Der instanceof-Operator

Der instanceof-Operator

Grundlegende Verwendung von instanceof

Object obj = "Hello, Java!";

if (obj instanceof String) {
    String s = (String) obj;
    System.out.println(s.toUpperCase()); // HELLO, JAVA!
}

Object num = Integer.valueOf(42);
System.out.println(num instanceof Integer); // true
System.out.println(num instanceof String);  // false

instanceof bei Vererbung

class Animal {}
class Dog extends Animal {}
class Cat extends Animal {}

Animal a = new Dog();

System.out.println(a instanceof Animal); // true
System.out.println(a instanceof Dog);    // true
System.out.println(a instanceof Cat);    // false

Pattern Matching mit instanceof (Java 16+)

Object shape = "circle";

// Old way
if (shape instanceof String) {
    String s = (String) shape;
    System.out.println(s.length());
}

// New way (Java 16+) — binding variable 's' is in scope
if (shape instanceof String s) {
    System.out.println(s.length()); // 6
}

Pattern Matching in einer Methode

sealed interface Shape permits Circle, Rectangle {}
record Circle(double radius) implements Shape {}
record Rectangle(double width, double height) implements Shape {}

static double area(Shape s) {
    if (s instanceof Circle c) {
        return Math.PI * c.radius() * c.radius();
    } else if (s instanceof Rectangle r) {
        return r.width() * r.height();
    }
    throw new IllegalArgumentException("Unknown shape");
}

System.out.println(area(new Circle(5)));         // 78.53...
System.out.println(area(new Rectangle(4, 6)));   // 24.0

instanceof mit null

String s = null;
System.out.println(s instanceof String); // false (not NPE)

Object o = null;
if (o instanceof Number n) {
    System.out.println(n.intValue());
} else {
    System.out.println("Not a number (or null)"); // prints this
}

Switch-Ausdrücke und Pattern Matching

static String describe(Object obj) {
    return switch (obj) {
        case Integer i -> "Integer: " + i;
        case Double d  -> "Double: " + d;
        case String s  -> "String of length " + s.length();
        case null      -> "null value";
        default        -> "Unknown: " + obj.getClass().getSimpleName();
    };
}

System.out.println(describe(42));       // Integer: 42
System.out.println(describe(3.14));     // Double: 3.14
System.out.println(describe("hello")); // String of length 5
System.out.println(describe(null));     // null value

ClassCastException ohne instanceof

Object obj = "I am a String";

try {
    Integer i = (Integer) obj; // ClassCastException!
} catch (ClassCastException e) {
    System.out.println("Cannot cast String to Integer");
}

// Safe version
if (obj instanceof Integer i) {
    System.out.println("Is integer: " + i);
} else {
    System.out.println("Not an integer");
}

Praxis: Verarbeiten einer gemischten Liste

import java.util.List;

List<Object> events = List.of(
    "User login", 42, 3.14, "Payment processed", 100
);

double numericSum = 0;
for (Object e : events) {
    if (e instanceof Integer n) numericSum += n;
    else if (e instanceof Double d) numericSum += d;
    else if (e instanceof String s) System.out.println("Log: " + s);
}
System.out.printf("Numeric sum: %.2f%n", numericSum); // 145.14

Versiegelte Klassen und vollständiges Matching

sealed interface Notification permits EmailNotification, SmsNotification {}
record EmailNotification(String to, String body) implements Notification {}
record SmsNotification(String phone, String text) implements Notification {}

static void send(Notification n) {
    switch (n) {
        case EmailNotification e ->
            System.out.println("Email to " + e.to() + ": " + e.body());
        case SmsNotification s ->
            System.out.println("SMS to " + s.phone() + ": " + s.text());
        // no default needed — compiler knows all cases are covered
    }
}

Bewachte Patterns

static String classify(Number n) {
    return switch (n) {
        case Integer i when i < 0  -> "negative int: " + i;
        case Integer i when i == 0 -> "zero";
        case Integer i             -> "positive int: " + i;
        case Double d              -> "double: " + d;
        default                    -> "other number";
    };
}

System.out.println(classify(-5));  // negative int: -5
System.out.println(classify(0));   // zero
System.out.println(classify(7));   // positive int: 7

Schnelltest

Object obj = null;
boolean result = obj instanceof String;
System.out.println(result);

Zusammenfassung: instanceof-Operator