Java 8 - Stream peek() method - BenchResources.Net

In this article, we will discuss Stream’s peek() method in details with examples

1. Stream peek() method :

This Stream method is an intermediate operation which reads given stream applying provided Consumer action on each elements, as they pass through the Stream
Stream.peek() method does nothing until a terminal operation is executed on the Stream
Stream’s peek() method is stateless which means it is non-interfering with other elements in the stream
For parallel stream pipelines, the action may be called at whatever time and in whatever thread the element is made available by the upstream operation
If the action modifies shared state, it is responsible for providing the required synchronization
API Note :- This method exists mainly to support debugging, where you want to see the elements as they flow past a certain point in a pipeline
Method signature :- Stream<T> peek(Consumer<? super T> action)

2. Stream peek() method examples :

2.1 Stream.peek() does nothing

A list contains integers numbers
We are applying peek() method to sneak through Stream elements to print to the console
But nothing gets printed to the console because an intermediate operation doesn’t get executed until a terminal operation is executed
Here, we don’t have a terminal operation

package net.bench.resources.stream.peek.example;

import java.util.Arrays;
import java.util.List;

public class StreamPeekMethod {

	public static void main(String[] args) {

		// list of integer numbers
		List<Integer> numbers = Arrays.asList(
				1,2,3,4,5,6,7
				);

		// Stream.peek()
		numbers // data source
		.stream() // get Stream
		.peek(System.out::println); // intermediate operation
	}
}

Output:

2.2 Stream.peek() with terminal operation

A list contains integers numbers
We are applying peek() method to sneak through Stream elements to print to the console
Here, all integers numbers with duplicates gets printed to console because of terminal operation collect()
collect() method collects elements and store it into Set thereby removing duplicates in the unordered/random fashion

package net.bench.resources.stream.peek.example;

import java.util.Arrays;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;

public class StreamPeekMethodWithTerminalOp {

	public static void main(String[] args) {

		// list of integer numbers
		List<Integer> numbers = Arrays.asList(
				1,
				2,2,
				5,5,5,
				7,7,7
				);

		// Stream.peek()
		System.out.println("Stream.peek() result :- ");
		Set<Integer> uniqueNumbers = 
				numbers // data source
				.stream() // get Stream
				.peek(System.out::println) // intermediate Op
				.collect(Collectors.toSet()) // terminal Op
				;

		// print to console
		System.out.println("\nPrinting Unique numbers :- ");
		uniqueNumbers.forEach(num -> System.out.print(" " + num));
	}
}

Output:

Stream.peek() result :- 
1
2
2
5
5
5
7
7
7

Printing Unique numbers :- 
 1 2 5 7

2.3 Stream.peek() with filter() and map() methods

A list contains String elements
First, we are filtering all String elements whose length is greater than 3 and then invoking peek() method to print the value to the console
Secondly, we are applying map() method to the already filtered values to map String elements to Upper Case and then invoking peek() method to print the value to the console
Finally, all filtered and upper case mapped String elements are collected into List using a terminal operation method collect()
This list is iterated/printed to the console using forEach() method

package net.bench.resources.stream.peek.example;

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class StreamPeekFilterMapMethod {

	public static void main(String[] args) {

		// order -> filter, peek, map, peek
		System.out.println("Filtering (str.length>3) and Mapping (str.UpperCase) :- \n");
		List<String> numbers = 
				Stream.of("one", "two", "three", "four") // data source
				.filter(e -> e.length() > 3) // filter
				.peek(e -> System.out.println("Filtered value: " + e))
				.map(String::toUpperCase) // map
				.peek(e -> System.out.println("Mapped value: " + e))
				.collect(Collectors.toList()); // terminal operation

		// print to console
		System.out.println("\n\nAfter filtering and mapping to UpperCase :- ");
		numbers.stream().forEach(System.out::println);
	}
}

Output:

Filtering (str.length>3) and Mapping (str.UpperCase) :- 

Filtered value: three
Mapped value: THREE
Filtered value: four
Mapped value: FOUR


After filtering and mapping to UpperCase :- 
THREE
FOUR

References:

Happy Coding !!
Happy Learning !!

1. Stream peek() method :

2. Stream peek() method examples :

2.1 Stream.peek() does nothing

2.2 Stream.peek() with terminal operation

2.3 Stream.peek() with filter() and map() methods

References:

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