Java Enum Best Practice

Table of Contents

• Java Enum
• Item 35: Use instance fields instead of ordinals
• Use EnumSet
• Implement interface
• Use enum for Singleton class
• Prefer equals() over ==
• Add abstract methods(constant-specific method implementations) to enum
• Create an enum with Case Insensitive String
• Misc Tips about Enum

Java Enum

• An enumerated type is a type whose legal values consist of a fixed set of constants
• An enum type is a subclass of java.Lang.Enum. An enum cannot extend any other class.
• effectively static and final
• enum implements Comparable and Serializable.
• we can add instance fields/methods into enum
• its valueOf(String) method translates a constant’s name into the constant itself.
• Avoid use and return mutable field
  • static state is bad.

Item 35: Use instance fields instead of ordinals

• Don’t rely on the ordinal or the order of elements in values
• it’s not a contract, and may change
• If needed, use an int field to declare the contract explicitly.
• if the order matters, or should not change, add tests to guarantee it.
• assertThat(EnumSet.allOf(MyEnum.class)).containsExactly(EnumA, EnumB, ...)

Use EnumSet

• UseEnumSet.of(EnumA, EnumB) to create a set to store enum values.
  • It provides better performance compared with HashSet<MyEnum>
• Use Set<MyEnum> as the interface, only use EnumSet when create the set.

Implement interface

public enum java.nio.file.LinkOption implements OpenOption, CopyOption {
  NOFOLLOW_LINKS;

  private LinkOption() {
  }
}

public enum StandardOpenOption implements OpenOption {
  READ,
  WRITE,
  APPEND,
  TRUNCATE_EXISTING,
  CREATE,
  CREATE_NEW,
  DELETE_ON_CLOSE,
  SPARSE,
  SYNC,
  DSYNC;

  private StandardOpenOption() {
  }
}

Use enum for Singleton class

• This enum will implement an interface, use the interface when needed.
• We can mock the enum where ever the interface has been used.

Prefer equals() over ==

• EqualsIncompatibleType can detect an equality test between objects with incompatible types always returns false.

Add abstract methods(constant-specific method implementations) to enum

• each enum constant (including added in the future) will have to implement this method which guarantees that all cases are covered

Create an enum with Case Insensitive String

private static MyEnume createMyEnum(String value) {
  for (MyEnume myEnum : MyEnume.values()) {
    if (Ascii.equalsIgnoreCase(myEnum.name(), value)) {
      return cacheHandler;
    }
  }
  throw new BadRequestException("Unknown CacheOperationType: " + cacheType);
}

Misc Tips about Enum

• Switches on enum types should either handle all values, or have a default case.
• Prefer switch over if/else
• Use inner enum
• Item 34: Use enums instead of int constants
• Use EnumMap