Konstruktorreferenzen

Was sind Konstruktorreferenzen?

import java.util.function.*;

class Person {
    String name;
    Person(String name) { this.name = name; }
    public String toString() { return "Person(" + name + ")"; }
}

// Function<String, Person> — calls Person(String)
Function<String, Person> factory = Person::new;
Person p = factory.apply("Alice");
System.out.println(p); // Person(Alice)

Konstruktor ohne Argumente als Supplier

class Counter {
    int count = 0;
    void inc() { count++; }
}

Supplier<Counter> newCounter = Counter::new;
Counter c1 = newCounter.get();
Counter c2 = newCounter.get(); // independent instances
c1.inc();
System.out.println(c1.count); // 1
System.out.println(c2.count); // 0

Konstruktor mit zwei Argumenten als BiFunction

record Point(int x, int y) {}

BiFunction<Integer, Integer, Point> makePoint = Point::new;
Point p = makePoint.apply(3, 4);
System.out.println(p); // Point[x=3, y=4]

Konstruktorreferenz in Stream.map

import java.util.*;
import java.util.stream.*;

List<String> names = List.of("Alice","Bob","Carol");

List<Person> people = names.stream()
    .map(Person::new)  // calls new Person(name) for each
    .collect(Collectors.toList());

people.forEach(System.out::println);
// Person(Alice)
// Person(Bob)
// Person(Carol)

Array-Konstruktorreferenz

import java.util.stream.*;

String[] arr = Stream.of("a","b","c")
    .toArray(String[]::new);

for (String s : arr) System.out.print(s + " ");
// a b c

int[] sizes = {3, 5, 2};
String[][] matrix = Arrays.stream(sizes)
    .mapToObj(String[]::new)
    .toArray(String[][]::new);

Muster für Factory-Schnittstellen

@FunctionalInterface
interface Factory<T> {
    T create(String config);
}

class DBConnection {
    String url;
    DBConnection(String url) { this.url = url; }
    public String toString() { return "DB(" + url + ")"; }
}

Factory<DBConnection> dbFactory = DBConnection::new;
DBConnection conn = dbFactory.create("jdbc:postgresql://localhost/mydb");
System.out.println(conn);

Dependency Injection mit Konstruktorreferenzen

import java.util.function.*;

class ServiceLocator {
    private final Map<String, Supplier<?>> registry = new HashMap<>();
    
    <T> void register(String name, Supplier<T> factory) {
        registry.put(name, factory);
    }
    
    @SuppressWarnings("unchecked")
    <T> T get(String name) { return (T) registry.get(name).get(); }
}

ServiceLocator loc = new ServiceLocator();
loc.register("counter", Counter::new);
Counter c = loc.get("counter");

Lambda oder Konstruktorreferenz auswählen

// Lambda:
Function<String, Person> f1 = name -> new Person(name);

// Constructor reference (preferred):
Function<String, Person> f2 = Person::new;

// When you need extra logic, stick with lambda:
Function<String, Person> f3 = name -> new Person(name.trim().toLowerCase());

Generische Typen und Konstruktorreferenzen

import java.util.*;
import java.util.function.*;

// ArrayList::new matches Supplier<ArrayList<String>>
Supplier<List<String>> listFactory = ArrayList::new;
List<String> l1 = listFactory.get();
List<String> l2 = listFactory.get(); // independent lists
l1.add("hello");
System.out.println(l2.size()); // 0

Stream-Collectors mit Konstruktorreferenzen

import java.util.stream.*;

// Collect into a LinkedList using a constructor reference supplier
LinkedList<String> linked = Stream.of("a","b","c")
    .collect(Collectors.toCollection(LinkedList::new));
System.out.println(linked.getFirst()); // a

Primitive Typen und Autoboxing

// Integer::new is deprecated in Java 9+, but illustrates the concept
// Use Integer.valueOf or just autoboxing in modern Java
Function<String, Integer> intParser = Integer::parseInt; // static method ref
System.out.println(intParser.apply("42")); // 42

Kurzer Check

Zusammenfassung: Konstruktorreferenzen