String Immutability and the String Pool

String Immutability and the String Pool

Why Strings are Immutable

String s = "hello";
s.toUpperCase(); // creates a new String, does NOT modify s
System.out.println(s); // still "hello"

String upper = s.toUpperCase(); // must capture the result
System.out.println(upper); // HELLO

The String Pool (Interning)

String a = "hello"; // goes into pool
String b = "hello"; // same object from pool
String c = new String("hello"); // forces new object outside pool

System.out.println(a == b);       // true  (same pool reference)
System.out.println(a == c);       // false (different objects)
System.out.println(a.equals(c));  // true  (same content)

== vs equals()

String input = new String("admin"); // from user input, not pooled
String role  = "admin";

// Dangerous: fails because different objects
if (input == role) System.out.println("Same"); // not printed!

// Correct: compares content
if (input.equals(role)) System.out.println("Equal!"); // prints

// Null-safe: put constant first to avoid NPE
if ("admin".equals(input)) System.out.println("Admin"); // prints

intern(): Manually Pool a String

String s1 = new String("interned");
String s2 = s1.intern();
String s3 = "interned"; // already in pool

System.out.println(s2 == s3); // true (both point to pool)
System.out.println(s1 == s3); // false (s1 is off-pool)

// Modern use: prefer String.intern() only when you need
// identity-based caching with thousands of repeated strings

String Concatenation Behavior

// Compiler optimizes this to a StringBuilder automatically:
String result = "Hello, " + "World" + "!";

// In loops: compiler does NOT optimize — use StringBuilder explicitly!
List<String> words = List.of("a", "b", "c", "d");
String bad = ""; // O(n^2) — new object each iteration
for (String w : words) bad += w;

StringBuilder sb = new StringBuilder();
for (String w : words) sb.append(w); // O(n) — efficient
String good = sb.toString();
System.out.println(good); // abcd

Comparing Strings Correctly

String a = "Java";
String b = "java";

System.out.println(a.equals(b));             // false
System.out.println(a.equalsIgnoreCase(b));   // true
System.out.println(a.compareToIgnoreCase(b)); // 0 (equal)
System.out.println(a.compareTo(b));           // negative (J < j in Unicode)

String Hashcode Caching

String key = "product:12345";
// hashCode computed once and cached:
int h1 = key.hashCode();
int h2 = key.hashCode(); // returns cached value
System.out.println(h1 == h2); // true

// Strings are ideal HashMap keys because:
// 1. hashCode is consistent (immutable content)
// 2. hashCode is cached (fast repeated lookups)
// 3. equals is well-defined (content-based)
Map<String, Integer> map = new HashMap<>();
map.put("key", 42);
System.out.println(map.get("key")); // 42

String is Final

// String is final — this will not compile:
// class MutableString extends String {} // compile error

// You cannot override String behavior.
// If you need custom string-like behavior, use a wrapper:
record ProductSku(String value) {
    ProductSku { if (!value.matches("[A-Z]{3}-[0-9]{4}")) throw new IllegalArgumentException(); }
    @Override public String toString() { return value; }
}

String.format vs Concatenation

String name = "Alice";
int age = 30;
double score = 97.5;

// Concatenation — harder to read with many parts
String bad = "User: " + name + ", Age: " + age + ", Score: " + score;

// String.format — clear placeholders
String good = String.format("User: %s, Age: %d, Score: %.1f", name, age, score);

System.out.println(good);
// User: Alice, Age: 30, Score: 97.5

Null-Safe String Operations

String value = null;

// Null-safe length check
int len = (value != null) ? value.length() : 0;
System.out.println(len); // 0

// Null-safe comparison (put constant first)
boolean isAdmin = "admin".equals(value); // false, no NPE

// String.valueOf converts null to "null" string
System.out.println(String.valueOf(value)); // "null"
System.out.println(Objects.toString(value, "default")); // "default"

Quick Check

String a = "hello";
String b = new String("hello");
System.out.println(a.equals(b));
System.out.println(a == b);

Recap: String Immutability and the String Pool