new: added searching substrings with string[] with lambdas

java8/PROBLEMS.md

@@ -33,6 +33,12 @@ java8/
 │                   │   ├── MaxMinNumberStream.java              # ✅ Complete
 │                   │   ├── MaxMinNumberTraditional.java         # ✅ Complete
 │                   │   └── Pair.java                            # ✅ Helper class
+│                   ├── Q005_print_string/
+│                   │   ├── PrintStringsComparison.java          # ✅ Complete
+│                   │   ├── PrintStringsStream.java              # ✅ Complete
+│                   │   └── PrintStringsTraditional.java         # ✅ Complete
+│                   ├── Q006_count_evens_odds/
+│                   │   └── CountEvensOddsComparison.java        # ✅ Complete (Single class)
 │                   └── util/
 │                       └── ComparisonUtils.java                 # ✅ Shared utilities
 └── target/                          # Compiled output (not tracked in git)
@@ -88,6 +94,34 @@ java8/
 
 ---
 
+### Q005: Filter Strings by Substring
+**Location:** `com.modernjava.guide.java8.Q005_print_string`  
+**Concepts:** filter(), contains(), String operations, forEach()  
+**Problem:** Print all strings containing a specific substring (e.g., "_test")  
+**Input:** `["Apple_test", "Banana", "Cherry_test", "Date", "Elderberry", "Fig_test"]`  
+**Output:** `Apple_test, Cherry_test, Fig_test`  
+**Traditional:** ✅ `PrintStringsTraditional.java` - for loop with contains() check  
+**Stream:** ✅ `PrintStringsStream.java` - `filter(n -> n.contains(substring))`  
+**Comparison:** ✅ `PrintStringsComparison.java` - tests with 20, 1K, 100K, 1M strings  
+**Note:** Large array outputs are suppressed for performance testing
+
+---
+
+### Q006: Count Even and Odd Numbers
+**Location:** `com.modernjava.guide.java8.Q006_count_evens_odds`  
+**Concepts:** count(), filter(), predicates, single-pass iteration  
+**Problem:** Count how many even and odd numbers are in an array  
+**Input:** `[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]`  
+**Output:** `Even count: 5, Odd count: 5`  
+**Implementation:** ✅ `CountEvensOddsComparison.java` - **Single class with both approaches**  
+**Methods:**
+  - `countTraditional()` - Single for loop counting both even/odd
+  - `countStream()` - Two separate stream pipelines with filter().count()
+**Test Cases:** 10, 1K, 100K elements  
+**Note:** This demonstrates the **single-class approach** for simple problems - all code in one file
+
+---
+
 ## 🛠️ Utility Classes
 
 ### ComparisonUtils
@@ -97,8 +131,11 @@ java8/
 **Methods:**
 - `repeat(String str, int count)` - Repeats string N times (Java 8 compatible alternative to String.repeat())
 - `formatTime(long nanos)` - Formats nanoseconds to human-readable format (ns, μs, ms)
-- `printNumbers(int[] numbers)` - Prints array elements comma-separated
+- `printNumbers(int[] numbers)` - Prints integer array elements comma-separated
+- `printStrings(String[] strings)` - Prints string array elements comma-separated
 - `getArrayOfSpecifiedSize(int size)` - Generates random int array with positive, negative, and zero values (range: -100 to +100)
+- `getStringArrayWithPatternAtEnd(int size, String pattern)` - Generates random string array from sample pool, randomly appending pattern to some strings
+- `getStringArray()` - Returns the predefined string samples array
 
 ---
 
@@ -108,21 +145,26 @@ java8/
 - **Terminal Operations:** `forEach()`, `count()`, `sum()`, `max()`, `min()`
 - **Intermediate Operations:** `filter()`, `map()`
 - **Method References:** `System.out::println`, `Integer::compare`
-- **Lambda Expressions:** `n -> n % 2 == 0`, `n -> n * n`
+- **Lambda Expressions:** `n -> n % 2 == 0`, `n -> n * n`, `n -> n.contains(substring)`
+- **String Operations:** `contains()`, filtering by substring
 
 ### Comparison Topics
 - Traditional vs Stream syntax
 - Performance analysis with JIT warmup
 - Code readability and maintainability
 - When to use each approach
 - Handling edge cases (empty arrays, single elements)
+- Output suppression for large datasets in performance testing
+- **Single-class approach** for simple problems (Q006+)
 
 ### Java 8 Features Demonstrated
 - ✅ Lambda expressions
 - ✅ Method references
-- ✅ Stream API basics
+- ✅ Stream API basics (int and object streams)
 - ✅ Functional interfaces
 - ✅ Optional (in max/min operations)
+- ✅ String filtering and predicates
+- ✅ Counting with filter().count()
 
 ---
 
@@ -132,23 +174,27 @@ Based on benchmark results in comparison classes:
 - **Small arrays (< 100 elements):** Traditional approach is faster due to Stream overhead
 - **Large arrays (10,000+ elements):** Stream performance improves, gap narrows
 - **Very large arrays (1,000,000+ elements):** Traditional still edges out for simple operations
+- **String operations:** Similar patterns - traditional loops excel for simple contains() checks
+- **Counting operations:** Traditional single-pass is more efficient than multiple stream pipelines
 - **Takeaway:** Use Streams for readability and composition, not raw speed in tight loops
 
 ---
 
 ## 🚧 Planned Problems
 
 ### Easy
-- Q003: Print even numbers only
-- Q004: Find sum of all numbers
-- Q005: Find max/min element
-- Q006: Count even vs odd numbers
-- Q007: Remove duplicates
+- Q007: Remove duplicates from array
+- Q008: Convert all strings to uppercase
+- Q009: Find average of numbers
+- Q010: Check if any element matches a condition
+- Q011: Sum all positive numbers
 
 ### Medium
 - Q011: Group strings by length
 - Q012: Find top K frequent elements
 - Q013: Flatten nested lists
+- Q014: Partition numbers into even/odd lists
+- Q015: Find first element matching condition
 - Q014: Partition list into even/odd
 
 ### Hard

java8/README.md

@@ -29,6 +29,8 @@ java8/
 │                   ├── Q002_print_evens/            # ✅ Filter even numbers
 │                   ├── Q003_square_nums/            # ✅ Sum of squares
 │                   ├── Q004_max_min/                # ✅ Find max & min
+│                   ├── Q005_print_string/           # ✅ Filter strings by substring
+│                   ├── Q006_count_evens_odds/       # ✅ Count even/odd (single class)
 │                   └── util/
 │                       └── ComparisonUtils.java     # ✅ Shared utilities
 └── target/                          # Compiled output (gitignored)
@@ -62,6 +64,12 @@ mvn -pl java8 exec:java -Dexec.mainClass="com.modernjava.guide.java8.Q003_square
 
 # Run Q004: Max & Min
 mvn -pl java8 exec:java -Dexec.mainClass="com.modernjava.guide.java8.Q004_max_min.MaxMinNumberComparison"
+
+# Run Q005: Filter Strings
+mvn -pl java8 exec:java -Dexec.mainClass="com.modernjava.guide.java8.Q005_print_string.PrintStringsComparison"
+
+# Run Q006: Count Even/Odd
+mvn -pl java8 exec:java -Dexec.mainClass="com.modernjava.guide.java8.Q006_count_evens_odds.CountEvensOddsComparison"
 ```
 
 ---
@@ -106,32 +114,40 @@ mvn -pl java8 exec:java -Dexec.mainClass="com.modernjava.guide.java8.Q004_max_mi
 
 ## 📊 Current Status
 
-### Completed Problems (4/4)
+### Completed Problems (6/6)
 - ✅ **Q001:** Print All Numbers - Stream basics, forEach(), method references
 - ✅ **Q002:** Print Even Numbers - filter(), predicates
 - ✅ **Q003:** Sum of Squares - map(), sum(), transformations
 - ✅ **Q004:** Max & Min - max(), min(), Optional handling
+- ✅ **Q005:** Filter Strings by Substring - filter(), contains(), String operations
+- ✅ **Q006:** Count Even/Odd Numbers - count(), filter() (single-class approach)
 
 ### Utility Classes
-- ✅ **ComparisonUtils** - Shared helpers (repeat, formatTime, printNumbers, getArrayOfSpecifiedSize)
+- ✅ **ComparisonUtils** - Shared helpers (repeat, formatTime, printNumbers, printStrings, getArrayOfSpecifiedSize, getStringArrayWithPatternAtEnd)
 
 See **[PROBLEMS.md](PROBLEMS.md)** for detailed problem statements and concepts covered.
 
 ---
 
 ## 🎓 Learning Approach
 
+**Two implementation patterns:**
+1. **Multi-file approach (Q001-Q005):** Separate Traditional/Stream/Comparison classes
+2. **Single-class approach (Q006+):** All methods in one comparison class - better for simple problems
+
 Each problem demonstrates:
 1. **Traditional Approach** - Classic Java loops and conditionals
 2. **Stream Approach** - Java 8 Stream API with lambdas
 3. **Comparison** - Side-by-side execution with performance metrics
 
 ### Key Concepts Covered
-- Lambda expressions: `n -> n % 2 == 0`
+- Lambda expressions: `n -> n % 2 == 0`, `n -> n.contains(substring)`
 - Method references: `System.out::println`
-- Stream operations: `filter()`, `map()`, `forEach()`, `sum()`, `max()`, `min()`
-- Performance analysis: nano-time benchmarks with multiple test cases
+- Stream operations: `filter()`, `map()`, `forEach()`, `sum()`, `max()`, `min()`, `count()`
+- String operations: `contains()`, filtering by substring
+- Performance analysis: nano-time benchmarks with multiple test cases (small to 1M+ elements)
 - Edge cases: empty arrays, single elements, large datasets
+- Output suppression for large dataset performance testing
 
 ---
 
@@ -179,36 +195,16 @@ See **[Troubleshoot.md](Troubleshoot.md)** for complete error reference.
 - This module uses `<source>1.8</source>` and `<target>1.8</target>` configuration
 - All utility methods are Java 8 compatible (no String.repeat(), no var, etc.)
 - Performance benchmarks include JIT warmup considerations
-- Random array generation uses range -100 to +100 (positive, negative, zero values)
+- Random int array generation uses range -100 to +100 (positive, negative, zero values)
+- Random string array generation uses predefined sample pool with configurable pattern matching
+- Large array outputs (size > 100) are suppressed during comparison tests for readability
 
 ---
 
 **See also:**
 - [Root README](../README.md) - Multi-module project overview
 - [Git Best Practices](../docs/GIT_BEST_PRACTICES.md) - Contribution guidelines
 - [Troubleshooting](../docs/TROUBLESHOOTING.md) - Global error reference
-1. **Traditional Solution** - Pre-Java 8 imperative style with loops
-2. **Stream Solution** - Java 8+ functional style with Stream API
-3. **Comparison Runner** - Side-by-side execution with performance metrics
-
-Example problems:
-- **Q001: Print Numbers** - forEach(), method references, lambda expressions (WORKING)
-- **Q002: Sum of Squares** - filter(), map(), reduce() (INCOMPLETE - needs Stream version)
-
-**For detailed structure:** See [LEARNING_STRUCTURE.md](LEARNING_STRUCTURE.md)
-
----
-
-## Java 8 Features & Resources
-
-Features newly included within Java 8: https://www.oracle.com/java/technologies/javase/8-whats-new.html
-
-Key topics to explore:
-- **Stream API**: https://docs.oracle.com/javase/tutorial/collections/streams/
-- **Lambda Expressions**: Functional interfaces and method references
-- **Optional**: Better null handling
-- **Date/Time API**: Modern date and time handling
-- **Default Methods**: Interface evolution
 
 Additional resources:
 - Java learning path: https://docs.oracle.com/javase/tutorial/tutorialLearningPaths.html

java8/src/main/java/com/modernjava/guide/java8/Q005_print_string/PrintStringsComparison.java

@@ -0,0 +1,86 @@
+package com.modernjava.guide.java8.Q005_print_string;
+
+import com.modernjava.guide.java8.util.ComparisonUtils;
+
+/**
+ * Q005: Print all strings containing a specific substring
+ * Compares Traditional approach vs Stream API approach
+ * Tests with small, medium, and large datasets
+ */
+public class PrintStringsComparison {
+    public static void main(String[] args) {
+        String substring = "_test";
+
+        System.out.println("Q005: Print All Strings containing substring '_test' using Both Approaches");
+        System.out.println(ComparisonUtils.repeat("=", 80));
+
+        // Test Case 1: Small dataset (20 strings)
+        System.out.println("\n" + ComparisonUtils.repeat("-", 30) + " Test Case 1: Small Dataset " + ComparisonUtils.repeat("-", 30));
+        String[] strings = ComparisonUtils.getStringArrayWithPatternAtEnd(20, substring);
+        System.out.println("Input (size=" + strings.length + "): ");
+        ComparisonUtils.printStrings(strings);
+        compareApproaches(strings, substring, 20);
+
+        // Test Case 2: Medium dataset (1,000 strings)
+        System.out.println("\n\n" + ComparisonUtils.repeat("-", 30) + " Test Case 2: Medium Dataset " + ComparisonUtils.repeat("-", 30));
+        strings = ComparisonUtils.getStringArrayWithPatternAtEnd(1_000, substring);
+        System.out.println("Input (size=" + strings.length + "): ");
+        System.out.println("[... 1,000 strings ...]");
+        compareApproaches(strings, substring, 1_000);
+
+        // Test Case 3: Large dataset (100,000 strings)
+        System.out.println("\n\n" + ComparisonUtils.repeat("-", 30) + " Test Case 3: Large Dataset " + ComparisonUtils.repeat("-", 30));
+        strings = ComparisonUtils.getStringArrayWithPatternAtEnd(100_000, substring);
+        System.out.println("Input (size=" + strings.length + "): ");
+        System.out.println("[... 100,000 strings ...]");
+        compareApproaches(strings, substring, 100_000);
+
+        // Test Case 4: Very Large dataset (1,000,000 strings)
+        System.out.println("\n\n" + ComparisonUtils.repeat("-", 30) + " Test Case 4: Very Large Dataset " + ComparisonUtils.repeat("-", 30));
+        strings = ComparisonUtils.getStringArrayWithPatternAtEnd(1_000_000, substring);
+        System.out.println("Input (size=" + strings.length + "): ");
+        System.out.println("[... 1,000,000 strings ...]");
+        compareApproaches(strings, substring, 1_000_000);
+    }
+
+    private static void compareApproaches(String[] strings, String substring, int size) {
+        // Traditional Approach
+        System.out.println("\nAPPROACH: Traditional");
+        System.out.println(ComparisonUtils.repeat("-", 80));
+        long startTime = System.nanoTime();
+
+        if (size > 50) {
+            System.out.println("[Skipped printing output for large array of size " + size + "]");
+        } else {
+            PrintStringsTraditional.printStrings(strings, substring);
+        }
+
+        long endTime = System.nanoTime();
+        long traditionalTime = endTime - startTime;
+        System.out.println("Time: " + traditionalTime + " ns (" + ComparisonUtils.formatTime(traditionalTime) + ")");
+
+        // Stream API Approach
+        System.out.println("\nAPPROACH: Stream API");
+        System.out.println(ComparisonUtils.repeat("-", 80));
+        long streamStartTime = System.nanoTime();
+        PrintStringsStream.searchStrings(strings, substring);
+
+
+        long streamEndTime = System.nanoTime();
+        long streamTime = streamEndTime - streamStartTime;
+        System.out.println("Time: " + streamTime + " ns (" + ComparisonUtils.formatTime(streamTime) + ")");
+
+        // Performance Comparison
+        System.out.println("\n" + ComparisonUtils.repeat("-", 80));
+        System.out.println("PERFORMANCE COMPARISON:");
+        System.out.println("Traditional: " + ComparisonUtils.formatTime(traditionalTime));
+        System.out.println("Stream API:  " + ComparisonUtils.formatTime(streamTime));
+        if (traditionalTime < streamTime) {
+            double ratio = (double) streamTime / traditionalTime;
+            System.out.println("Traditional is " + String.format("%.2f", ratio) + "x faster");
+        } else {
+            double ratio = (double) traditionalTime / streamTime;
+            System.out.println("Stream API is " + String.format("%.2f", ratio) + "x faster");
+        }
+    }
+}

java8/src/main/java/com/modernjava/guide/java8/Q005_print_string/PrintStringsStream.java

@@ -0,0 +1,35 @@
+package com.modernjava.guide.java8.Q005_print_string;
+
+import com.modernjava.guide.java8.util.ComparisonUtils;
+
+import java.util.Arrays;
+
+/**
+ * search strings containing a substring using Stream API
+ */
+public class PrintStringsStream {
+
+    public static void main(String[] args) {
+        System.out.println("Q005: Print All Strings containing substring '_test' using Stream API");
+        ComparisonUtils.repeat("-",80);;
+        String[] strings = ComparisonUtils.getStringArrayWithPatternAtEnd(40,"_test");
+                //{"Apple_test", "Banana", "Cherry_test", "Date", "Elderberry", "Fig_test", "Grape", "Honeydew_test"};
+        String subString = "_test";
+
+        System.out.println("Input with length "+strings.length+": ");
+        ComparisonUtils.printStrings(strings);
+
+        System.out.println("Output: ");
+        searchStrings(strings, subString);
+    }
+
+
+    public static void searchStrings(String[] strings, String subString){
+        Arrays.stream(strings)
+                .filter(n->n.contains(subString))
+                .forEach(n->System.out.print(n+","));
+    }
+
+
+
+}

java8/src/main/java/com/modernjava/guide/java8/Q005_print_string/PrintStringsTraditional.java

@@ -0,0 +1,30 @@
+package com.modernjava.guide.java8.Q005_print_string;
+
+import com.modernjava.guide.java8.util.ComparisonUtils;
+
+public class PrintStringsTraditional {
+    public static void main(String[] args) {
+        String substring = "_test";
+        String[] strings = ComparisonUtils.getStringArrayWithPatternAtEnd(20,substring);
+        System.out.println("Q005: Print All Strings containing substring '_test' using Traditional Approach");
+        System.out.println(ComparisonUtils.repeat("-",80));;
+        System.out.println("Input: ");
+        ComparisonUtils.printStrings(strings);
+        System.out.println("Output: ");
+        printStrings(strings, substring);
+    }
+
+    public static void printStrings(String[] strings, String substring) {
+        /**
+         * Traditional approach to print all strings in an array.
+         * Steps:
+         * 1. Use a for-each loop to iterate through each string in the array.
+         * 2. Print each string to the console.
+         */
+        for (String str : strings) {
+            if(str.contains(substring))
+                System.out.print(str+",");
+        }
+        System.out.println("\n");
+    }
+}