Pattern matching in Java: InstanceOf (JEP 305)

Pattern Matching is a new preview feature in Java 14.

instanceof

To better understand this new feature, let's see how the instanceof operator works . If you are already familiar with it, feel free to move on to the next section.

In short, it checks to see if a given object belongs to a given type. As a result of this check, it returns either true or false .

    if(animal instanceof Cat) {
      // It is a cat! Do something
    } else {
      // It is not a cat, do something else
    }

true, . false.

instanceof

. Animal — , : Cat Dog.

public String getAnimalSound(Animal animal) {
    if(animal instanceof Cat) {
        Cat cat = (Cat)animal;
        return cat.meow();
    } else if (animal instanceof Dog) {
        Dog dog = (Dog)animal;
        return dog.bark();
    }
    throw new UnknownAnimalException();
}

animal . animal Cat, , «». Dog, . Animal, , :

1. -, , animal , instanceof.
2. Cat Dog.
3. animal .

Cat Dog. .

, . . , , Cat Dog.

, Java 14 instanceof, JEP 305. , .

Java 14 .

// Before Java 14
if(animal instanceof Cat) {
    Cat cat = (Cat)animal;
    return cat.meow();
}

//After
if(animal instanceof Cat cat) {
    return cat.meow();
}

:

1. cat, .
2. . cat, Cat.
3. cat if.

, , .

, if:

if(animal instanceof Cat cat) {
    return cat.meow();
} else {
    // Can't use cat here
}
// Can't use cat here either

, if, , AND/OR.

if(animal instanceof Cat cat && cat.isAlive()) {
    return cat.meow();
}

instanceof, &&, cat, Cat, Animal.

IDEA

, IntelliJ IDEA , 2020.1 ( Java 14, Records Switch).

!

, JDK 14.

instanceof Java 14. (Preview feature). ?

VM — Java SE, , , . JDK ; , Java SE.

JDK , , Java SE , , . , ( ), ( ), .

JDK, . . , , , .

IntelliJ IDEA

IntelliJ IDEA File → Project Structure.

, , javac:

javac --release 14 --enable-preview ...

. --enable-preview

java --enable-preview ...

Maven

Maven :

<build>
    <plugins>
        <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>
                <release>14</release>
                <compilerArgs>
                    --enable-preview
                </compilerArgs>
```14</source>
                <target>14</target>
            </configuration>
        </plugin>
        <plugin>
            <artifactId>maven-surefire-plugin</artifactId>
            <configuration>
                <argLine>--enable-preview</argLine>
            </configuration>
        </plugin>
        <plugin>
            <artifactId>maven-failsafe-plugin</artifactId>
            <configuration>
                <argLine>--enable-preview</argLine>
            </configuration>
        </plugin>
    </plugins>
</build>

Java 14

Records
Enhanced Switch
Text Blocks