Empty object mode (3.14-3.20)

Introduce
In empty object mode （Null Object Pattern） in , An empty object replaces NULL Object instance checking .Null Object is not a check null , It's about reacting to a relationship that doesn't do anything . In this way Null Objects can also provide default behavior when data is not available .

In empty object mode , We create an abstract class that specifies various operations to be performed and an entity class that extends that class , Also create an empty object class that does not implement anything for this class , This empty object class will be used seamlessly where null values need to be checked .

Realization
We will create a definition operation （ ad locum , It's the name of the customer ） Of AbstractCustomer abstract class , And expanded AbstractCustomer Entity class of class . Factory CustomerFactory Returns... Based on the name passed by the customer RealCustomer or NullCustomer object .

NullPatternDemo, Our demo class uses CustomerFactory To demonstrate the use of the empty object pattern .

step 1

Create an abstract class .

AbstractCustomer.java

public abstract class AbstractCustomer {

    protected String name;

    public abstract boolean isNil();

    public abstract String getName();

}

step 2

Create entity classes that extend the above classes .

RealCustomer.java

public class RealCustomer extends AbstractCustomer {

    public RealCustomer(String name) {
        this.name = name;
    }

    @Override
    public boolean isNil() {
        return false;
    }

    @Override
    public String getName() {
        return name;
    }

}

NullCustomer.java

public class NullCustomer extends AbstractCustomer {

    @Override
    public String getName() {
        return "Not Available in Customer Database";
    }

    @Override
    public boolean isNil() {
        return true;
    }

}

step 3

establish  CustomerFactory  class .

CustomerFactory.java

public class CustomerFactory {

    public static final String[] names = {"Rob", "Joe", "Julie"};

    public static AbstractCustomer getCustomer(String name) {
        for (int i = 0; i < names.length; i++) {
            if (names[i].equalsIgnoreCase(name)) {
                return new RealCustomer(name);
            }
        }
        return new NullCustomer();
    }

}

step 4

Use  CustomerFactory, Based on the name passed by the customer , To get  RealCustomer  or  NullCustomer  object .

NullPatternDemo.java

public class NullPatternDemo {

    public static void main(String[] args) {
        AbstractCustomer customer1 = CustomerFactory.getCustomer("Rob");
        AbstractCustomer customer2 = CustomerFactory.getCustomer("Bob");
        AbstractCustomer customer3 = CustomerFactory.getCustomer("Julie");
        AbstractCustomer customer4 = CustomerFactory.getCustomer("Laura");

        System.out.println("Customers");
        System.out.println(customer1.getName());
        System.out.println(customer2.getName());
        System.out.println(customer3.getName());
        System.out.println(customer4.getName());
    }

}

step 5

Verify output .

Customers
Rob
Not Available in Customer Database
Julie
Not Available in Customer Database