Computing the Maximum of Items in a Collection

To determine the maximum item in a collection call its max() function with no arguments.

However, if you need to find the maximum from a subset of items in a collection based on a particular condition, then pass a closure to max() that identifies the expression for which to find the maximum value. For example, to find the maximum item in the following list of users based on the number of accesses they've made to a system, do the following:

def users = [
             'smuench':[name:'Steve', badge:'A123', accesses: 1001],
             'sburns':[name:'Steve', badge:'C789', accesses: 52],
             'qbronson':[name:'Quello', badge:'Z231', accesses: 152],
             'jevans':[name:'Joe', badge:'B456', accesses: 1001]
            ]
// Return the map entry with the maximum value based on accesses
def maxUser = users.max { it.value.accesses }

The max() function returns the first item having the maximum accesses value of 1001, which is the map entry corresponding to smuench. However, to return all users having the maximum value requires first determining the maximum value of accesses and then finding all map entries having that value for their accesses property. This code looks like:

// Find the maximum value of the accesses property
def maxAccesses = users.max { it.value.accesses }.value.accesses
// Return all map entries having that value for accesses
def usersWithMaxAccesses = users.findAll{ it.value.accesses == maxAccesses }

There is often more than one way to solve a problem. Another way to compute the maximum number of accesses would be to first collect() all the accesses values, then call max() on that collection of numbers. That alternative approach looks like this:

// Find the maximum value of the accesses property
def maxAccesses = users.collect{ it.value.accesses }.max()

Using either approach to find the maximum accesses value, the resulting map produced is:

[
  smuench:[name:Steve, badge:A123, accesses:1001],
  jevans:[name:Joe, badge:B456, accesses:1001]
]

If the collection whose maximum element you seek requires a custom comparison to be done correctly, then you can pass the same kind of two-parameter comparator closure that the sort() function supports.