Groovy - Map collectEntries(Map collector, Closure transform) method

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Description

Groovy Map collectEntries(Map collector, Closure transform) method iterates through the Map entries and transform them using transform closure to collector Map with transformed entries.

Syntax

public Map collectEntries(Map collector, Closure transform)

Parameters

• collector − the collector Map to contain the transformed entries.

• transform − a transformer closure taking one argument as Map.Entry or two arguments as key value pairs.

Return Value

collector map with transformed entries

Example - Transforming entries of a Map of String and String

Following is an example of the usage of this method −

main.groovy

// define a map
def map = ["A" : "Apple", "B" : "Banana", "C": "Carrot"] 

// transform entries of map to Map using one argument closure
result = map.collectEntries([:]){ entry -> [entry.value, entry.key] }

println(result)

// transform entries of map to Map using two arguments closure
result = map.collectEntries(["M":"MANGO"]){ key,value ->  [key, value.toUpperCase()] }

println(result)

Output

When we run the above program, we will get the following result −

[Apple:A, Banana:B, Carrot:C]
[M:MANGO, A:APPLE, B:BANANA, C:CARROT]

Example - Transforming entries of a Map of Integer and Integer

Following is an example of the usage of this method −

main.groovy

// define a map
def map = [1 : 11, 2 : 12, 3: 13] 

// transform entries of map to Map using one argument closure
result = map.collectEntries([:]){ entry -> [entry.value, entry.key] }

println(result)

// transform entries of map to Map using two arguments closure
result = map.collectEntries([:]){ key,value -> [value, key * value] }

println(result)

Output

When we run the above program, we will get the following result −

[11:1, 12:2, 13:3]
[11:11, 12:24, 13:39]

Example - Tranforming entries of a Map of Integer and Object

Following is an example of the usage of this method −

main.groovy

// define a map
def map = [1 : new Student(1, "Julie"), 2 : new Student(2, "Robert")] 

// transform entries of map to Map using one argument closure
result = map.collectEntries([:]){ entry -> [entry.value, entry.key] }

println(result)

// transform entries of map to List using two arguments closure
result = map.collectEntries([:]){ key,value -> [value, key] }

println(result)

class Student{
   int rollNo
   String name

   Student(int rollNo, String name){
      this.rollNo = rollNo
      this.name = name
   }

   @Override
   public boolean equals(Object obj) {
      Student s = (Student)obj
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name)
   }
   
   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]"
   }
}

Output

When we run the above program, we will get the following result −

[[ 1, Julie ]:1, [ 2, Robert ]:2]
[[ 1, Julie ]:1, [ 2, Robert ]:2]