Observer is deprecated in Java 9. What should we use instead of it?

java.util.Observable is out! Instead, use PropertyChangeSupport for property change listener mechanisms:

import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;

public class ObservableBean {
    private final PropertyChangeSupport pcs = new PropertyChangeSupport(this);
    private String value;

    // Subscribe to value changes like it's your favourite YouTube channel
    public void addListener(PropertyChangeListener listener) {
        pcs.addPropertyChangeListener(listener);
    }

    // Where the magic happens. Or not? Let's see...
    public void setValue(String newValue) {
        String oldValue = this.value;
        this.value = newValue;
        pcs.firePropertyChange("value", oldValue, newValue);
    }
}

// Usage
ObservableBean bean = new ObservableBean();
bean.addListener(evt -> System.out.println("Changed: " + evt.getNewValue()));
bean.setValue("Hello"); // Hello to the Observable world
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For asynchronous and reactive data flows, start using java.util.concurrent.Flow. It has mastered the art of non-blocking performance and loves to apply back-pressure as needed:

import java.util.concurrent.Flow.Subscriber;
import java.util.concurrent.Flow.Subscription;
import java.util.concurrent.SubmissionPublisher;

// Publisher: data's personal Uber driver
SubmissionPublisher<String> publisher = new SubmissionPublisher<>();

// Subscriber: Always waiting eagerly for the next data delivery
Subscriber<String> subscriber = new Subscriber<>() {
    public void onSubscribe(Subscription subscription) {
        subscription.request(1); // Just one for now, I'm on diet
    }
    public void onNext(String item) {
        System.out.println(item); // Let's see what we got!
    }
    public void onError(Throwable t) { t.printStackTrace(); } // Oopsie Daisy
    public void onComplete() { System.out.println("Done"); } // That's a wrap, folks! 
};

publisher.subscribe(subscriber); // Connect subscriber, let the fun begin!
publisher.submit("data"); // Start the data flow
publisher.close(); // Bye Bye, Publisher. You've been great!
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