Stream Transformation: map(..) method

map(..) method mainly used for data transformation.

it help us to map data from one form to other form.

 Stream<R> map(Function<? super T,? extends R> mapper)
Returns a stream consisting of the results of applying the given function to the elements of this stream.

Function is FunctionalInterface that takes one input and returns another input.

String a[]={"Ram","Shyam","Mohan","BhanuPratap"};

Arrays.stream(a)
       .map(x->x.length())
       .forEach(System.out::println);

Result:

3 5 5 11

You can simply replace the expression x->x.length() with your function.

Function<String,Integer> getStrLength=(x)->x.length();

String a[]={"Ram","Shyam","Mohan","BhanuPratap"};
Arrays.stream(a)
       .map(getStrLength)

Let’s consider a use-case, we have an Employee List and we just want a list of All Employee name OR email ID THEN we can map Employee object to List of Names or List of emails.

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Get list of Names(using Lambda Expression)

List<EmployeeBean> empList=AppUtil.getEmpList();

List<String> empNameList=empList.stream()
       .map(x->x.getName()).collect(Collectors.toList());

empNameList.forEach(System.out::println);


Result: 
Krish
Marina
Saloni
Saloni
Saloni
Amir

getList of all emails(using Lambda Expression):

List<String> empEmailList=AppUtil.getEmpList()
       .stream()
       .map(x->x.getEmail()).collect(Collectors.toList());

empNameList.forEach(System.out::println);

Get list of Names(using Method Reference)

List<EmployeeBean> empList=AppUtil.getEmpList();

List<String> empNameList=empList.stream()
       .map(EmployeeBean::getName).collect(Collectors.toList());

empNameList.forEach(System.out::println);


Result:
Krish
Marina
Saloni
Saloni
Saloni
Amir