A built in observer pattern. It seems it is no different than my callback implementation in Java

1. Subject.java

package com.chp2.observerpattern.custom;

/**
 * The objects who will broadcast events will implement this interface
 */
public interface Subject {
    public void registerObserver(Observer o);
    public void removeObserver(Observer o);
    public void notifyObservers();
}

2. Observer.java

package com.chp2.observerpattern.custom;

/**
 * The objects who will be subscribers to the broadcaster will implement this interface.
 */
public interface Observer {
    public void update(float temp,float humidity,float pressure);
}

3. DisplayElement.java

package com.chp2.observerpattern.custom;

/**
 * All 3 displays will implement this interface
 */
public interface DisplayElement {
    public void display();
}

4. WhetherData.java

package com.chp2.observerpattern.custom;

import java.util.ArrayList;

/**
 * This class is the broadcaster
 */
public class WheatherData implements Subject{

    private ArrayList<Observer> observers;
    private float temperature;
    private float hummidity;
    private float pressure;
    
    public WheatherData(){
        observers = new ArrayList<>();
    }

    @Override
    public void registerObserver(Observer o) {
        observers.add(o);
    }

    @Override
    public void removeObserver(Observer o) {
        if(observers.contains(o)){
            observers.remove(o);
        }
    }

    @Override
    public void notifyObservers() {
        for (Observer b_Observer : observers) {
            b_Observer.update(temperature, hummidity, pressure);
        }
    }
    
    public void measurementsChanged(){
        notifyObservers();
    }
    
    public void setMeasurements(float temp,float hum,float pres){
        this.temperature = temp;
        this.hummidity = hum;
        this.pressure = pres;
        measurementsChanged();
    }
    
}

5. CurrentCondisitonsDisplay.java

package com.chp2.observerpattern.custom;

public class CurrentConditionsDisplay implements Observer, DisplayElement{

    private float temperature;
    private float humidity;
    private Subject whetherData;
    
    public void registerObserver(Subject wd){
        this.whetherData = wd;
        this.whetherData.registerObserver(this);
    }
    
    @Override
    public void update(float temp, float humidity, float pressure) {
        this.temperature = temp;
        this.humidity = humidity;
        display();
    }

    @Override
    public void display() {
        StringBuilder sb = new StringBuilder();
        sb.append("Current conditions : " ).append(temperature).append(" F degrees and hummidity ").append(humidity);
        System.out.println(sb.toString());
        
    }
    
}

6. Testter.java

    @Test
    public void test_observer(){
        WheatherData wd = new WheatherData();
        CurrentConditionsDisplay cd = new CurrentConditionsDisplay();
        
        cd.registerObserver(wd);
        
        wd.setMeasurements(80, 65, 30.4f);
        wd.setMeasurements(82, 70, 29.2f);
        wd.setMeasurements(78, 90, 29.2f);
    }

7. Output.txt

Current conditions : 80.0 F degrees and hummidity 65.0
Current conditions : 82.0 F degrees and hummidity 70.0
Current conditions : 78.0 F degrees and hummidity 90.0