Can I add and remove elements of enumeration at runtime in Java

Question

It is possible to add and remove elements from an enum in Java at runtime   For example  could I read in the labels and constructor arguments of an enum from a file      saua  it s just a question of whether it can be done out of interest really  I was hoping there d be some neat way of altering the running bytecode  maybe using BCEL or something  I ve also followed up with this question because I realised I wasn t totally sure when an enum should be used   I m pretty convinced that the right answer would be to use a collection that ensured uniqueness instead of an enum if I want to be able to alter the contents safely at runtime

User · Answer

A working example in widespread use is in modded Minecraft. See EnumHelper.addEnum() methods on Github

However, note that in rare situations practical experience has shown that adding Enum members can lead to some issues with the JVM optimiser. The exact issues may vary with different JVMs. But broadly it seems the optimiser may assume that some internal fields of an Enum, specifically the size of the Enum's .values() array, will not change. See issue discussion. The recommended solution there is not to make .values() a hotspot for the optimiser. So if adding to an Enum's members at runtime, it should be done once and once only when the application is initialised, and then the result of .values() should be cached to avoid making it a hotspot.

The way the optimiser works and the way it detects hotspots is obscure and may vary between different JVMs and different builds of the JVM. If you don't want to take the risk of this type of issue in production code, then don't change Enums at runtime.

User · Answer

You could try to assign properties to the ENUM you re trying to create and statically contruct it by using a loaded properties file  Big hack  but it works

User · Answer

I faced this problem on the formative project of my young career    The approach I took was to save the values and the names of the enumeration externally  and the end goal was to be able to write code that looked as close to a language enum as possible   I wanted my solution to look like this   enum HatType       BASEBALL      BRIMLESS      INDIANA JONES    HatType mine   HatType BASEBALL      prints  BASEBALL  System out println mine toString         prints true System out println mine equals HatType BASEBALL      And I ended up with something like this      in a file somewhere     1 -- gt  BASEBALL    2 -- gt  BRIMLESS    3 -- gt  INDIANA JONES  HatDynamicEnum hats   HatEnumRepository retrieve     HatEnumValue mine   hats valueOf  BASEBALL        prints  BASEBALL  System out println mine toString         prints true System out println mine equals hats valueOf  BASEBALL       Since my requirements were that it had to be possible to add members to the enum at run-time  I also implemented that functionality   hats addEnum  BATTING PRACTICE     HatEnumRepository storeEnum hats    hats   HatEnumRepository retrieve     HatEnumValue justArrived   hats valueOf  BATTING PRACTICE       file now reads     1 -- gt  BASEBALL    2 -- gt  BRIMLESS    3 -- gt  INDIANA JONES    4 -- gt  BATTING PRACTICE   I dubbed it the Dynamic Enumeration  pattern   and you read about the original design and its revised edition    The difference between the two is that the revised edition was designed after I really started to grok OO and DDD  The first one I designed when I was still writing nominally procedural DDD  under time pressure no less

User · Answer

I faced this problem on the formative project of my young career    The approach I took was to save the values and the names of the enumeration externally  and the end goal was to be able to write code that looked as close to a language enum as possible   I wanted my solution to look like this   enum HatType       BASEBALL      BRIMLESS      INDIANA JONES    HatType mine   HatType BASEBALL      prints  BASEBALL  System out println mine toString         prints true System out println mine equals HatType BASEBALL      And I ended up with something like this      in a file somewhere     1 -- gt  BASEBALL    2 -- gt  BRIMLESS    3 -- gt  INDIANA JONES  HatDynamicEnum hats   HatEnumRepository retrieve     HatEnumValue mine   hats valueOf  BASEBALL        prints  BASEBALL  System out println mine toString         prints true System out println mine equals hats valueOf  BASEBALL       Since my requirements were that it had to be possible to add members to the enum at run-time  I also implemented that functionality   hats addEnum  BATTING PRACTICE     HatEnumRepository storeEnum hats    hats   HatEnumRepository retrieve     HatEnumValue justArrived   hats valueOf  BATTING PRACTICE       file now reads     1 -- gt  BASEBALL    2 -- gt  BRIMLESS    3 -- gt  INDIANA JONES    4 -- gt  BATTING PRACTICE   I dubbed it the Dynamic Enumeration  pattern   and you read about the original design and its revised edition    The difference between the two is that the revised edition was designed after I really started to grok OO and DDD  The first one I designed when I was still writing nominally procedural DDD  under time pressure no less

User · Answer

Behind the curtain  enums are POJOs with a private constructor and a bunch of public static final values of the enum s type  see here for an example   In fact  up until Java5  it was considered best-practice to build your own enumeration this way  and Java5 introduced the enum keyword as a shorthand  See the source for Enum lt T gt  to learn more   So it should be no problem to write your own  TypeSafeEnum  with a public static final array of constants  that are read by the constructor or passed to it   Also  do yourself a favor and override equals  hashCode and toString  and if possible create a values method  The question is how to use such a dynamic enumeration    you can t read the value  PI 3 14  from a file to create enum MathConstants and then go ahead and use MathConstants PI wherever you want

User · Answer

You could try to assign properties to the ENUM you re trying to create and statically contruct it by using a loaded properties file  Big hack  but it works

User · Answer

No  enums are supposed to be a complete static enumeration   At compile time  you might want to generate your enum  java file from another source file of some sort  You could even create a  class file like this   In some cases you might want a set of standard values but allow extension  The usual way to do this is have an interface for the interface and an enum that implements that interface for the standard values  Of course  you lose the ability to switch when you only have a reference to the interface

User · Answer

You could try to assign properties to the ENUM you re trying to create and statically contruct it by using a loaded properties file  Big hack  but it works

User · Answer

You can load a Java class from source at runtime   Using JCI  BeanShell or JavaCompiler   This would allow you to change the Enum values as you wish   Note  this wouldn t change any classes which referred to these enums so this might not be very useful in reality

User · Answer

A working example in widespread use is in modded Minecraft. See EnumHelper.addEnum() methods on Github

However, note that in rare situations practical experience has shown that adding Enum members can lead to some issues with the JVM optimiser. The exact issues may vary with different JVMs. But broadly it seems the optimiser may assume that some internal fields of an Enum, specifically the size of the Enum's .values() array, will not change. See issue discussion. The recommended solution there is not to make .values() a hotspot for the optimiser. So if adding to an Enum's members at runtime, it should be done once and once only when the application is initialised, and then the result of .values() should be cached to avoid making it a hotspot.

The way the optimiser works and the way it detects hotspots is obscure and may vary between different JVMs and different builds of the JVM. If you don't want to take the risk of this type of issue in production code, then don't change Enums at runtime.

User · Answer

You can load a Java class from source at runtime   Using JCI  BeanShell or JavaCompiler   This would allow you to change the Enum values as you wish   Note  this wouldn t change any classes which referred to these enums so this might not be very useful in reality

User · Answer

No  enums are supposed to be a complete static enumeration   At compile time  you might want to generate your enum  java file from another source file of some sort  You could even create a  class file like this   In some cases you might want a set of standard values but allow extension  The usual way to do this is have an interface for the interface and an enum that implements that interface for the standard values  Of course  you lose the ability to switch when you only have a reference to the interface

User · Answer

Behind the curtain  enums are POJOs with a private constructor and a bunch of public static final values of the enum s type  see here for an example   In fact  up until Java5  it was considered best-practice to build your own enumeration this way  and Java5 introduced the enum keyword as a shorthand  See the source for Enum lt T gt  to learn more   So it should be no problem to write your own  TypeSafeEnum  with a public static final array of constants  that are read by the constructor or passed to it   Also  do yourself a favor and override equals  hashCode and toString  and if possible create a values method  The question is how to use such a dynamic enumeration    you can t read the value  PI 3 14  from a file to create enum MathConstants and then go ahead and use MathConstants PI wherever you want

User · Answer

I needed to do something like this  for unit testing purposes   and I came across this - the EnumBuster  http   www javaspecialists eu archive Issue161 html  It allows enum values to be added  removed and restored   Edit  I ve only just started using this  and found that there s some slight changes needed for java 1 5  which I m currently stuck with    Add array copyOf static helper methods  e g  take these 1 6 versions  http   www docjar com html api java util Arrays java html  Change EnumBuster undoStack to a Stack lt Memento gt    In undo    change undoStack poll   to undoStack isEmpty     null   undoStack pop     The string VALUES FIELD needs to be  ENUM VALUES  for the java 1 5 enums I ve tried so far

User · Answer

You can load a Java class from source at runtime   Using JCI  BeanShell or JavaCompiler   This would allow you to change the Enum values as you wish   Note  this wouldn t change any classes which referred to these enums so this might not be very useful in reality

User · Answer

Behind the curtain  enums are POJOs with a private constructor and a bunch of public static final values of the enum s type  see here for an example   In fact  up until Java5  it was considered best-practice to build your own enumeration this way  and Java5 introduced the enum keyword as a shorthand  See the source for Enum lt T gt  to learn more   So it should be no problem to write your own  TypeSafeEnum  with a public static final array of constants  that are read by the constructor or passed to it   Also  do yourself a favor and override equals  hashCode and toString  and if possible create a values method  The question is how to use such a dynamic enumeration    you can t read the value  PI 3 14  from a file to create enum MathConstants and then go ahead and use MathConstants PI wherever you want

User · Answer

Behind the curtain  enums are POJOs with a private constructor and a bunch of public static final values of the enum s type  see here for an example   In fact  up until Java5  it was considered best-practice to build your own enumeration this way  and Java5 introduced the enum keyword as a shorthand  See the source for Enum lt T gt  to learn more   So it should be no problem to write your own  TypeSafeEnum  with a public static final array of constants  that are read by the constructor or passed to it   Also  do yourself a favor and override equals  hashCode and toString  and if possible create a values method  The question is how to use such a dynamic enumeration    you can t read the value  PI 3 14  from a file to create enum MathConstants and then go ahead and use MathConstants PI wherever you want

User · Answer

No  enums are supposed to be a complete static enumeration   At compile time  you might want to generate your enum  java file from another source file of some sort  You could even create a  class file like this   In some cases you might want a set of standard values but allow extension  The usual way to do this is have an interface for the interface and an enum that implements that interface for the standard values  Of course  you lose the ability to switch when you only have a reference to the interface

User · Answer

I faced this problem on the formative project of my young career    The approach I took was to save the values and the names of the enumeration externally  and the end goal was to be able to write code that looked as close to a language enum as possible   I wanted my solution to look like this   enum HatType       BASEBALL      BRIMLESS      INDIANA JONES    HatType mine   HatType BASEBALL      prints  BASEBALL  System out println mine toString         prints true System out println mine equals HatType BASEBALL      And I ended up with something like this      in a file somewhere     1 -- gt  BASEBALL    2 -- gt  BRIMLESS    3 -- gt  INDIANA JONES  HatDynamicEnum hats   HatEnumRepository retrieve     HatEnumValue mine   hats valueOf  BASEBALL        prints  BASEBALL  System out println mine toString         prints true System out println mine equals hats valueOf  BASEBALL       Since my requirements were that it had to be possible to add members to the enum at run-time  I also implemented that functionality   hats addEnum  BATTING PRACTICE     HatEnumRepository storeEnum hats    hats   HatEnumRepository retrieve     HatEnumValue justArrived   hats valueOf  BATTING PRACTICE       file now reads     1 -- gt  BASEBALL    2 -- gt  BRIMLESS    3 -- gt  INDIANA JONES    4 -- gt  BATTING PRACTICE   I dubbed it the Dynamic Enumeration  pattern   and you read about the original design and its revised edition    The difference between the two is that the revised edition was designed after I really started to grok OO and DDD  The first one I designed when I was still writing nominally procedural DDD  under time pressure no less

User · Answer

I needed to do something like this  for unit testing purposes   and I came across this - the EnumBuster  http   www javaspecialists eu archive Issue161 html  It allows enum values to be added  removed and restored   Edit  I ve only just started using this  and found that there s some slight changes needed for java 1 5  which I m currently stuck with    Add array copyOf static helper methods  e g  take these 1 6 versions  http   www docjar com html api java util Arrays java html  Change EnumBuster undoStack to a Stack lt Memento gt    In undo    change undoStack poll   to undoStack isEmpty     null   undoStack pop     The string VALUES FIELD needs to be  ENUM VALUES  for the java 1 5 enums I ve tried so far

User · Answer

You could try to assign properties to the ENUM you re trying to create and statically contruct it by using a loaded properties file  Big hack  but it works

User · Answer

No  enums are supposed to be a complete static enumeration   At compile time  you might want to generate your enum  java file from another source file of some sort  You could even create a  class file like this   In some cases you might want a set of standard values but allow extension  The usual way to do this is have an interface for the interface and an enum that implements that interface for the standard values  Of course  you lose the ability to switch when you only have a reference to the interface

User · Answer

You can load a Java class from source at runtime   Using JCI  BeanShell or JavaCompiler   This would allow you to change the Enum values as you wish   Note  this wouldn t change any classes which referred to these enums so this might not be very useful in reality

User · Answer

I faced this problem on the formative project of my young career    The approach I took was to save the values and the names of the enumeration externally  and the end goal was to be able to write code that looked as close to a language enum as possible   I wanted my solution to look like this   enum HatType       BASEBALL      BRIMLESS      INDIANA JONES    HatType mine   HatType BASEBALL      prints  BASEBALL  System out println mine toString         prints true System out println mine equals HatType BASEBALL      And I ended up with something like this      in a file somewhere     1 -- gt  BASEBALL    2 -- gt  BRIMLESS    3 -- gt  INDIANA JONES  HatDynamicEnum hats   HatEnumRepository retrieve     HatEnumValue mine   hats valueOf  BASEBALL        prints  BASEBALL  System out println mine toString         prints true System out println mine equals hats valueOf  BASEBALL       Since my requirements were that it had to be possible to add members to the enum at run-time  I also implemented that functionality   hats addEnum  BATTING PRACTICE     HatEnumRepository storeEnum hats    hats   HatEnumRepository retrieve     HatEnumValue justArrived   hats valueOf  BATTING PRACTICE       file now reads     1 -- gt  BASEBALL    2 -- gt  BRIMLESS    3 -- gt  INDIANA JONES    4 -- gt  BATTING PRACTICE   I dubbed it the Dynamic Enumeration  pattern   and you read about the original design and its revised edition    The difference between the two is that the revised edition was designed after I really started to grok OO and DDD  The first one I designed when I was still writing nominally procedural DDD  under time pressure no less

[java] Can I add and remove elements of enumeration at runtime in Java

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