Java Most Asked Stream API Coding Questions

1. Filter Even Numbers Problem: Given a list of integers, return a list containing only even numbers.

2.Find Maximum

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✅ Problem

Find maximum value in list.

Example:

List<Integer> numbers = Arrays.asList(10, 5, 20, 8);

Expected Output:

20

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✅ Code

Optional<Integer> max = numbers.stream()
                               .max(Integer::compare);

1️⃣ numbers.stream()

List → Stream me convert kiya
Taaki Stream operations use kar saken.

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2️⃣ .max(Integer::compare)

Yahi important part hai 👇

❓ max() kya karta hai?

Stream ka max() method list ka largest element return karta hai.

Lekin ek problem hai:

👉 Stream ko kaise pata chale kaun bada hai?

Isliye usko chahiye:

Comparator

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🔥 Simple Version (Aise Likho Toh Clear Hoga)

Optional<Integer> max = numbers.stream()
        .max((a, b) -> a.compareTo(b));

Ya:

Optional<Integer> max = numbers.stream()
        .max(Integer::compareTo);

Same cheez hai.

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❓ Ye Optional Kyu Aaya?

Very important 🔥

Socho agar list empty ho:

List<Integer> numbers = new ArrayList<>();

T

Optional<Integer>

💎 Final Simple Meaning

numbers.stream().max(Integer::compare);

3.Sum of Elements
Problem: Calculate the sum of elements in a list of integers.

int sum = numbers.stream() 
.mapToInt(Integer::intValue) 
.sum(); 

4.List of Names to Uppercase
Problem: Convert all strings in a list to uppercase.