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Stream map in Java with examples
Stream map Function mappe returns a stream consisting of the results of applying the given function to the elements of this stream.

Stream map Function mapper is an intermediate operation. These operations are always lazy. Intermediate operations are invoked on a Stream instance and after they finish their processing, they give a Stream instance as output.

The map function is a method in the Stream class that represents a functional programming concept. In simple words, the map is used to transform one object into other by applying a function.

That's why the Stream.map Function mapper takes a function as an argument. For example, by using the map function, you can convert a list of String into a List of Integer by applying the Integer.valueOf method to each String on the input list.

All you need is a mapping function to convert one object to the other. Then, the map function will do the transformation for you. It is also an intermediate Stream operation, which means you can call other Stream methods, like a filter, or collect on this to create a chain of transformations.

The Java Stream API provides a functional approach to processing collections of objects. The Java Stream API was added in Java 8 along with several other functional programming features. This Java Stream tutorial will explain how these functional streams work, and how you use them.

How to Use Map and Filter in Java 8
You need a good example to understand any new concept. Since String and Integer are the most common data type in Java, I have chosen an example that is both simple and interesting.

I have a list of String: numbers e.g. "1", "2", "3", "4", "5", "6" . I want to process this list and need another List of Integer with just even numbers.

In order to find the even numbers, I first need to convert a List of String to a List of Integer. For that, I can use the map method of java.util.Stream class. But, before that, we need a Stream as a map as defined in the java.util.stream class.

This is not difficult at all, since you can get the stream from any collection, e.g. List or Set, by calling the stream method, which is defined in the java.util.Collection interface.

The map Function mapper method takes a Function, technically speaking, an object of java.util.function.Function interface. This function is then applied to each element of Stream to convert it into the type you want.

Because we need to convert a String to an Integer, we can pass either the Integer.parseInt or Integer.valueOf method to the map function.

The map will then return a Stream of Integer that contains both even and odd numbers. To select just even numbers, we can use the filter method.

It takes a predicate object which is technically a function to convert an object to boolean. We pass an object and it will return true or false. The filter, then uses that information to include the object in the result stream.

To include only even numbers, we call filter number - number%2==0 which means each number will be divided by two, and if there is no remainder, it will be selected. This is the same logic we have used while solving coding problems to check if a given number is even or odd in Java.

We are almost done. But, so far, we only have the Stream of even integers — not the List of even Integers and that's why we need to use them.

Since we need a List, I called collect Collectors.toList , which will accumulate all even numbers into a List and return.

Now, you may be thinking: how does it know to return List of Integer? Well, we need to get that information by Type inference, because we have already specified that information by storing the result into a List Integer

Introduced in Java 8, the Stream API is used to process collections of objects. A stream is a sequence of objects that supports various methods which can be pipelined to produce the desired result.
The features of Java stream are –

A stream is not a data structure instead it takes input from the Collections, Arrays or I/O channels.
Streams don’t change the original data structure, they only provide the result as per the pipelined methods.
Each intermediate operation is lazily executed and returns a stream as a result, hence various intermediate operations can be pipelined. Terminal operations mark the end of the stream and return the result.
Different Operations On Streams-
Intermediate Operations:

map: The map method is used to returns a stream consisting of the results of applying the given function to the elements of this stream.