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Er. Bhupendra
Er. Bhupendra

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STREAM API IN JAVA

#java #programming #tutorial #beginners

❓ numbers.stream() sabse pehle kyu likhte hain?

🔷 Simple Answer

Kyuki:

Stream operations sirf Stream object par hi lagte hain,
List par directly nahi.

🔍 Thoda Deep Samjho

numbers kya hai?

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
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Ye ek List object hai.

List ke paas methods hote hain:

  • add()
  • remove()
  • size()

But List ke paas filter() method nahi hota ❌

🔥 filter() kiske paas hota hai?

Stream ke paas.

Isliye pehle:

numbers.stream()
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Ye List ko convert karta hai → Stream me

🔷 Real Flow 

List → Stream → Process → Result
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🔥 Example Samjho

Galat code:

numbers.filter(n -> n % 2 == 0);  // ❌ ERROR
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Kyuki List me filter method hi nahi hai.

Sahi code:

numbers.stream()   // List → Stream
       .filter(n -> n % 2 == 0)
       .toList();
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🔷 Internal Concept

stream() method defined hai:

Collection interface me
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Jab tum numbers.stream() likhte ho:

  • JVM ek Stream object create karta hai
  • Data ko internally pipeline me daal deta hai
  • Phir tum filter/map laga sakte ho

🔥 Example Flow Diagram 

numbers (List)
   ↓
stream()
   ↓
Stream Object
   ↓
filter()
   ↓
collect()
   ↓
Result List
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🔥 Important Interview Point

👉 Stream ek pipeline hai
👉 Stream original data ko change nahi karta
👉 Stream ek baar use hota hai
👉 stream() source hota hai

🔷 Ek Real Life Example

Socho:

numbers = raw vegetables 🥕🥦

stream() = conveyor belt machine start

filter() = sorting machine

collect() = final packed box

Machine start kiye bina processing nahi hoga 😄

🎯 Final Clear Answer

numbers.stream() isliye sabse pehle likhte hain kyuki:

✔ Stream methods List par directly available nahi hote
✔ Processing Stream object par hoti hai
✔ stream() source hota hai
✔ Ye pipeline start karta hai

Agar chaho toh next main samjha sakta hoon:

  • Stream internally kaise kaam karta hai?
  • stream() vs parallelStream() difference?
  • stream() call karte hi kya hota hai memory me?

Tum kis level ka deep explanation chahte ho? 😎

Main tumhe clear roadmap + master template de raha hoon — jisse tum Stream API ka pura concept pakad jao 💪

🔷 1️⃣ Stream API Kya Hai?

👉 Stream API Java 8 me introduce hua
Part of: java.util.stream package

👉 Stream API ka kaam hai:

Collection ke data ko functional style me process karna

Matlab:

  • Filter karna
  • Sort karna
  • Map karna
  • Group karna
  • Count karna
  • Aggregate karna

Without writing complex loops.

🔷 2️⃣ Stream API Kyu Hai? (Why Introduced?)

Before Java 8:

for(int i=0; i<list.size(); i++){
    if(list.get(i) % 2 == 0){
        result.add(list.get(i));
    }
}
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❌ Verbose
❌ Complex
❌ Hard to maintain

Stream API ne diya:

list.stream()
    .filter(n -> n % 2 == 0)
    .toList();
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✅ Short
✅ Clean
✅ Readable
✅ Functional Programming Support
✅ Parallel processing possible

🔷 3️⃣ Stream API Kya Nahi Hai?

❌ It is NOT a data structure
❌ It does NOT store data
❌ It does NOT modify original collection

Stream sirf processing pipeline hai.

🔷 4️⃣ Stream Ka Master Structure (Golden Template)

Har Stream 3 part me hota hai:

1️⃣ Source
2️⃣ Intermediate Operation
3️⃣ Terminal Operation
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Template:

collection.stream()
          .intermediateOperation()
          .terminalOperation();
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Example:

numbers.stream()          // Source
       .filter(n -> n % 2 == 0)   // Intermediate
       .collect(Collectors.toList());  // Terminal
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🔷 5️⃣ Stream Types (Important)

1️⃣ Stream → Object Stream

2️⃣ IntStream

3️⃣ LongStream

4️⃣ DoubleStream

Primitive streams better performance dete hain.

🔷 6️⃣ Intermediate Operations (Must Know List 🔥)

Ye sab lazy hote hain.

Method Kaam
filter() condition check
map() transform
flatMap() flatten
distinct() duplicate remove
sorted() sort
limit() first n
skip() skip n
peek() debug

🔷 7️⃣ Terminal Operations (Must Know 🔥)

Ye stream ko close kar dete hain.

Method Kaam
collect() convert
forEach() iterate
count() count
reduce() aggregation
anyMatch() condition
allMatch() condition
findFirst() first element

🔷 8️⃣ Most Important Concepts (Interview Level)

🔥 1. filter() 

list.stream()
    .filter(x -> x > 10)
    .toList();
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🔥 2. map() 

list.stream()
    .map(x -> x * 2)
    .toList();
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🔥 3. reduce() 

int sum = list.stream()
              .reduce(0, (a, b) -> a + b);
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🔥 4. collect() with Collectors 

Collectors.toList()
Collectors.toSet()
Collectors.groupingBy()
Collectors.counting()
Collectors.joining()
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🔷 9️⃣ Stream vs ParallelStream 

list.parallelStream()
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⚡ Multi-core processing
⚠️ Not always faster
⚠️ Order not guaranteed

🔷 🔟 Internal Working (Very Important)

Stream Pipeline:

Data → Stream → Intermediate → Terminal → Result
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👉 Lazy evaluation
👉 One-time use
👉 Does not store data

🎯 Ab Tumhe Kya Kya Padhna Padega? (Complete Roadmap)

Step 1: Basic

  • Lambda Expression
  • Functional Interface
  • Predicate
  • Function
  • Consumer
  • Supplier

Step 2: Stream Basics

  • stream()
  • filter()
  • map()
  • collect()

Step 3: Advanced

  • reduce()
  • groupingBy()
  • partitioningBy()
  • flatMap()
  • Optional class

Step 4: Parallel Stream + Performance

🚀 Master Learning Strategy (How To Study)

🔥 Rule 1:

Har concept ke 5 question solve karo.

🔥 Rule 2:

Ek hi problem ko:

  • for loop se karo
  • stream se karo

🔥 Rule 3:

Daily 30 min practice.

💎 20 Must Practice Questions

  1. Filter even numbers
  2. Find max number
  3. Find second highest
  4. Count strings length > 5
  5. Convert list to uppercase
  6. Group by age
  7. Remove duplicates
  8. Sum of list
  9. Find duplicate elements
  10. Find first non-repeated character ... etc

🎓 Interview Me Puchne Wale Tough Questions

  • Difference between map and flatMap?
  • What is lazy evaluation?
  • Why stream cannot be reused?
  • Difference between intermediate and terminal?
  • What is reduce?
  • How groupingBy works internally?
  • How parallel stream works?

🔥 Final Master Template (Yaad Karlo) 

Stream = Functional Data Processing Pipeline

1. Source
2. Intermediate (lazy)
3. Terminal (execute)
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