Strings

01

What is a String?

  • A String is a class in java.lang, even though Java lets you create one with literal syntax like a primitive.
  • Immutable. Once built, its characters never change., , + —
  • Backed by a char sequence. Internally a String wraps an array of characters, which is exactly why it's fixed-length once created.
02

Creating Strings

There are two ways to create a String, and they behave differently under the hood — which matters the moment you start comparing them.

String literal

String a = "hello";

Checked against a special pool of Strings first. If "hello" already exists there, the reference is reused — no new object is created.

The new keyword

String b = new String("hello");

Always creates a fresh object on the heap and memory area

03

Immutability & the String Pool

Because Strings can't change, Java is free to let identical literals safely share one copy. That shared cache is the String Pool.

Two literals

String x = "cat";
String y = "cat";
x and y points to same memory area
x == y  →  true

One literal, one new

String p = "cat";
String q = new String("cat");
p and q points to different memory area
p == q  →  false
Why concatenation "changing" a String is an illusion 
String greeting = "Hi";
greeting = greeting + " there";  // "Hi" is untouched — a brand-new
                                 // String "Hi there" is created,
                                 // and greeting is re-pointed to it
04

Comparing Strings

This is the single most common source of String bugs: == and .equals() ask two different questions.

== — same object?

String a = new String("cat");
String b = new String("cat");
a == b;   // false — two different objects

.equals() — same characters?

String a = new String("cat");
String b = new String("cat");
a.equals(b);   // true — same character content
Rule of thumb Always compare String content with .equals(). Reserve == for checking whether two references point to the exact same object — rarely what you actually want with Strings.
05

Common Methods

A handful of methods cover the vast majority of everyday String work — every one of them returns something new rather than editing in place.

MethodWhat it doesExample
length()Number of characters"hello".length() → 5
charAt(i)Character at index i"hello".charAt(1) → 'e'
substring(a, b)Characters from index a up to, not including, b"hello".substring(1,4) → "ell"
indexOf(s)First position of s, or -1 if absent"hello".indexOf("l") → 2
toUpperCase() / toLowerCase()New String with case changed"Hi".toUpperCase() → "HI"
trim() / strip()New String with leading/trailing whitespace removed" hi ".trim() → "hi"
replace(a, b)New String with all occurrences of a replaced by b"cat".replace('c','b') → "bat"
split(regex)Breaks the String into an array by a delimiter"a,b,c".split(",") → [a, b, c]
contains(s)True if s appears anywhere inside"hello".contains("ell") → true