Java Stream map()

Java 8 Stream.map() converts Stream<X> to Stream<Y>. For each object of type X, a new object of type Y is created and put in the new Stream.

1. Stream map() API

1.1. Method Syntax

The Stream map() method has the following syntax.

<R> Stream<R> map(Function<? super T,? extends R> mapper)

• R represents the element type of the new stream.
• mapper is a non-interfering, stateless function to apply to each element which produces a stream of new values.
• The method returns a new stream of objects of type R.

Stream interface has three more similar methods which produce IntStream, LongStream and DoubleStream respectively after the map operation.

If the streams created after map() operations are given return types then consider using these functions directly.

IntStream mapToInt(ToIntFunction<? super T> mapper)
LongStream mapToLong(ToLongFunction<? super T> mapper)
DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper)

1.2. Description

• The map() is an intermediate operation. It returns a new Stream as return value.
• The map() operation takes a Function, which is called for each value in the input stream and produces one result value sent to the output stream.
• The mapper function used for transformation is a stateless function (does not store the information of previously processed objects) and returns only a single value.
• The map() method is used when we want to convert a Stream of X to Stream of Y.
• The mapped stream is closed after its contents have been placed into the new output stream.
• map() operation does not flatten the stream as flatMap() operation does.

2. Stream map() Example

Example 1: Converting a Stream of Strings to a Stream of Integers

In this example, we will convert a Stream<String> to Stream<Integer>. Here the mapper function Integer::valueOf() takes one string from the Stream at a time, and convert the String to an Integer.

It then put the Integer into another stream which is then collected using Collectors.toList().

List<String> listOfStrings = Arrays.asList("1", "2", "3", "4", "5");
 
List<Integer> listOfIntegers = listOfStrings.stream()
        .map(Integer::valueOf)
        .collect(Collectors.toList());
 
System.out.println(listOfIntegers);

Program output.

[1, 2, 3, 4, 5]

Example 2: Finding all distinct salaries among all employees

Java example to find all possible distinct salaries for a List of employees.

List<Employee> employeesList = Arrays.asList(
                                    new Employee(1, "Alex", 100),
                                    new Employee(2, "Brian", 100),
                                    new Employee(3, "Charles", 200),
                                    new Employee(4, "David", 200),
                                    new Employee(5, "Edward", 300),
                                    new Employee(6, "Frank", 300)
                                );
  
List<Double> distinctSalaries = employeesList.stream()
                        .map( e -> e.getSalary() )
                        .distinct()
                        .collect(Collectors.toList());

System.out.println(distinctSalaries);

Program output.

[100.0, 200.0, 300.0]

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