Java Design Patterns Tutorial - Java Design Pattern - Null Object Pattern

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In Null Object pattern, a business-meaningless object is created incase of null object.

We use the business-meaningless object to replace the null pointer check.

We call the a business-meaningless object Null Object.

Null object provides default behaviour when data is not available.

In Null Object pattern, we usually create an abstract class to specify the various operations.

Both Null Object and concreate classes will extends this abstract class.

The Null Object class just provide empty logic.

Example

abstract class AbstractEmployee {
   protected String name;
   public abstract boolean isNull();
   public abstract String getName();
}// w ww  .ja  va2 s .c o  m
class Programmer extends AbstractEmployee {
   public Programmer(String name) {
      this.name = name;    
   }
   @Override
   public String getName() {
      return name;
   }
   @Override
   public boolean isNull() {
      return false;
   }
}
class NullCustomer extends AbstractEmployee {
   @Override
   public String getName() {
      return "Not Available";
   }

   @Override
   public boolean isNull() {
      return true;
   }
}
class EmployeeFactory {
   public static final String[] names = {"Rob", "Joe", "Jack"};
   public static AbstractEmployee getCustomer(String name){
      for (int i = 0; i < names.length; i++) {
         if (names[i].equalsIgnoreCase(name)){
            return new Programmer(name);
         }
      }
      return new NullCustomer();
   }
}
public class Main {
   public static void main(String[] args) {

      AbstractEmployee emp = EmployeeFactory.getCustomer("Rob");
      AbstractEmployee emp2 = EmployeeFactory.getCustomer("Bob");
      AbstractEmployee emp3 = EmployeeFactory.getCustomer("Jack");
      AbstractEmployee emp4 = EmployeeFactory.getCustomer("Tom");

      System.out.println(emp.getName());
      System.out.println(emp2.getName());
      System.out.println(emp3.getName());
      System.out.println(emp4.getName());
   }
}

The code above generates the following result.

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