Java中的模式匹配：InstanceOf（JEP 305）

模式匹配是Java 14中的新预览功能。

实例

为了更好地理解此新功能，让我们看看instanceof运算符是如何工作的。如果您已经熟悉它，请继续进行下一部分。

简而言之，它检查给定对象是否属于给定类型。作为此检查的结果，它返回true或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

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