☕ Java String Guide — Complete Programs

📋 Table of Contents

1String Creation & Memory 2length, charAt, indexOf 3substring, trim, replace 4Case & Comparison 5split, join, format 6StringBuilder & StringBuffer 7Type Conversion Methods 8Interview Programs

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Topic 1: String Creation & Memory Model

String kya hai? Kaise banti hai? Memory mein kahan jaati hai?

📖 Key Concepts

String immutable hai — ek baar bani, change nahi hoti. Koi bhi operation NAYA object banata hai.
String Literal → String Pool mein jaata hai (same value = same object)
new String() → Heap mein HAMESHA naya object banta hai
== reference compare karta hai, .equals() content compare karta hai

⚠️ Golden Rule: Strings compare karne ke liye HAMESHA .equals() use karo, kabhi == nahi!

StringCreation1.java

Program 1

▶ Output

s1 == s2 : true ← same pool reference s1 == s3 : false ← alag object (heap) s3 == s4 : false ← dono alag heap objects s1.equals(s2): true s1.equals(s3): true ← content same hai s3.equals(s4): true

StringCreation2.java

Program 2

▶ Output

Before: Hello Hash before: 12345678 After: Hello World Hash after: 87654321 ← Hash change = NAYA object bana! a = JAVA ← naya object b = Java ← unchanged! String IMMUTABLE hai!

StringCreation3.java

Program 3

▶ Output

Char[] to String: Java Char array: B a c k e n d int→String: "42" double→String: "99.99" Shortcut: "42" String→int: 123 (check: 124)

StringCreation4.java

Program 4

▶ Output

empty.isEmpty() : true blank.isEmpty() : false blank.isBlank() : true ← Java 11+ empty.length() : 0 nullStr is null — safe check! "Java".equals(null): false ← no crash! s1 == s2: false (heap vs pool) s2 == s3: true (dono pool)

StringCreation5.java

Program 5

▶ Output

JSON: { "name": "Rahul", "course": "Java Backend", "age": 21 } ------------------------------ Java Java Java ------------------------------ SMS: Dear Rahul, your OTP is 9876. Valid for 5 mins.

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Topic 2: length(), charAt(), indexOf()

String ke andar characters ko access karna aur dhundhna

📖 Key Concepts

length() — total characters count (spaces bhi count hoti hain)
charAt(i) — index i par ka character (0 se start)
indexOf(x) — pehli occurrence ka index, nahi mila toh -1
lastIndexOf(x) — aakhri occurrence ka index
contains(s) — substring hai ya nahi → boolean

⚠️ Note: indexOf() return karta hai -1 agar char/string nahi mili. Hamesha check karo: if (s.indexOf("x") != -1)

StringMethods1.java

Program 1

▶ Output

String: "Java Backend Course" length(): 19 charAt(0): J charAt(5): B charAt(last): e indexOf('a'): 1 indexOf("Backend"): 5 indexOf("xyz"): -1 ← not found lastIndexOf('a'): 6 lastIndexOf('e'): 18 indexOf('a', 2): 6 ← search from index 2 contains("Backend"): true

StringMethods2.java

Program 2

▶ Output

Original : Hello Reversed : olleH Sentence : Java Programming Language Vowels : 8 Email: hello.world@gmail.com Dots: 2

StringMethods3.java

Program 3

▶ Output

"racecar" palindrome: true All occurrences of "an" in "banana": index: 1 index: 3 URL: https://www.google.com startsWith("https"): true endsWith(".com"): true

StringMethods4.java

Program 4

▶ Output

String : "Hello World 123 Java 456" Letters: 14 Digits : 6 Spaces : 4 Others : 0 Sentence: "Java is very powerful" Words : 4

StringMethods5.java

Program 5

▶ Output

Char frequency in: "programming" --------------------- 'p' → 1 █ 'r' → 2 ██ 'o' → 1 █ 'g' → 2 ██ 'a' → 1 █ 'm' → 2 ██ 'i' → 1 █ 'n' → 1 █

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Topic 3: substring(), trim(), replace()

String se parts nikalna, spaces hatana, aur content replace karna

📖 Key Concepts

substring(start) — start se end tak extract karo
substring(start, end) — end index EXCLUDED hai!
trim() — leading/trailing whitespace hatao
replace(old, new) — sabhi occurrences replace (no regex)
replaceAll(regex, new) — regex pattern use karta hai
replaceFirst(regex, new) — sirf pehli occurrence

⚠️ Yaad rakho: substring(0, 4) mein 4 index EXCLUDED hai — sirf 0,1,2,3 ke chars milenge!

SubstringReplace1.java

Program 1

▶ Output

Java World World Hello Java Last 5: World First 5: Hello Email : student@gmail.com User : student Domain : gmail.com

SubstringReplace2.java

Program 2

▶ Output

SubstringReplace3.java

Program 3

▶ Output

replace 'a'→'X': bXnXnX replace Java→Python: I love Python. Python is awesome! replaceFirst: I love Python. Java is awesome! Before: |Hello World Java| After: |Hello World Java| ← multiple spaces → one Remove digits: JavaBackendCourse Clean phone: 919876543210

SubstringReplace4.java

Program 4

▶ Output

Password: mySecret123 Masked : my*******23 Original : rahul kumar sharma Title Case : Rahul Kumar Sharma URL : https://api.example.com/users/profile Host: https://api.example.com Path: /users/profile

SubstringReplace5.java

Program 5

▶ Output

camelCase : backendJavaDevelopmentCourse snake_case: backend_java_development_course Back to camel: backendJavaDevelopmentCourse First : Hello Middle: Beautiful Last : World

🔴

Topic 4: Case & Comparison Methods

String ka case change karna aur correct tarike se compare karna

📖 Key Concepts

toUpperCase() / toLowerCase() — case convert karo (naya object banta hai)
equals() — case-sensitive comparison → boolean
equalsIgnoreCase() — case ignore karke compare (login ke liye)
compareTo() — Negative=pehle, Zero=equal, Positive=baad mein
matches(regex) — full string ko regex se validate karo

CompareCase1.java

Program 1

▶ Output

Original : Java Backend toUpperCase : JAVA BACKEND toLowerCase : java backend s1.equals(s2) : false s1.equals(s3) : true s1.equalsIgnoreCase(s2) : true ✅ Login Successful!

CompareCase2.java

Program 2

▶ Output

"apple".compareTo("banana") = -1 ← pehle aata hai "apple".compareTo("apple") = 0 ← equal "banana".compareTo("apple") = 1 ← baad mein aata hai Before: Rahul Amit Priya Vikram Deepa After: Amit Deepa Priya Rahul Vikram Smallest: Apple | Largest: Mango

CompareCase3.java

Program 3

▶ Output

"Java".equals(a): true "Java".equals(b): false ← null safe, no crash! Objects.equals(a, "Java"): true Objects.equals(b, "Java"): false Objects.equals(null,null): true "9876543210" valid phone: true "test@gmail.com" valid email: true

CompareCase4.java

Program 4

▶ Output

"listen" and "silent" anagram: true Anagram pairs: triangle + integral = ✅ Anagram hello + world = ❌ Not Anagram race + care = ✅ Anagram java + avaj = ✅ Anagram

CompareCase5.java

Program 5

▶ Output

Text: Java is POWERFUL and Java is FUN. JAVA rocks! "java" (case-insensitive) count: 3 Original : Hello World JAVA SwapCase : hELLO wORLD java Upper: 5 | Lower: 11

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Topic 5: split(), join(), format()

String ko todna, milana, aur format karna

📖 Key Concepts

split(delimiter) — String → String[]; delimiter ek regex hai
split(delimiter, limit) — max limit parts mein split
String.join(sep, parts) — parts ko separator se jodo
String.format("%s %d %.2f") — %s=string, %d=int, %f=float, %-10s=left-align

⚠️ Trap: split(".") GALAT hai! Dot regex mein "any char" hota hai. Sahi likho: split("\\.")

SplitJoinFormat1.java

Program 1

▶ Output

Parts: 4 [0] = Apple [1] = Banana [2] = Mango [3] = Orange Words: [Java] [is] [the] [best] Version parts: [Java] [17] [0] [2] Split with limit 3: → one → two → three:four:five

SplitJoinFormat2.java

Program 2

▶ Output

SplitJoinFormat3.java

Program 3

▶ Output

SplitJoinFormat4.java

Program 4

▶ Output

=== Employee Details === Name : Rahul Age : 21 City : Indore Role : Java Developer Salary : 75000 === URL Parts === Full URL: https://api.example.com/users/123/orders └─ users └─ 123 └─ orders Original: Java is very powerful Reversed: powerful very is Java

SplitJoinFormat5.java

Program 5

▶ Output

Text: "the quick brown fox jumps over the lazy dog the fox" Total words: 11 Word Frequencies: ------------------------------ the → 3 ★★★ quick → 1 ★ brown → 1 ★ fox → 2 ★★ jumps → 1 ★ over → 1 ★ lazy → 1 ★ dog → 1 ★

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Topic 6: StringBuilder & StringBuffer

Mutable strings — performance ke liye zaroori!

📖 String vs StringBuilder vs StringBuffer

Feature	String	StringBuilder	StringBuffer
Mutable?	No	Yes	Yes
Thread-safe?	Yes	No	Yes
Performance (loops)	Slow ❌	Fast ⚡	Medium
Use When	Fixed values	Single thread	Multi-thread

⚡ Rule: Loop ke andar String concatenation (s += "x") AVOID karo! Hamesha StringBuilder use karo — 100x faster hai!

StringBuilder1.java

Program 1

▶ Output

Iterations : 50000 String time : 1200 ms ← bahut slow! StringBuilder : 2 ms ← blazing fast! ⚡ Both equal? : true

StringBuilder2.java

Program 2

▶ Output

Initial : Hello append : Hello World! insert(6) : Hello Java World! delete : Hello World! replace : Hi World! reverse : !dlroW iH setCharAt : Hi World! deleteAt : H World! length : 8 charAt(0) : H

StringBuilder3.java

Program 3

▶ Output

Java Backend Development 2024 Original: HelloWorld Reversed: dlroWolleH Palindrome check: madam → ✅ Palindrome racecar → ✅ Palindrome hello → ❌ Not Palindrome level → ✅ Palindrome

StringBuilder4.java

Program 4

▶ Output

<ul class="topics"> <li>Java Basics</li> <li>OOP Concepts</li> <li>String Class</li> <li>Collections</li> </ul> Query: SELECT * FROM users WHERE 1=1 AND name = 'Rahul' AND city = 'Indore' AND age >= 18;

StringBuilder5.java

Program 5

▶ Output

StringBuffer: ⚡ Thread-Safe firstIndexOf : 0 lastIndexOf : 19 substring(8,12): Java ╔═══════════════════════════════════════════╗ ║ Use String → Fixed/readonly values║ ║ Use StringBuilder → Single thread, loops║ ║ Use StringBuffer → Multi-thread apps ║ ╚═══════════════════════════════════════════╝

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Topic 7: Type Conversion Methods

Primitives ko String mein aur String ko primitives mein convert karna

📖 Key Conversions

Primitive → String: String.valueOf(x) — recommended way for all types
String → int: Integer.parseInt("123")
String → double: Double.parseDouble("3.14")
String → boolean: Boolean.parseBoolean("true")
Special: Integer.toBinaryString(10), toHexString(), toOctalString()

⚠️ Exception: Integer.parseInt("abc") → NumberFormatException! Invalid input ke liye hamesha try-catch use karo!

StringConversion1.java

Program 1

▶ Output

42 3.14 true A Decimal : 255 Binary : 1010 Hex : ff Octal : 10 int : 124 ← arithmetic works! double : 199.98 boolean: true long : 9876543210

StringConversion2.java

Program 2

▶ Output

Safe parseInt results: "123" → ✅ 123 "456.78" → ❌ Not valid int "hello" → ❌ Not valid int "99" → ✅ 99 "" → ❌ Not valid int "2147483648" → ❌ Not valid int ← int overflow! isNumeric check: 42 → ✅ Numeric 3.14 → ✅ Numeric -99 → ✅ Numeric abc → ❌ Not numeric

StringConversion3.java

Program 3

▶ Output

=== Simple Calculator === 45 + 67 = 112 100 - 37 = 63 12 * 8 = 96 100 / 4 = 25 Price : ₹1499.99 Discount : ₹269.99 Final : ₹1230.00

StringConversion4.java

Program 4

▶ Output

Bytes of "Java": 74 97 118 97 From bytes: Java SwapCase: Hello World → hELLO wORLD int max: 2147483647 long max: 9223372036854775807 Big sum : 18888888887

StringConversion5.java

Program 5

▶ Output

Hello Rahul! Score: 92/100. Grade: A+ ╔══════════════════════════════╗ ║ Name Score Grade ║ ╠══════════════════════════════╣ ║ Rahul 85 A ║ ║ Priya 92 A+ ║ ║ Amit 67 B ║ ║ Deepa 78 B+ ║ ║ Vikram 55 C ║ ╠══════════════════════════════╣ ║ Average: 75.4 ║ ╚══════════════════════════════╝

🩷

Topic 8: Interview Programs

Exam aur interview mein aane wale classic String problems

📖 Common Approaches

Two-pointer — Palindrome, reverse check (start+end)
indexOf loop — Sabhi occurrences dhundne ke liye
Sort + Compare — Anagram check
StringBuilder — String building operations
charAt + frequency — Character counting

StringInterview1.java

Program 1

▶ Output

First non-repeating in "aabcddbe": 'c' Remove duplicates: programming → progamin "abcde" rotation of "cdeab": true

StringInterview2.java

Program 2

▶ Output

Original : aaabbbccddddee Compressed: a3b3c2d4e2 Shorter? : true Original : Java is very powerful Reversed : powerful very is Java Longest word: programming

StringInterview3.java

Program 3

▶ Output

Strings: flower, flow, flight Longest common prefix: "fl" All unique chars: abcdef → ✅ All unique backend → ❌ Has duplicates java → ❌ Has duplicates swift → ✅ All unique

StringInterview4.java

Program 4

▶ Output

Count of "to": 2 Palindrome results: madam → ✅ java → ❌ level → ✅ backend → ❌ racecar → ✅ civic → ✅ "Hello World JAVA 2024!" Upper=7 Lower=7 Digit=4 Space=2 Other=1

StringInterview5.java

Program 5

▶ Output

Pangram: ✅ "The quick brown fox jumps over the lazy..." Pangram: ❌ (missing: bcdfhijkmnopqruvwxyz) "Java is the best programming language" Sentence: Hello, Java World! How are you? Punctuations: 3

📋 String Methods — Quick Reference

length()

Total characters ki count

→ int

charAt(i)

Index i par ka character

→ char

indexOf(x)

Pehli occurrence ka index (-1 if not found)

→ int

lastIndexOf(x)

Aakhri occurrence ka index

→ int

substring(s,e)

s se e-1 tak extract (end excluded)

→ String

toUpperCase()

Sabhi chars UPPERCASE mein

→ String

toLowerCase()

Sabhi chars lowercase mein

→ String

trim()

Leading/trailing spaces remove

→ String

strip() ☕11

trim() + Unicode whitespace support

→ String

replace(old,new)

Sabhi occurrences replace karo

→ String

replaceAll(rx,n)

Regex pattern se replace

→ String

split(delim)

Delimiter se todna → array

→ String[]

equals(s)

Content compare (case-sensitive)

→ boolean

equalsIgnoreCase

Case ignore karke compare

→ boolean

compareTo(s)

Lexicographic: neg/0/pos

→ int

contains(s)

Substring hai ya nahi?

→ boolean

startsWith(s)

Is prefix se shuru hai?

→ boolean

endsWith(s)

Is suffix se khatam hai?

→ boolean

isEmpty()

length == 0?

→ boolean

isBlank() ☕11

Empty ya sirf whitespace?

→ boolean

toCharArray()

char[] mein convert karo

→ char[]

String.valueOf(x)

Koi bhi type → String

→ String

String.join(d,...)

Parts ko delimiter se jodo

→ String

String.format(p)

Printf-style formatted string

→ String

intern()

String Pool mein daalo

→ String

matches(regex)

Full string regex se match?

→ boolean

repeat(n) ☕11

n baar repeat karo

→ String

getBytes()

byte[] mein convert karo

→ byte[]

⭐ Exam Tips — Yeh ZAROOR yaad rakho!

✅ String IMMUTABLE hai — koi bhi method NAYA object banata hai

✅ String literal → String Pool | new String() → Heap

✅ == sirf reference | .equals() content compare

✅ substring(start, end) mein end index EXCLUDED hai

✅ indexOf() return karta hai -1 agar nahi mila

✅ Loop mein String concat SLOW → StringBuilder use karo

✅ StringBuilder = fast, single-thread | StringBuffer = thread-safe

✅ split(".") GALAT → split("\\.") sahi hai!

✅ Null String pe method → NullPointerException!

✅ Null-safe: "Java".equals(str) — literal pehle!

✅ compareTo(): negative=pehle, 0=equal, positive=baad

✅ parseInt("abc") → NumberFormatException → try-catch!

⚠️ Common Mistakes — Exam mein yeh mat karo!

Mistake	❌ Galat	✅ Sahi
String comparison	`if (s1 == s2)`	`if (s1.equals(s2))`
Null-safe compare	`s.equals("Java")` ← NPE if null	`"Java".equals(s)` ← safe
substring end index	substring(0,4) = 5 chars sochna	substring(0,4) = 4 chars (0,1,2,3)
Loop concatenation	`for(...) s += "x";` ← slow!	`sb.append("x");`
parseInt on float	`parseInt("12.5")` → Exception!	`parseDouble("12.5")` = 12.5
split dot	`split(".")` → empty result!	`split("\\.")` → correct
Immutability forget	`s.toUpperCase();` ← s unchanged!	`s = s.toUpperCase();` ← assign back

☕ Java String — Complete Programs Guide

📋 Table of Contents

Topic 1: String Creation & Memory Model

📖 Key Concepts

Topic 2: length(), charAt(), indexOf()

📖 Key Concepts

Topic 3: substring(), trim(), replace()

📖 Key Concepts

Topic 4: Case & Comparison Methods

📖 Key Concepts

Topic 5: split(), join(), format()

📖 Key Concepts

Topic 6: StringBuilder & StringBuffer

📖 String vs StringBuilder vs StringBuffer

Topic 7: Type Conversion Methods

📖 Key Conversions

Topic 8: Interview Programs

📖 Common Approaches

📋 String Methods — Quick Reference

⭐ Exam Tips — Yeh ZAROOR yaad rakho!

⚠️ Common Mistakes — Exam mein yeh mat karo!