Open-Closed Principle (OCP)

1. Overview

Software entities such as Classes, Modules, Methods, and etc, should be open for extension but closed for modification.

2. Intuition

Open for Extension: Extend existing behavior

Closed for Modification: Existing code remains unchanged

3. Example

3.1 Violate OCP

abstract class Subscriber {}
class CallHistory {
    public static class Call {
        private LocalDateTime begin;
        private long duration;
        private Long subscriberId;
        public Call(Long subscriberId, LocalDateTime begin, long duration) {
            this.begin = begin;
            this.duration = duration;
            this.subscriberId = subscriberId;
        }
        public LocalDateTime getBegin() {
            return begin;
        }
        public long getDuration() {
            return duration;
        }
        public Long getSubscriberId() {
            return subscriberId;
        }
    }
    private static final Map<Long, List<Call>> CALLS = new Hashtable<>();
    public static List<Call> getCurrentCalls(Long subscriberId) {
        if(!CALLS.containsKey(subscriberId)) {
            return Collections.emptyList();
        }
        return CALLS.get(subscriberId);
    }
    public static void addSession(Long subscriberId, LocalDateTime begin, long duration) {
        List<Call> calls;
        if(!CALLS.containsKey(subscriberId)) {
            calls =  new LinkedList<>();
            CALLS.put(subscriberId, calls);
        } else {
            calls = CALLS.get(subscriberId);
        }
        calls.add(new Call(subscriberId, begin, duration));
    }
}
class InternetSessionHistory {
    public static class InternetSession {
        private LocalDateTime begin;
        private Long subscriberId;
        private Long dataUsed;
        public InternetSession(Long subscriberId, LocalDateTime begin, long dataUsed) {
            this.begin = begin;
            this.dataUsed = dataUsed;
            this.subscriberId = subscriberId;
        }
        public LocalDateTime getBegin() {
            return begin;
        }
        public long getDataUsed() {
            return dataUsed;
        }
        public Long getSubscriberId() {
            return subscriberId;
        }
    }
    private static final Map<Long, List<InternetSession>> SESSIONS = new HashMap<>();
    public synchronized static List<InternetSession> getCurrentSessions(Long subscriberId) {
        if(!SESSIONS.containsKey(subscriberId)) {
            return Collections.emptyList();
        }
        return SESSIONS.get(subscriberId);
    }
    public synchronized static void addSession(Long subscriberId, LocalDateTime begin, long dataUsed) {
        List<InternetSession> sessions;
        if(!SESSIONS.containsKey(subscriberId)) {
            sessions = new LinkedList<>();
            SESSIONS.put(subscriberId, sessions);
        } else {
            sessions = SESSIONS.get(subscriberId);
        }
        sessions.add(new InternetSession(subscriberId, begin, dataUsed));
    }
}
class ISPSubscriber {
    // Duplication fields
    private Long subscriberId;
    private String address;
    private Long phoneNumber;
    private int baseRate;
    private long freeUsage;
    public ISPSubscriber() {

    }
    // Common functionality
    public double calculateBill() {
        List<InternetSessionHistory.InternetSession> sessions = InternetSessionHistory.getCurrentSessions(subscriberId);
        long totalData = sessions.stream().mapToLong(InternetSessionHistory.InternetSession::getDataUsed).sum();
        long chargeableData = totalData - freeUsage;
        return chargeableData*baseRate/100;
    }
    public Long getSubscriberId() {
        return subscriberId;
    }
    public void setSubscriberId(Long subscriberId) {
        this.subscriberId = subscriberId;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }
    public Long getPhoneNumber() {
        return phoneNumber;
    }
    public void setPhoneNumber(Long phoneNumber) {
        this.phoneNumber = phoneNumber;
    }
    public int getBaseRate() {
        return baseRate;
    }
    public void setBaseRate(int baseRate) {
        this.baseRate = baseRate;
    }
    public long getFreeUsage() {
        return freeUsage;
    }
    public void setFreeUsage(long freeUsage) {
        this.freeUsage = freeUsage;
    }
}
class PhoneSubscriber {
    // Duplication fields
    private Long subscriberId;
    private String address;
    private Long phoneNumber;
    private int baseRate;
    // Common functionality
    public double calculateBill() {
        List<CallHistory.Call> sessions = CallHistory.getCurrentCalls(subscriberId);
        long totalDuration = sessions.stream().mapToLong(CallHistory.Call::getDuration).sum();
        return totalDuration*baseRate/100;
    }
    public Long getSubscriberId() {
        return subscriberId;
    }
    public void setSubscriberId(Long subscriberId) {
        this.subscriberId = subscriberId;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }
    public Long getPhoneNumber() {
        return phoneNumber;
    }
    public void setPhoneNumber(Long phoneNumber) {
        this.phoneNumber = phoneNumber;
    }
    public int getBaseRate() {
        return baseRate;
    }
    public void setBaseRate(int baseRate) {
        this.baseRate = baseRate;
    }
}

3.2 Follow OCP

abstract class Subscriber {
    // Aggregate common fields
    protected Long subscriberId;
    protected String address;
    protected Long phoneNumber;
    public Long getSubscriberId() {
        return subscriberId;
    }
    public void setSubscriberId(Long subscriberId) {
        this.subscriberId = subscriberId;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }
    public Long getPhoneNumber() {
        return phoneNumber;
    }
    public void setPhoneNumber(Long phoneNumber) {
        this.phoneNumber = phoneNumber;
    }
    // Aggregate common functionality
    public abstract double calculateBill();
}
class CallHistory {
    public static class Call {
        private LocalDateTime begin;
        private long duration;
        private Long subscriberId;
        public Call(Long subscriberId, LocalDateTime begin, long duration) {
            this.begin = begin;
            this.duration = duration;
            this.subscriberId = subscriberId;
        }
        public LocalDateTime getBegin() {
            return begin;
        }
        public long getDuration() {
            return duration;
        }
        public Long getSubscriberId() {
            return subscriberId;
        }
    }
    private static final Map<Long, List<CallHistory.Call>> CALLS = new Hashtable<>();
    public static List<CallHistory.Call> getCurrentCalls(Long subscriberId) {
        if(!CALLS.containsKey(subscriberId)) {
            return Collections.emptyList();
        }
        return CALLS.get(subscriberId);
    }
    public static void addSession(Long subscriberId, LocalDateTime begin, long duration) {
        List<CallHistory.Call> calls;
        if(!CALLS.containsKey(subscriberId)) {
            calls =  new LinkedList<>();
            CALLS.put(subscriberId, calls);
        } else {
            calls = CALLS.get(subscriberId);
        }
        calls.add(new CallHistory.Call(subscriberId, begin, duration));
    }
}
class InternetSessionHistory {

    public static class InternetSession {

        private LocalDateTime begin;

        private Long subscriberId;

        private Long dataUsed;

        public InternetSession(Long subscriberId, LocalDateTime begin, long dataUsed) {
            this.begin = begin;
            this.dataUsed = dataUsed;
            this.subscriberId = subscriberId;
        }
        public LocalDateTime getBegin() {
            return begin;
        }
        public long getDataUsed() {
            return dataUsed;
        }
        public Long getSubscriberId() {
            return subscriberId;
        }

    }
    private static final Map<Long, List<InternetSessionHistory.InternetSession>> SESSIONS = new Hashtable<>();

    public static List<InternetSessionHistory.InternetSession> getCurrentSessions(Long subscriberId) {
        if(!SESSIONS.containsKey(subscriberId)) {
            return Collections.emptyList();
        }
        return SESSIONS.get(subscriberId);
    }

    public static void addSession(Long subscriberId, LocalDateTime begin, long dataUsed) {
        List<InternetSessionHistory.InternetSession> sessions;
        if(!SESSIONS.containsKey(subscriberId)) {
            sessions = new LinkedList<>();
            SESSIONS.put(subscriberId, sessions);
        } else {
            sessions = SESSIONS.get(subscriberId);
        }
        sessions.add(new InternetSessionHistory.InternetSession(subscriberId, begin, dataUsed));
    }
}
class ISPSubscriber extends Subscriber {
    private int baseRate;
    private long freeUsage;
    //only for demonstration
    @Override
    public double calculateBill() {
        List<InternetSessionHistory.InternetSession> sessions = InternetSessionHistory.getCurrentSessions(subscriberId);
        long totalData = sessions.stream().mapToLong(InternetSessionHistory.InternetSession::getDataUsed).sum();
        long chargeableData = totalData - freeUsage;

        if(chargeableData <= 0) {
            return 0;
        }
        return chargeableData*baseRate/100;
    }
    public int getBaseRate() {
        return baseRate;
    }
    public void setBaseRate(int baseRate) {
        this.baseRate = baseRate;
    }
    public long getFreeUsage() {
        return freeUsage;
    }
    public void setFreeUsage(long freeUsage) {
        this.freeUsage = freeUsage;
    }
}
class PhoneSubscriber extends Subscriber {
    private int baseRate;
    //only for demonstration - open for extension
    @Override
    public double calculateBill() {
        List<CallHistory.Call> sessions = CallHistory.getCurrentCalls(subscriberId);
        long totalDuration = sessions.stream().mapToLong(CallHistory.Call::getDuration).sum();
        return totalDuration*baseRate/100;
    }
    public int getBaseRate() {
        return baseRate;
    }
    public void setBaseRate(int baseRate) {
        this.baseRate = baseRate;
    }
}

4. Reference

https://en.wikipedia.org/wiki/Open%E2%80%93closed_principle