README.md

Enum

Introduction

• Lets you create an ordered list of constants as a type,
• Mainly used for declaring constant,
• Simple structure:

  <access-modifier> enum <enum-type-name> {
   // List of comma separated names of enum constants
  }

• Ex:

  public enum Gender {
      MALE, FEMALE, OTHERS ; // these are enum constant
  }

• It is a convention to name the enum constants in uppercase,
• Java implements an enum type as a class,
• The compiler does a lot of work for an enum type and generates code for it that is essentially a class,
• Can't create object of enum type. But,
• It can have constructor which is called by compiler only,
• Can be used in switch statement,

Enum constant

• All enum constants are objects of the same enum type,
• In above Gender enum: MALE, FEMALE, OTHERS are enum constant,
• Name is assigned to each enum constant,
• Order number is assigned to each enum constant called ordinal,
• The ordinal starts with zero & it is incremented by one for enum constant,

  System.out.println(Gender.MALE.name()); // MALE
  System.out.println(Gender.FEMALE.name()); // FEMALE
  System.out.println(Gender.OTHERS.name()); // OTHERS
  
  System.out.println(Gender.MALE.ordinal());   // 0
  System.out.println(Gender.FEMALE.ordinal()); // 1
  System.out.println(Gender.OTHERS.ordinal()); // 2

• Each enum constant can have different body. See CommandList.java,

Enum again

• An enum type is actually a class type. So,
• You can declare pretty much everything inside an enum type body that you can declare inside a class body,
• Can't add a public or protected constructor to an enum type. If you do so, they will be automatically converted to private during compilation,
• Ex: See MyWeekDay.java:

  public enum MyWeekDay {
      SATURDAY(true),SUNDAY(false),MONDAY(false),
      TUESDAY(false), WEDNESDAY(false),THURSDAY(false), FRIDAY(false);
  
      // see above constant carefully. They all are objects. 
      // We are actually using constructor of this class to create constant.
  
      private final boolean isHoliday;
      private MyWeekDay(boolean isHoliday){
          this.isHoliday = isHoliday;
      }
  
      public boolean isHoliday() {
          return isHoliday;
      }
  
  }

• Example usage: See Test.java:

  private static int countWorkingDays(List<MyWeekDay> days){
      int counter = 0;
  
      for(MyWeekDay day : days){
          if(day.isHoliday()) continue;
  
          counter++;
      }
  
      return counter;
  }

  Calling is like:

  ArrayList<MyWeekDay> days = new ArrayList<>();
  days.add(SATURDAY);
  days.add(MONDAY);
  days.add(THURSDAY);
  
  System.out.println(countWorkingDays(days)); // 2

Comparing Two Enum Constants

• Two enum constant can be compared in 3 ways:
  • compareTo() method
    • Returns difference in their ordinal,
    • Must be of same type,
  • equals() method
    • An enum constant is equal only to itself,
    • For comparing two enum constants of different types,
  • == operator,
    • Must be of same type,
• Ex:

  Gender gender1 = Gender.MALE;
  Gender gender2 = Gender.FEMALE;
  Gender gender3 = Gender.MALE;
  
  MyWeekDay day1 = SATURDAY;
  MyWeekDay day2 = MONDAY;
  MyWeekDay day3 = MONDAY;
  
  // compareTo
  System.out.println(gender1.compareTo(gender2)); // -1
  System.out.println(gender1.compareTo(gender3)); // 0
  
  // equals
  System.out.println(gender1.equals(day1)); // false
  System.out.println(day1.equals(day2)); // false
  System.out.println(day2.equals(day2)); // true
  
  // ==
  System.out.println(gender1 == gender3); // true
  System.out.println(day1 == day3); // false

Nested enum type

• You can declare a nested enum type inside a class, an interface, or another enum type,
• Nested enum types are implicitly static,

  public class Test {
      ...
      //nested enum
      public enum NestedEnum{
          MY_VALUE1,MY_VALUE2;
      }
      ...
  }

Implementing interface

• An enum type may implement interfaces,
• An enum type is never inherited by another enum type. So,
• You must provide implementation for all abstract methods in that interface,
• See CommandList.java, Executable.java:

  public interface Executable {
      void execute();
  }

  public enum CommandList implements Executable{
      RUN{
          @Override
          public void doNothing() { }
      },
      JUMP{
          @Override
          public void doNothing() { }
      };
  
      @Override
      public void execute() {
          if(this.equals(RUN)){
              System.out.println("Running");
          }
          else if(this.equals(JUMP)){
              System.out.println("Jumping");
          }
      }
      public abstract void doNothing();
  }
  ```java
  Calling is like:

  RUN.execute(); // Running JUMP.execute(); // Jumping

Abstract method in enum

• Enum can have abstract method. But,
• You must have to override it in every constant,
• See doNothing() in CommandList.java above;

Reverse Lookup for Enum Constants

• We can get enum constant by name of ordinal,

  System.out.println(MyWeekDay.valueOf("SATURDAY")); // SATURDAY
  System.out.println(MyWeekDay.values()[0]); // SATURDAY

Range of Enum Constants

• Can find all enum constant by performing search,
• Abstract class EnumSet is used for this,
• Ex:

  EnumSet<MyWeekDay> days = EnumSet.allOf(MyWeekDay.class);
  printArray(days.toArray()); // SATURDAY SUNDAY MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY
  
  EnumSet<MyWeekDay> workingDays = EnumSet.range(SUNDAY,THURSDAY);
  printArray(workingDays.toArray()); // SUNDAY MONDAY TUESDAY WEDNESDAY THURSDAY
  
  EnumSet<MyWeekDay> holidays = EnumSet.complementOf(workingDays);
  printArray(holidays.toArray()); // SATURDAY FRIDAY 

ENUM IS MAINLY USED FOR DECLARING CONSTANT & IT SHOULD BE ENOUGH FOR YOU