Master IntStream in Java: From Basics to Real Use Case

IntStream is a primitive stream in Java (from java.util.stream) that works specifically with int values.

👉 It is part of Java 8 Streams and helps you:

• Avoid boxing/unboxing (Integer ↔ int)
• Perform fast numeric operations like sum(), average(), max()

How IntStream is Created

✔ Common Ways

1. IntStream.of(...)
2. Arrays.stream(int[])
3. list.stream().mapToInt(...)

Complete Example (Interview Ready)

Problem:

Find the average salary of employees whose age > 30

Full Code Example

import java.util.*;
import java.util.stream.*;class Employee {
    int age;
    int salary;    Employee(int age, int salary) {
        this.age = age;
        this.salary = salary;
    }    public int getAge() {
        return age;
    }    public int getSalary() {
        return salary;
    }
}public class Main {
    public static void main(String[] args) {        List<Employee> employees = Arrays.asList(
            new Employee(25, 30000),
            new Employee(35, 50000),
            new Employee(40, 70000)
        );        double avgSalary = employees.stream()
            .filter(e -> e.getAge() > 30)     // Stream<Employee>
            .mapToInt(e -> e.getSalary())     // IntStream ✅
            .average()                        // OptionalDouble
            .orElse(0);        System.out.println("Average Salary: " + avgSalary);
    }
}

import java.util.*;
import java.util.stream.*;class Employee {
    int age;
    int salary;    Employee(int age, int salary) {
        this.age = age;
        this.salary = salary;
    }    public int getAge() {
        return age;
    }    public int getSalary() {
        return salary;
    }
}public class Main {
    public static void main(String[] args) {        List<Employee> employees = Arrays.asList(
            new Employee(25, 30000),
            new Employee(35, 50000),
            new Employee(40, 70000)
        );        double avgSalary = employees.stream()
            .filter(e -> e.getAge() > 30)     // Stream<Employee>
            .mapToInt(e -> e.getSalary())     // IntStream ✅
            .average()                        // OptionalDouble
            .orElse(0);        System.out.println("Average Salary: " + avgSalary);
    }
}

Step-by-Step Flow

1. employees.stream()
   → Stream<Employee>
2. .filter(e -> e.getAge() > 30)
   → still Stream<Employee>
3. .mapToInt(e -> e.getSalary())
   → 🔥 converts to IntStream
4. .average()
   → returns OptionalDouble
5. .orElse(0)
   → gives final double

Output

Average Salary: 60000.0

Another Simple Example (Very Important)

IntStream numbers = IntStream.of(10, 20, 30, 40);int sum = numbers.sum();System.out.println(sum); // 100

Key Interview Points

• IntStream is for primitive int values
• Created using:
  • mapToInt()
  • IntStream.of()
  • Arrays.stream()
• Supports operations:
  • sum(), average(), max(), min()

Final One-Line Answer (Interview)

“IntStream is a primitive stream of integers used for efficient numeric operations, typically created using mapToInt() when converting from object streams.”