Java Streams API

Java Streams API (Java 8+) provides a functional way to process collections of data. Streams let you filter, map, sort, and reduce data with clean, readable code — replacing verbose loops with elegant operations. This is a foundational concept in enterprise application development that professional developers rely on daily. The explanations below are written to be beginner-friendly while covering the depth and nuance that comes from real-world Java experience. Take your time with each section and practice the examples

45 min•By Priygop Team•Last updated: Feb 2026

Streams API

Example

import java.util.*;
import java.util.stream.*;

public class StreamsDemo {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
        
        // Filter: keep only even numbers
        List<Integer> evens = numbers.stream()
            .filter(n -> n % 2 == 0)
            .collect(Collectors.toList());
        System.out.println("Evens: " + evens);
        
        // Map: transform each element
        List<Integer> squares = numbers.stream()
            .map(n -> n * n)
            .collect(Collectors.toList());
        System.out.println("Squares: " + squares);
        
        // Reduce: combine into single result
        int sum = numbers.stream()
            .reduce(0, Integer::sum);
        System.out.println("Sum: " + sum);
        
        // Chaining operations
        double avgOfSquaredEvens = numbers.stream()
            .filter(n -> n % 2 == 0)
            .map(n -> n * n)
            .mapToInt(Integer::intValue)
            .average()
            .orElse(0);
        System.out.println("Avg of squared evens: " + avgOfSquaredEvens);
        
        // String streams
        List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "Anna", "Alex");
        
        List<String> aNames = names.stream()
            .filter(name -> name.startsWith("A"))
            .sorted()
            .map(String::toUpperCase)
            .collect(Collectors.toList());
        System.out.println("A-names (sorted, uppercase): " + aNames);
        
        // Count, min, max
        System.out.println("\nCount > 5: " + numbers.stream().filter(n -> n > 5).count());
        System.out.println("Max: " + numbers.stream().max(Integer::compare).orElse(0));
        System.out.println("Min: " + numbers.stream().min(Integer::compare).orElse(0));
    }
}

Try It Yourself: Student Analytics

Try It Yourself: Student AnalyticsJava

Java Editor

✓ ValidTab = 2 spaces

import java.util.*;
import java.util.stream.*;

public class StudentAnalytics {
    public static void main(String[] args) {
        // Student data: Name, Grade, Age
        String[][] data = {
            {"Alice", "92", "20"}, {"Bob", "78", "22"}, {"Charlie", "95", "21"},
            {"Diana", "65", "23"}, {"Eve", "88", "20"}, {"Frank", "72", "24"},
            {"Grace", "91", "21"}, {"Henry", "83", "22"}, {"Ivy", "97", "20"}
        };
        
        // Convert to lists
        List<String> names = Arrays.stream(data).map(d -> d[0]).collect(Collectors.toList());
        List<Integer> grades = Arrays.stream(data).map(d -> Integer.parseInt(d[1])).collect(Collectors.toList());
        
        // Average grade
        double avg = grades.stream().mapToInt(Integer::intValue).average().orElse(0);
        System.out.println("Class Average: " + String.format("%.1f", avg));
        
        // Top students (>= 90)
        System.out.println("\nHonor Roll (>= 90):");
        for (int i = 0; i < data.length; i++) {
            int grade = Integer.parseInt(data[i][1]);
            if (grade >= 90) {
                System.out.println("  " + data[i][0] + ": " + grade);
            }
        }
        
        // Highest and lowest
        int maxGrade = grades.stream().max(Integer::compare).orElse(0);
        int minGrade = grades.stream().min(Integer::compare).orElse(0);
        System.out.println("\nHighest: " + maxGrade);
        System.out.println("Lowest: " + minGrade);
        
        // Grade distribution
        long aCount = grades.stream().filter(g -> g >= 90).count();
        long bCount = grades.stream().filter(g -> g >= 80 && g < 90).count();
        long cCount = grades.stream().filter(g -> g >= 70 && g < 80).count();
        long fCount = grades.stream().filter(g -> g < 70).count();
        
        System.out.println("\n=== Grade Distribution ===");
        System.out.println("A (90+): " + aCount + " students");
        System.out.println("B (80-89): " + bCount + " students");
        System.out.println("C (70-79): " + cCount + " students");
        System.out.println("F (<70): " + fCount + " students");
    }
}

import java.util.*;
import java.util.stream.*;

public class StudentAnalytics {
    public static void main(String[] args) {
        // Student data: Name, Grade, Age
        String[][] data = {
            {"Alice", "92", "20"}, {"Bob", "78", "22"}, {"Charlie", "95", "21"},
            {"Diana", "65", "23"}, {"Eve", "88", "20"}, {"Frank", "72", "24"},
            {"Grace", "91", "21"}, {"Henry", "83", "22"}, {"Ivy", "97", "20"}
        };
        
        // Convert to lists
        List<String> names = Arrays.stream(data).map(d -> d[0]).collect(Collectors.toList());
        List<Integer> grades = Arrays.stream(data).map(d -> Integer.parseInt(d[1])).collect(Collectors.toList());
        
        // Average grade
        double avg = grades.stream().mapToInt(Integer::intValue).average().orElse(0);
        System.out.println("Class Average: " + String.format("%.1f", avg));
        
        // Top students (>= 90)
        System.out.println("\nHonor Roll (>= 90):");
        for (int i = 0; i < data.length; i++) {
            int grade = Integer.parseInt(data[i][1]);
            if (grade >= 90) {
                System.out.println("  " + data[i][0] + ": " + grade);
            }
        }
        
        // Highest and lowest
        int maxGrade = grades.stream().max(Integer::compare).orElse(0);
        int minGrade = grades.stream().min(Integer::compare).orElse(0);
        System.out.println("\nHighest: " + maxGrade);
        System.out.println("Lowest: " + minGrade);
        
        // Grade distribution
        long aCount = grades.stream().filter(g -> g >= 90).count();
        long bCount = grades.stream().filter(g -> g >= 80 && g < 90).count();
        long cCount = grades.stream().filter(g -> g >= 70 && g < 80).count();
        long fCount = grades.stream().filter(g -> g < 70).count();
        
        System.out.println("\n=== Grade Distribution ===");
        System.out.println("A (90+): " + aCount + " students");
        System.out.println("B (80-89): " + bCount + " students");
        System.out.println("C (70-79): " + cCount + " students");
        System.out.println("F (<70): " + fCount + " students");
    }
}

Output

Click ▶ Run to see the result

Edit the code on the left, then click Run

Java|48 lines|2160 chars|✓ Valid syntax

UTF-8

Quick Quiz: Streams

Java Streams API

45 min•By Priygop Team•Last updated: Feb 2026

Streams API

Example

import java.util.*;
import java.util.stream.*;

public class StreamsDemo {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
        
        // Filter: keep only even numbers
        List<Integer> evens = numbers.stream()
            .filter(n -> n % 2 == 0)
            .collect(Collectors.toList());
        System.out.println("Evens: " + evens);
        
        // Map: transform each element
        List<Integer> squares = numbers.stream()
            .map(n -> n * n)
            .collect(Collectors.toList());
        System.out.println("Squares: " + squares);
        
        // Reduce: combine into single result
        int sum = numbers.stream()
            .reduce(0, Integer::sum);
        System.out.println("Sum: " + sum);
        
        // Chaining operations
        double avgOfSquaredEvens = numbers.stream()
            .filter(n -> n % 2 == 0)
            .map(n -> n * n)
            .mapToInt(Integer::intValue)
            .average()
            .orElse(0);
        System.out.println("Avg of squared evens: " + avgOfSquaredEvens);
        
        // String streams
        List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "Anna", "Alex");
        
        List<String> aNames = names.stream()
            .filter(name -> name.startsWith("A"))
            .sorted()
            .map(String::toUpperCase)
            .collect(Collectors.toList());
        System.out.println("A-names (sorted, uppercase): " + aNames);
        
        // Count, min, max
        System.out.println("\nCount > 5: " + numbers.stream().filter(n -> n > 5).count());
        System.out.println("Max: " + numbers.stream().max(Integer::compare).orElse(0));
        System.out.println("Min: " + numbers.stream().min(Integer::compare).orElse(0));
    }
}

Try It Yourself: Student Analytics

Try It Yourself: Student AnalyticsJava

Java Editor

✓ ValidTab = 2 spaces

import java.util.*;
import java.util.stream.*;

public class StudentAnalytics {
    public static void main(String[] args) {
        // Student data: Name, Grade, Age
        String[][] data = {
            {"Alice", "92", "20"}, {"Bob", "78", "22"}, {"Charlie", "95", "21"},
            {"Diana", "65", "23"}, {"Eve", "88", "20"}, {"Frank", "72", "24"},
            {"Grace", "91", "21"}, {"Henry", "83", "22"}, {"Ivy", "97", "20"}
        };
        
        // Convert to lists
        List<String> names = Arrays.stream(data).map(d -> d[0]).collect(Collectors.toList());
        List<Integer> grades = Arrays.stream(data).map(d -> Integer.parseInt(d[1])).collect(Collectors.toList());
        
        // Average grade
        double avg = grades.stream().mapToInt(Integer::intValue).average().orElse(0);
        System.out.println("Class Average: " + String.format("%.1f", avg));
        
        // Top students (>= 90)
        System.out.println("\nHonor Roll (>= 90):");
        for (int i = 0; i < data.length; i++) {
            int grade = Integer.parseInt(data[i][1]);
            if (grade >= 90) {
                System.out.println("  " + data[i][0] + ": " + grade);
            }
        }
        
        // Highest and lowest
        int maxGrade = grades.stream().max(Integer::compare).orElse(0);
        int minGrade = grades.stream().min(Integer::compare).orElse(0);
        System.out.println("\nHighest: " + maxGrade);
        System.out.println("Lowest: " + minGrade);
        
        // Grade distribution
        long aCount = grades.stream().filter(g -> g >= 90).count();
        long bCount = grades.stream().filter(g -> g >= 80 && g < 90).count();
        long cCount = grades.stream().filter(g -> g >= 70 && g < 80).count();
        long fCount = grades.stream().filter(g -> g < 70).count();
        
        System.out.println("\n=== Grade Distribution ===");
        System.out.println("A (90+): " + aCount + " students");
        System.out.println("B (80-89): " + bCount + " students");
        System.out.println("C (70-79): " + cCount + " students");
        System.out.println("F (<70): " + fCount + " students");
    }
}

import java.util.*;
import java.util.stream.*;

Output

Click ▶ Run to see the result

Edit the code on the left, then click Run

Java|48 lines|2160 chars|✓ Valid syntax

UTF-8

Java Streams API

Streams API

Try It Yourself: Student Analytics

Quick Quiz: Streams

Topics in This Module

Java Streams API

Streams API

Try It Yourself: Student Analytics

Quick Quiz: Streams

Topics in This Module