new: added filtering even numbers #4

durgesh137 · 2025-12-01T19:01:58Z

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Copilot

Pull request overview

This PR adds filtering and printing of even numbers functionality by implementing Q002 problem set, creating a reusable utility class, and refactoring existing code to use it. The PR adds four new Java files and updates three documentation files.

Key Changes

Created ComparisonUtils utility class with repeat() and formatTime() helper methods
Implemented Q002 (Print Even Numbers) with traditional, stream, and comparison approaches
Refactored Q001 to use the new ComparisonUtils instead of local methods

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
`ComparisonUtils.java`	New utility class providing string repeat and time formatting helpers for comparison classes
`PrintEvensTraditional.java`	Traditional for-loop implementation for filtering and printing even numbers
`PrintEvensStream.java`	Stream API implementation demonstrating filter operation with even numbers
`PrintEvensComparison.java`	Comparison runner with performance benchmarks for traditional vs stream approaches
`PrintNumbersComparison.java`	Refactored to use `ComparisonUtils` methods instead of local implementations
`SumOfSquareOfNumber.java`	Empty placeholder class for Q003
`README.md`	Updated with documentation references and quick commands
`PROBLEMS.md`	New file documenting completed and planned problems
`PROBLEM_TEMPLATE.md`	Removed duplicate problem list that now exists in PROBLEMS.md

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Copilot · 2025-12-01T19:05:46Z

java8/src/main/java/com/modernjava/guide/java8/Q002_print_evens/PrintEvensStream.java

+        Stream<Integer> integerStream = numbers.stream()
+                .filter(n -> n % 2 == 0);//filter even numbers
+
+        //filter even numbers stream is acquired, now can be printed either using method reference or lambda
+        printNumbersWithLambda(integerStream);
+        printNumsWithMethodReference(integerStream);
+
+    }
+
+    private static void printNumsWithMethodReference(Stream<Integer> integerStream) {
+        integerStream.forEach(System.out::println);
+    }
+
+    private static void printNumbersWithLambda(Stream<Integer> integerStream) {
+        integerStream.forEach(n->   System.out.println(n + ","));


Stream can only be consumed once. The second call to printNumsWithMethodReference(integerStream) will throw IllegalStateException: stream has already been operated upon or closed because the stream was already consumed by printNumbersWithLambda(). Create a new stream for each operation or remove one of the method calls.

Suggested change

Stream<Integer> integerStream = numbers.stream()

.filter(n -> n % 2 == 0);//filter even numbers

//filter even numbers stream is acquired, now can be printed either using method reference or lambda

printNumbersWithLambda(integerStream);

printNumsWithMethodReference(integerStream);

}

private static void printNumsWithMethodReference(Stream<Integer> integerStream) {

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

}

private static void printNumbersWithLambda(Stream<Integer> integerStream) {

integerStream.forEach(n-> System.out.println(n + ","));

// Each method gets its own stream to avoid IllegalStateException

printNumbersWithLambda(numbers);

printNumsWithMethodReference(numbers);

}

private static void printNumsWithMethodReference(List<Integer> numbers) {

numbers.stream()

.filter(n -> n % 2 == 0)

.forEach(System.out::println);

}

private static void printNumbersWithLambda(List<Integer> numbers) {

numbers.stream()

.filter(n -> n % 2 == 0)

.forEach(n -> System.out.println(n + ","));

Copilot · 2025-12-01T19:05:46Z

java8/src/main/java/com/modernjava/guide/java8/Q002_print_evens/PrintEvensStream.java

+    }
+
+    private static void printNumbersWithLambda(Stream<Integer> integerStream) {
+        integerStream.forEach(n->   System.out.println(n + ","));


[nitpick] Inconsistent spacing around lambda arrow operator. There should be a space before and after ->, not multiple spaces after. Change to n -> System.out.println(n + \",\").

Suggested change

integerStream.forEach(n-> System.out.println(n + ","));

integerStream.forEach(n -> System.out.println(n + ","));

Copilot · 2025-12-01T19:05:47Z

java8/README.md

- **Sum of Squares of Odd Numbers**: Combining filter, map, and reduce operations
- More coming soon...
+- **Q001: Print Numbers** - forEach(), method references, lambda expressions (WORKING)
+- **Q002: Sum of Squares** - filter(), map(), reduce() (INCOMPLETE - needs Stream version)


The README describes Q002 as 'Sum of Squares', but the actual implementation in this PR is 'Print Even Numbers'. Update the description to match the actual Q002 implementation: - **Q002: Print Even Numbers** - filter(), forEach() (COMPLETE).

Suggested change

- **Q002: Sum of Squares** - filter(), map(), reduce() (INCOMPLETE - needs Stream version)

- **Q002: Print Even Numbers** - filter(), forEach() (COMPLETE)

Copilot · 2025-12-01T19:05:47Z

java8/PROBLEMS.md

+## Planned Problems
+
+### Easy
+- Q003: Print even numbers only


Q003 is listed as 'Print even numbers only' in the Planned Problems section, but Q002 already implements printing even numbers. This appears to be a duplicate. Update to a different problem or clarify the distinction.

Suggested change

- Q003: Print even numbers only

- Q003: Print odd numbers only

Copilot · 2025-12-01T19:05:47Z

java8/src/main/java/com/modernjava/guide/java8/Q002_print_evens/PrintEvensComparison.java

+        for (Integer number : numbers) {
+            if(number % 2 == 0) {
+                // No output for performance testing
+                number.toString();


Calling toString() on a statement by itself has no effect and will be optimized away by the compiler. For accurate performance testing, assign the result to a volatile variable or use a method like Objects.requireNonNull() to ensure the operation isn't eliminated.

Copilot · 2025-12-01T19:05:47Z

java8/src/main/java/com/modernjava/guide/java8/Q002_print_evens/PrintEvensComparison.java

+    private static void printStreamSilent(List<Integer> numbers) {
+        numbers.stream()
+                .filter(n -> n % 2 == 0)
+                .forEach(n -> n.toString());


Calling toString() within forEach has no side effects and may be optimized away. For accurate performance testing, use a volatile variable to store results or use a method that ensures the operation isn't eliminated by the JIT compiler.

Copilot · 2025-12-01T19:05:48Z

java8/src/main/java/com/modernjava/guide/java8/Q002_print_evens/PrintEvensTraditional.java

+public class PrintEvensTraditional {
+    public static void main(String[] args) {
+        System.out.println("Print Even numbers in the list - Traditional way");
+        List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10,1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10);


[nitpick] Missing space after comma before 1 (between 10,1). Add space for consistent formatting.

Suggested change

List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10,1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10);

List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10, 1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10);

Copilot · 2025-12-01T19:05:48Z

java8/src/main/java/com/modernjava/guide/java8/Q002_print_evens/PrintEvensStream.java

+public class PrintEvensStream {
+    public static void main(String[] args) {
+        System.out.println("Print all Even numbers in the list - Functional way");
+        List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10,1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10);


[nitpick] Missing space after comma before 1 (between 10,1). Add space for consistent formatting.

Suggested change

List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10,1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10);

List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10, 1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10);

Copilot · 2025-12-01T19:05:48Z

java8/src/main/java/com/modernjava/guide/java8/Q002_print_evens/PrintEvensComparison.java

+public class PrintEvensComparison {
+
+    public static void main(String[] args) {
+        List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10,1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10);


[nitpick] Missing space after comma before 1 (between 10,1). Add space for consistent formatting.

Suggested change

List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10,1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10);

List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10, 1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10);

new: added filtering even numbers

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Copilot AI review requested due to automatic review settings December 1, 2025 19:01

Copilot started reviewing on behalf of durgesh137 December 1, 2025 19:02 View session

durgesh137 merged commit aff9fbe into main Dec 1, 2025
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Copilot finished reviewing on behalf of durgesh137 December 1, 2025 19:04

Copilot AI reviewed Dec 1, 2025

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	integerStream.forEach(n-> System.out.println(n + ","));
	integerStream.forEach(n -> System.out.println(n + ","));

	- Q002: Sum of Squares - filter(), map(), reduce() (INCOMPLETE - needs Stream version)
	- Q002: Print Even Numbers - filter(), forEach() (COMPLETE)

	- Q003: Print even numbers only
	- Q003: Print odd numbers only

	List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10,1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10);
	List<Integer> numbers = Arrays.asList(1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10, 1, 2, 1, 3, 2, 4, 5, 6, 2, 2, 7, 8, 4, 9, 10);

new: added filtering even numbers #4

new: added filtering even numbers #4

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durgesh137 commented Dec 1, 2025

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