groovy safely find a key in a map and return its value

Question

I want to find a specific key in a given map  If the key is found  I then want to get the value of that key from the map   This is what I managed so far   def mymap    name  Gromit   likes  cheese   id 1234   def x   mymap find  it key     likes     if x      println x value   This works  the output is  cheese  as expected  Great  but I don t want to do x value at the end  and I don t want to do if x   I want x to directly contain the value somehow   I can t get the value directly into x like this   def mymap    name  Gromit   likes  cheese   id 1234   def x   mymap find  it key     likesZZZ    value  println x   Because the find closure is null in this case  this results in a Null Pointer Exception  Of course  the above code snippet works when it key     likes   but I am not sure that I will always find the target key in the map   What is a  Groovier  and safe way to do this on a map    Check if a map has a given key  And if so  get the value of this key

User · Answer

Groovy maps can be used with the property property  so you can just do  def x   mymap likes  If the key you are looking for  for example  likes key   contains a dot itself  then you can use the syntax  def x   mymap  likes key

User · Answer

The whole point of using Maps is direct access   If you know for sure that the value in a map will never be Groovy-false  then you can do this   def mymap    name  Gromit   likes  cheese   id 1234  def key    likes   if mymap key         println mymap key      However  if the value could potentially be Groovy-false  you should use   if mymap containsKey key         println mymap key      The easiest solution  though  if you know the value isn t going to be Groovy-false  or you can ignore that   and want a default value  is like this   def value   mymap key      default    All three of these solutions are significantly faster than your examples  because they don t scan the entire map for keys   They take advantage of the HashMap  or LinkedHashMap  design that makes direct key access nearly instantaneous

User · Answer

In general  this depends what your map contains  If it has null values  things can get tricky and containsKey key  or get key  default  should be used to detect of the element really exists  In many cases the code can become simpler you can define a default value   def mymap    name  Gromit   likes  cheese   id 1234  def x1   mymap get  likes     nothing specified    println  x value    x       Note also that containsKey   or get   are much faster than setting up a closure to check the element mymap find  it key     likes     Using closure only makes sense if you really do something more complex in there   You could e g  do this   mymap find      it  is the default parameter   if  it key     likes   return false   println  x value    it value      return true    stop searching     Or with explicit parameters   mymap find  key value - gt     key     likes    return false   println  x value    value      return true    stop searching

User · Answer

The reason you get a Null Pointer Exception is because there is no key likesZZZ in your second example  Try   def mymap    name  Gromit   likes  cheese   id 1234  def x   mymap find  it key     likes    value if x      println  x value    x

User · Answer

def mymap    name  Gromit   id 1234  def x   mymap find  it key     likes     value if x      println  x value    x    println x getClass   name      checks for null and does not create an exception in Groovy  If the key does not exist  the result will be a org codehaus groovy runtime NullObject

[dictionary] groovy: safely find a key in a map and return its value

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