Referencer til instansmetoder på en bestemt instans

// System.out is a specific PrintStream instance
List<String> names = List.of("Alice","Bob","Carol");
names.forEach(System.out::println); // System.out is the captured instance

class Greeter {
    private String prefix;
    Greeter(String prefix) { this.prefix = prefix; }
    String greet(String name) { return prefix + ", " + name + "!"; }
}

Greeter g = new Greeter("Hello");
java.util.function.Function<String, String> greetFn = g::greet;

System.out.println(greetFn.apply("Alice")); // Hello, Alice!
System.out.println(greetFn.apply("Bob"));   // Hello, Bob!

StringBuilder sb = new StringBuilder("Hello World");
java.util.function.Supplier<String> toString = sb::toString;

System.out.println(toString.get()); // Hello World
sb.append(" !");
System.out.println(toString.get()); // Hello World ! (sb is live)

String prefix = "Java";
java.util.function.Predicate<String> startsWith = prefix::equals;
// Hmm, this checks if prefix.equals(candidate).
// More useful:
Predicate<String> startsWithJ = s -> s.startsWith("J");

// But a specific instance Predicate:
StringBuilder pattern = new StringBuilder("hello");
Predicate<String> equalsHello = ((CharSequence)"hello")::equals;
System.out.println(equalsHello.test("hello")); // true
System.out.println(equalsHello.test("world")); // false

class EventHandler {
    void onEvent(String event) {
        System.out.println("Handling: " + event);
    }
}

EventHandler handler = new EventHandler();
List<String> events = List.of("click","hover","submit");
events.forEach(handler::onEvent); // captures the specific handler instance

import java.util.logging.*;

Logger log = Logger.getLogger("MyApp");

List<String> messages = List.of("Starting","Processing","Done");
messages.stream()
    .map(String::toUpperCase)
    .forEach(log::info); // log is the specific Logger instance

List<String> list = new ArrayList<>(List.of("A","B"));
java.util.function.Supplier<Integer> sizeRef = list::size;

System.out.println(sizeRef.get()); // 2
list.add("C");
System.out.println(sizeRef.get()); // 3 (reflects live state)

// Specific instance: s is captured at binding
String s = "hello";
Predicate<String> p1 = s::equalsIgnoreCase; // String::equalsIgnoreCase bound to 'hello'

// Arbitrary instance: receiver is the stream element
Function<String, String> p2 = String::toUpperCase; // receiver IS the element

class Counter {
    private int count = 0;
    synchronized void increment(String ignored) { count++; }
    int get() { return count; }
}

Counter c = new Counter();
List<String> tasks = List.of("t1","t2","t3");
tasks.forEach(c::increment);
System.out.println(c.get()); // 3

StringBuilder sb = null;
// Reference is created without error:
java.util.function.Supplier<String> ref = sb::toString;
try {
    ref.get(); // NPE thrown here
} catch (NullPointerException e) {
    System.out.println("NPE at invocation");
}