
}
}
}

Questions & Answers
• Thanks for attending my talk.
haim.michael@lifemichael.com
+972 54 6655837
https://blog.lifemichael.com

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