Null Object pattern

Remarks#

Null Object is an object with no referenced value or with defined neutral behaviour. Its purpose is to remove the need of null pointer/reference check.

Null Object Pattern (C++)

Assuming a abstract class:

class ILogger {
    virtual ~ILogger() = default;
    virtual Log(const std::string&) = 0;
};

Instead of

void doJob(ILogger* logger) {
    if (logger) {
         logger->Log("[doJob]:Step 1");
    }
    // ...
    if (logger) {
         logger->Log("[doJob]:Step 2");
    }
    // ...
    if (logger) {
         logger->Log("[doJob]:End");
    }
}

void doJobWithoutLogging()
{
    doJob(nullptr);
}

You may create a Null Object Logger:

class NullLogger : public ILogger
{
    void Log(const std::string&) override { /* Empty */ }
};

and then change doJob in the following:

void doJob(ILogger& logger) {
    logger.Log("[doJob]:Step1");
    // ...
    logger.Log("[doJob]:Step 2");
    // ...
    logger.Log("[doJob]:End");
}

void doJobWithoutLogging()
{
    NullLogger logger;
    doJob(logger);
}

Null Object Java using enum

Given an interface:

public interface Logger {
   void log(String message);
}

Rather than usage:

public void doJob(Logger logger) {
    if (logger != null) {
       logger.log("[doJob]:Step 1");
    }
    // ...
    if (logger != null) {
       logger.log("[doJob]:Step 2");
    }
    // ...
    if (logger != null) {
       logger.log("[doJob]:Step 3");
    }
}

public void doJob() {
    doJob(null); // Without Logging
}

Because null objects have no state, it makes sense to use a enum singleton for it, so given a null object implemented like so:

public enum NullLogger implements Logger {
    INSTANCE;

    @Override
    public void log(String message) {
        // Do nothing
    }
}

You can then avoid the null checks.

public void doJob(Logger logger) {
    logger.log("[doJob]:Step 1");
    // ...
    logger.log("[doJob]:Step 2");
    // ...
    logger.log("[doJob]:Step 3");
}

public void doJob() {
    doJob(NullLogger.INSTANCE);
}