Map is a well known functional programming concept which is incorporated into Java 8. Map is a function defined in java.util.stream.Streams class, which is used to transform each element of stream. Because of this property you can use Map in Java 8 to transform a Collection, List, Set or Map. For example, if you have a list of String and you want to convert all of them into upper case, how will you do this? Prior to Java 8, there is no function to do this. You had to iterate through List using a for loop or foreach loop and transform each element. In Java 8, you get stream, which allow you to apply many functional programming operator like map, reduce and filter. By using Map function you can apply any function to every element of Collection. It can be any predefined function or a user defined function. You not only can use lambda expression but also you can also use method references. Some examples of Map in Java 8 is to convert a list of integers and then square of each number. Map function is also an intermediate operation and it return a stream of transformed element. Stream API also provides methods like mapToDouble(), mapToInt(), and mapToLong() which returns DoubleStream, IntStream and LongStream, which are specialized stream for double, int and long data types. You can collect the result of transformation by using Collectors class, which provides several method to collect the result of transformation into List, Set or any Collection.