java.util.Observer

By / 27 May 2025 / 3 minutes of reading

java.util.Observer is an interface that represents the observer in the Observer design pattern.

  • Objects that implement this interface can subscribe to an Observable object and be notified of any state changes.
  • It works with java.util.Observable to implement the publish-subscribe model.

Deprecated since Java 9 — replaced by modern event systems and reactive frameworks like Java Flow API, RxJava, and JavaBeans PropertyChangeListener.

Interface Signature

public interface Observer {
    void update(Observable o, Object arg);
}Code language: PHP (php)

Methods

The above method is invoked by Observable.notifyObservers().

Simple Program

import java.util.Observable;
import java.util.Observer;

class WeatherData extends Observable {
    public void temperatureChanged(float temp) {
        setChanged(); // Mark as changed
        notifyObservers(temp); // Notify all observers
    }
}

class TemperatureDisplay implements Observer {
    @Override
    public void update(Observable o, Object arg) {
        System.out.println("Temperature updated: " + arg + "°C");
    }
}

public class WeatherApp {
    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData();
        TemperatureDisplay display = new TemperatureDisplay();

        weatherData.addObserver(display);

        weatherData.temperatureChanged(28.5f);
        weatherData.temperatureChanged(32.0f);
    }
}

Output

Temperature updated: 28.5°C
Temperature updated: 32.0°CCode language: CSS (css)

Problem Statement:

LotusJavaPrince is developing a Banking Notification System.Mahesh subscribes to different bank alerts: low balance, large withdrawal, and loan EMI due.When the system updates, all observers (Mahesh’s alert handlers) must receive notifications instantly.

import java.util.Observable;
import java.util.Observer;

// Observable class (Bank System)
class BankSystem extends Observable {
    public void triggerAlert(String message) {
        setChanged();
        notifyObservers(message);
    }
}

// Observer 1: Balance Alert
class BalanceAlert implements Observer {
    public void update(Observable o, Object arg) {
        System.out.println("Balance Alert for Mahesh → " + arg);
    }
}

// Observer 2: Withdrawal Alert
class WithdrawalAlert implements Observer {
    public void update(Observable o, Object arg) {
        System.out.println("Withdrawal Alert for Mahesh → " + arg);
    }
}

// Observer 3: Loan EMI Alert
class LoanEMIAlert implements Observer {
    public void update(Observable o, Object arg) {
        System.out.println("Loan EMI Alert for Mahesh → " + arg);
    }
}

public class BankAlertApp {
    public static void main(String[] args) {
        BankSystem bank = new BankSystem();

        BalanceAlert balanceObserver = new BalanceAlert();
        WithdrawalAlert withdrawalObserver = new WithdrawalAlert();
        LoanEMIAlert loanObserver = new LoanEMIAlert();

        // Register observers
        bank.addObserver(balanceObserver);
        bank.addObserver(withdrawalObserver);
        bank.addObserver(loanObserver);

        // Trigger alerts
        bank.triggerAlert("Balance below ₹1000 for Mahesh.");
        bank.triggerAlert("Mahesh withdrew ₹50,000.");
        bank.triggerAlert("Loan EMI of ₹7,500 due for Mahesh.");
    }
}

Output

Balance Alert for Mahesh → Balance below ₹1000 for Mahesh.
Withdrawal Alert for Mahesh → Balance below ₹1000 for Mahesh.
Loan EMI Alert for Mahesh → Balance below ₹1000 for Mahesh.
Balance Alert for Mahesh → Mahesh withdrew ₹50,000.
Withdrawal Alert for Mahesh → Mahesh withdrew ₹50,000.
Loan EMI Alert for Mahesh → Mahesh withdrew ₹50,000.
Balance Alert for Mahesh → Loan EMI of ₹7,500 due for Mahesh.
Withdrawal Alert for Mahesh → Loan EMI of ₹7,500 due for Mahesh.
Loan EMI Alert for Mahesh → Loan EMI of ₹7,500 due for Mahesh.

Even though Observer is outdated, learning it builds a strong OO foundation and helps understand event-based architecture, which is critical for designing systems

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