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Java Enum

Java's enum not only includes names but can also have fields like other classes.

Define Enum​

Only have names:

public enum Day {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
    THURSDAY, FRIDAY, SATURDAY 
}

Fields:

public enum InstanceType {
   TINY("t2.nano", 1, 0.5),
   MEDUIM("t2.medium", 2, 4),
   LARGE("t2.large", 2, 8); // NOTE: it is semicolon, not comma
   
   private final String spec;
   private final int vcpu;
   private final double memory;
}

Enum cannot extend other class because it is already inheriting from java.lang.Enum. But Enum can implement interface.

Get all enum values​

Java Enum has builtin in values() method (generated at compile time) to get all enum values. Other languages like go requires handwritten code or external generator.

for(InstanceType type : InstanceType.values()) {
    System.out.println(type);
}

Parse enum from string​

During compile time a valueOf method is generated, but it requires the string to be exact same as enum name. If you defined enum Color { RED, GREEN }, then Color.valueOf("RED") will work but Color.valueOf("red") will throw IllegalArgumentException

java.lang.IllegalArgumentException: No enum constant dev.at15.javadocs.EnumTest.Color.Red

References​