Strings

A String in Java is:

An immutable sequence of characters
Stored in String Pool for memory optimization
Represented internally as a char[] (Java 8) or byte[] with coder (Java 9+)

Once created, a String cannot be modified.

Why String is Immutable?

Security (used in URLs, DB connections, class loading)
Thread safety
Caching & String Pool optimization
HashCode caching (important for HashMap)

String Pool

String literals go to String Pool
new String("abc") creates object in heap
intern() moves string to pool

String a = "java";
String b = "java";          // same reference
String c = new String("java"); // different reference

Common String Operations

🔹 Length & Access

length()
charAt(int index)
substring(int begin)
substring(int begin, int end)

🔹 Comparison

equals()
equalsIgnoreCase()
compareTo()
compareToIgnoreCase()

⚠️ Never use == for content comparison.

🔹 Searching

contains()
indexOf()
lastIndexOf()
startsWith()
endsWith()

🔹 Modification (Creates New String)

toUpperCase()
toLowerCase()
trim()
strip() // Java 11+
stripLeading()
stripTrailing()
replace()
replaceAll()
replaceFirst()

🔹 Splitting & Joining

split()
String.join()

Java 11+ String Enhancements (Very Important)

isBlank() // checks whitespace only
lines() // stream of lines
repeat(int n)
strip() // Unicode-aware trim

Example:

” “.isBlank(); // true

“ab\ncd”.lines();

Java 12+ / 15+ Updates (FYI)

Compact Strings (internal optimization)
Text Blocks (“”” “””) – not a String method, but related

String json = """
{
"name": "Java"
}
""";

Mutable Alternatives

🔹 StringBuilder

Mutable
Faster
Not thread-safe

🔹 StringBuffer

Mutable
Thread-safe
Slower

Use StringBuilder in almost all cases.

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