NullObject.java

// Null object pattern

package behavioral.nullobject;

interface Log
{
    void info(String msg);
    void warn(String msg);
}

class ConsoleLog implements Log
{

    @Override
    public void info(String msg) {
        System.out.println(msg);
    }

    @Override
    public void warn(String msg) {
        System.out.println("WARNING: " + msg);
    }
}

class BankAccount
{
    private Log log;
    private int balance;

    public BankAccount(Log log) {
        this.log = log;
    }

    public void deposit(int amount)
    {
        balance += amount;

        log.info("Deposited " + amount); // hard dependency on 'Log'
    }
}

// Null object
final class NullLog implements Log
{
    // Leave methods empty and fields default value
    @Override
    public void info(String msg) {

    }

    @Override
    public void warn(String msg) {

    }
}

class NullObjectDemo
{
    public static void main(String[] args) {
        Log log = new NullLog();
        BankAccount account = new BankAccount(log);

        account.deposit(100);
    }
}