Strings in Java

In Java, the String class represents immutable sequences of characters. The JDK provieds a rich set of APIs—predefined classes and methods—for common operations.

Creating String Objects

• Literal assignment:
  String name = "AWei";
• Empty string via constructor:
  String s1 = new String();
• From another string:
  String s2 = new String("Like");
• From a byte array:
  byte[] data = {97, 98, 99, 100};<br></br> String s4 = new String(data);
• From a char array:
  char[] letters = {'a', 'b', 'c'};<br></br> String s3 = new String(letters);

String Comparison

Using == compares object references, not content. For value comparison, use:

String a = "abc";
String b = new String("abc");
boolean eq1 = a.equals(b);          // true
boolean eq2 = a.equalsIgnoreCase("ABC"); // true (case-insensitive)

Always prefer equals() or equalsIgnoreCase() for content checks.

StringBuilder

StringBuilder is a mutable sequence of characters, ideal for frequent modifications.

Basic Usage

StringBuilder sb = new StringBuilder();
StringBuilder sb2 = new StringBuilder("initial");

sb.append(42)
  .append(true)
  .reverse();

int len = sb.length();
String result = sb.toString(); // convert back to String

Chaining (Fluent Interface)

Methods like append() return the StringBuilder instance, enabling method chaining:

String reversed = new StringBuilder()
    .append(input)
    .reverse()
    .toString();

StringJoiner

StringJoiner constructs sequences of strings separated by a deilmiter, with optional prefix and suffix.

Constructors

StringJoiner sj1 = new StringJoiner("---");                // delimiter only
StringJoiner sj2 = new StringJoiner(",", "[", "]");        // delimiter, prefix, suffix

Methods

sj2.add("apple").add("banana");
int count = sj2.length();      // total character count
String output = sj2.toString(); // e.g., "[apple,banana]"

ArrayList

ArrayList is a resizable array implementation of the List interface.

Declaration and Initialization

ArrayList<String> list = new ArrayList<>();

When printed, it displays contents enclosed in square brackets (e.g., [item1, item2]).

Common Operations

list.add("newItem");               // add element
list.remove("oldItem");            // remove by value
list.remove(0);                    // remove by index (returns removed element)
list.set(1, "updated");            // replace element at index
String val = list.get(2);          // retrieve element
int size = list.size();            // number of elements

Primitive Wrappers

Use wrapper classes when working with collections:

• int → Integer
• char → Character
• Other primitives: capitalize first letter (e.g., double → Double)