Pattern Matching in Java 14

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Java 14 Pattern Matching
Oleksandr Navka
Lead Software Engineer, Consultant, GlobalLogic

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1. Updated instanceof
2. Updated switch-case
3. Java Records
Agenda

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Тагир Валеев
JetBrains
Pattern matching
и его воображаемые друзья

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When someone match something :)
What is pattern matching?

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Any pattern
• _
Constant pattern
• 1
• true
• null
Type pattern
• String s
Var pattern
• var i = 1
Deconstruction pattern
• Optional(String s)
• Node(Node left, Node right)
What are the patterns?

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public class Animal {
}
class Dog extends Animal {
public void bark() {
}
}
class Cat extends Animal {
public void meow() {
}
}

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public static void say(Animal animal) {
if (animal instanceof Dog) {
Dog dog = (Dog) animal;
dog.bark();
} else if (animal instanceof Cat) {
Cat cat = (Cat) animal;
cat.meow();
}
}

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if (animal instanceof Dog) {
Dog dog = (Dog) animal;
dog.bark();
} else if (animal instanceof Cat) {
Cat cat = (Cat) animal;
cat.meow();
}
}

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if (animal instanceof Dog dog) {
dog.bark();
} else if (animal instanceof Cat cat) {
cat.meow();
}
}

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if (animal instanceof Dog dog) {
dog.bark();
} else if (animal instanceof Cat cat) {
cat.meow();
}
}

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Before:
expression istanceof Type
Now:
expression instanceof Pattern

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if (obj instanceof String s) {
// can use s here
} else {
// can't use s here
}

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if (obj instanceof String s) {
// can use s here
} else {
// can't use s here
}
binding variable

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if (!(obj instanceof String s)) {
} else {
s.contains("a")
}

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if(obj instanceof String s && s.length() > 5) {
System.out.println(s.toUpperCase());
}

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if(obj instanceof String s || s.length() > 5) {
}

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if(!(obj instanceof String s) || s.length() > 5) {
System.out.println("Do something");
} else {
}

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System.out.println("Do something");
} else {
}

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throw new IllegalArgumentException();
}

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}

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s.contains(..)
} else {
s.contains(..)
}

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private String s = "abc";
s.contains(..)
} else {
s.contains(..)
}

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1. if (obj instanceof String s) {s.trim()}
2. if (!(obj instanceof String s)) {} else {s.contains("a")}
3. if(obj instanceof String s && s.length() > 5) {}
4. if(obj instanceof String s || s.length() > 5) {}
Binding variable scope

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5. if(!(obj instanceof String s) || s.length() > 5) {}
6. if(!(obj instanceof String s) || s.length() > 5) {} else {s.toUpperCase());}
7. if(!(obj instanceof String s) || s.length() > 5) {
}

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1. instanceof has got type pattern matching
2. instanceof the first java feature that got pattern matching
3. Be aware about scoping
Instanceof Summary

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public enum ProgrammingParadigm {OBJECT_ORIENTED, PROCEDURAL, FUNCTIONAL}
public static List<String> getProgrammingLanguage(ProgrammingParadigm programmingParadigm) {
List<String> languages = new ArrayList<>();
switch (programmingParadigm) {
case OBJECT_ORIENTED:
languages.add("Java");
languages.add("C++");
break;
case FUNCTIONAL:
languages.add("Haskel");
break;
case PROCEDURAL:
languages.add("Pascal");
break;
}
return languages;
}
Old version of switch

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break;
case FUNCTIONAL:
break;
case PROCEDURAL:
break;
}
return languages;
}

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break;
case FUNCTIONAL:
break;
case PROCEDURAL:
break;
}
return languages;
}
Input type limitations:
byte, short, char, and int
Enum Types, String

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break;
case FUNCTIONAL:
break;
case PROCEDURAL:
break;
}
return languages;
}
Can’t
get
result

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return List.of("Java", "C++");
case FUNCTIONAL:
return List.of("Haskel");
case PROCEDURAL:
return List.of("Pascal");
default:
return List.of();
}
}

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return List.of("Java", "C++");
case FUNCTIONAL:
return List.of("Haskel");
case PROCEDURAL:
return List.of("Pascal");
default:
return List.of();
}
}

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Disadvantages:
1. Always remember about break
2. Input data type limitation
3. Can’t get result
4. Don’t understand all possible options for enum

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Pattern matching with switch case

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public void printObject(Object obj) {
switch (obj) {
case String s:
System.out.println("This is String" + s.trim());
break;
case Integer i:
System.out.println("This is Integer" + i);
break;
case Number n:
System.out.println("This is number"+ n);
break;
default:
System.out.println("I don't know");
}
}

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switch (obj) {
case String s:
break;
case Number n:
break;
case Integer i:
break;
default:
}
}

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switch (obj) {
case String s:
break;
default:
case Number n:
break;
case Integer i:
break;
}
}

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return switch (programmingParadigm) {
case OBJECT_ORIENTED -> List.of("Java", "C++");
case FUNCTIONAL -> List.of("Haskel");
case PROCEDURAL -> List.of("Pascal");
};
}
Updated switch case

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return switch (programmingParadigm) {
case OBJECT_ORIENTED -> {
System.out.println("You are master of polymorphism");
yield List.of("Java", "C++");
}
case FUNCTIONAL -> {
System.out.println("You are master of recursion");
yield List.of("Haskel");
}
case PROCEDURAL -> {
System.out.println("Try to use another paradigms");
yield List.of("Pascal");
}
};
}

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1. Don’t need to use break
2. Can return result
3. Don’t need to use default branch if all options described
4. Input data types have limitations
Summary of updated switch

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private record Car(int speed, String manufacturer) {
}
Java Records

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}
public static void main(String[] args) {
Car car = new Car(100, "Mercedes");
System.out.println(car.speed());
System.out.println(car.manufacturer());
}
Java Records

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}
Car car = new Car(100, "Mercedes");
System.out.println(car);
}
-------
Output:
Car[speed=100, manufacturer=Mercedes]
Java Records

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private static final class Car extends java.lang.Record {
private final int speed;
private final java.lang.String manufacturer;
public Car(int speed, java.lang.String manufacturer) { /* compiled code */ }
public java.lang.String toString() { /* compiled code */ }
public final int hashCode() { /* compiled code */ }
public final boolean equals(java.lang.Object o) { /* compiled code */ }
public int speed() { /* compiled code */ }
public java.lang.String manufacturer() { /* compiled code */ }
}

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}

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public abstract class Record {
protected Record() {}
@Override
public abstract boolean equals(Object obj);
@Override
public abstract int hashCode();
@Override
public abstract String toString();
}

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}

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}

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public Car {
if(speed < 0) {
}
}
}
var car = new Car(-2, "Mercedes");
System.out.println(car);
}
Canonical constructor

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public Car {
if(speed < 0) {
}
}
public Car(String manufacturer) {
this(0, manufacturer);
}
}
Canonical constructor

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}

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}

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1. Java 14 present first version of pattern matching
2. Type pattern matching present in instanceof operator
3. New Switch statement was released with some blanks for
pattern matching
4. Keep eye on Java Records and deconstruction patterns
Summary

Pattern Matching in Java 14

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