Creating a map from a list in Groovy can be a time-consuming task, but with the right shortcuts, it can be a breeze. In this article, we will explore a simple and efficient method for converting a list into a map using Groovy.
First, let's clarify what a map and a list are in Groovy. A map is a collection of key-value pairs, where each key is unique and associated with a value. On the other hand, a list is an ordered collection of elements, where each element is identified by its index.
Now, let's say we have a list of countries and their respective currencies:
```
def countryList = ["USA", "Canada", "Japan"]
def currencyList = ["USD", "CAD", "JPY"]
```
Our goal is to create a map where the countries are the keys, and the currencies are the values. Traditionally, we would create an empty map and iterate through the list to add each element as a key-value pair. However, with Groovy, we can achieve this in a single line of code using the `collectEntries` method.
The `collectEntries` method takes a closure as its parameter and returns a map with the elements of the list as key-value pairs. Let's see how we can use it to create our map:
```
def currencyMap = countryList.collectEntries { country -> [(country): currencyList[countryList.indexOf(country)]] }
```
Let's break down this code. We first call the `collectEntries` method on the `countryList` and pass in a closure. The `country` variable in the closure represents each element in the list. Inside the closure, we create a key-value pair where the key is the current country, and the value is retrieved from the `currencyList` using the `indexOf` method. This method returns the index of the current country in the `countryList`, which we use to get the corresponding currency from the `currencyList`.
In our example, the `collectEntries` method will first iterate through the `countryList` and set "USA" as the key and "USD" as the value, then "Canada" as the key and "CAD" as the value, and so on. Finally, the method returns a map with the key-value pairs.
We can also simplify the code further by using the `toMap` method. This method takes a list of key-value pairs as its parameter and returns a map. We can use this method in combination with the `transpose` method, which will convert our two lists into a list of key-value pairs.
```
def currencyMap = [countryList, currencyList].transpose().toMap()
```
This code is much shorter and easy to understand. We first create a list with our two lists as elements, then call the `transpose` method, which will return a list of key-value pairs like this: `[[USA, USD], [Canada, CAD], [Japan, JPY]]`. Finally, we use the `toMap` method to convert this list into a map.
In both cases, the result will be the same, and we will get the following map:
```
["USA": "USD", "Canada": "CAD", "Japan": "JPY"]
```
We can also use the `collectEntries` method to create a map with more complex key-value pairs. Let's say we have a list of countries and their populations:
```
def countryList = ["USA", "Canada", "Japan"]
def populationList = [328.2, 37.5, 126.5]
```
We can use the `collectEntries` method to create a map where the country is the key, and the population is the value, but with the population rounded to the nearest million:
```
def populationMap = countryList.collectEntries { country -> [(country): Math.round(populationList[countryList.indexOf(country)] / 1000000)] }
```
This will give us the following map:
```
["USA": 328, "Canada": 38, "Japan": 127]
```
In conclusion, the `collectEntries` method is a powerful shortcut for creating maps from lists in Groovy. It simplifies our code and makes it more readable. So next time you need to convert a list into a map, remember this handy shortcut and save yourself some time and effort.
