Strings

String is a class, and it is immutable

• String lives in java.lang. Once created, its char/byte content never changes. Methods like toUpperCase() return a new object.
• String literals ("hi") are interned in the string pool. new String("hi") forces a heap copy and is almost never what you want.
• Java 9+ stores compact strings as byte[] (LATIN-1 or UTF-16), but you still think in charAt / code points.

public class StringBasics {
    public static void main(String[] args) {
        String a = "java";
        String b = "java";
        String c = new String("java");
        System.out.println(a == b);        // true (same interned literal)
        System.out.println(a == c);        // false (different object)
        System.out.println(a.equals(c));   // true (same characters)
    }
}

== vs equals

• For references, == tests identity (same object). equals on String tests character content (and is null-safe only if you call it on the non-null side).

Core API you must know

• length(), charAt(i), substring(start, end) (end exclusive), indexOf, contains, split, trim / strip (Java 11), isBlank (Java 11), repeat (Java 11), formatted (Java 15).
• substring in modern JDKs copies (no shared backing array). Empty string "" has length 0 and is interned.

StringBuilder vs StringBuffer vs String

Concatenation, intern, and complexity

• s + t in a loop is O(n²) because each + may copy. The compiler rewrites *standalone* + chains to StringBuilder, but not across a loop. Use StringBuilder yourself.
• intern() returns the pooled instance. Do not intern every runtime string — it can leak metaspace/heap.
• String hashCode is cached. That is why strings make decent HashMap keys.
• Never compare strings with == in application code. Interviews will still ask when == happens to be true.

Reverse a string with StringBuilder

public class ReverseString {
    public static void main(String[] args) {
        String input = "LearnCodeGuru";
        String reversed = new StringBuilder(input).reverse().toString();
        System.out.println(reversed);
        System.out.println(input.equalsIgnoreCase("learncodeguru"));
    }
}

Build vs + in a loop

public class BuilderDemo {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        for (int i = 1; i <= 5; i++) {
            sb.append(i).append(' ');
        }
        System.out.println(sb.toString().strip());
    }
}