Strings In Depth

Reading Time: 12 Minutes
Difficulty: Beginner

Topic Summary

A String in Java is a sequence of characters used to store text like names, messages, or sentences. Strings in Java are immutable — once created, they cannot be changed. Java also uses a special memory area called the String Pool to save memory by reusing String objects.

What You'll Learn

What immutability means and why Strings are immutable
How the String pool works and saves memory
All important String methods with examples
The difference between == and .equals() for Strings
Why StringBuilder is faster than String for repeated changes

Prerequisites

Basic Java syntax (variables, data types)
Understanding of objects and classes

Explanation

What Is a String?

A String is a class in Java, not a primitive type. When you write:

String name = "Alice";

Java creates a String object that holds the characters A, l, i, c, e.

Immutability — What Does It Mean?

Immutable means "cannot be changed". Once a String object is created in memory, its value can never be modified. If you try to "change" a String, Java actually creates a brand new String object and leaves the old one in memory.

String s = "Hello";
s = s + " World";
// "Hello" is still in memory, Java created a new String "Hello World"

This behaviour is by design — it makes Strings thread-safe and predictable.

The String Pool

Java has a special area inside the Heap memory called the String Constant Pool (or just String Pool). When you create a String using double quotes, Java checks the pool first:

If the same String already exists → Java reuses it (no new object)
If it does NOT exist → Java creates a new one and adds it to the pool

String a = "Java";   // Created in pool
String b = "Java";   // Reused from pool — same reference!
String c = new String("Java"); // Created in heap, NOT pool

This is why a == b is true but a == c is false (they point to different memory locations).

`==` vs `.equals()` — The Most Common Mistake

== compares memory addresses (references)
.equals() compares actual content

Always use .equals() when comparing String values. Use == only when you intentionally want to check if two variables point to the exact same object.

Key String Methods

1. `length()` — Count Characters

Returns the number of characters in the String.

2. `charAt(index)` — Get Character at Position

Returns the character at a specific index (0-based).

3. `substring(start)` / `substring(start, end)` — Extract Part

Extracts a portion of the String. The end index is exclusive.

4. `indexOf(str)` — Find Position

Returns the index of the first occurrence of the given character or substring. Returns -1 if not found.

5. `contains(str)` — Check If Present

Returns true if the String contains the specified sequence of characters.

6. `replace(old, new)` — Replace Characters

Replaces all occurrences of a character or substring with another.

7. `toUpperCase()` / `toLowerCase()` — Change Case

Converts all characters to upper or lower case.

8. `trim()` — Remove Extra Spaces

Removes leading and trailing whitespace from the String.

9. `split(delimiter)` — Break Into Array

Splits the String around matches of the given delimiter, returning a String array.

10. `equals(str)` / `equalsIgnoreCase(str)` — Compare

equals() is case-sensitive. equalsIgnoreCase() ignores case differences.

StringBuilder vs String — Performance

Every time you concatenate Strings using +, Java creates a new String object. Do this thousands of times (in a loop) and your program slows down because of repeated object creation and garbage collection.

StringBuilder is a mutable sequence of characters. You can change it without creating new objects every time. This makes it much faster for building Strings in loops.

Feature	String	StringBuilder
Mutable	❌ No	✅ Yes
Thread-safe	✅ Yes	❌ No
Performance	Slower in loops	Much faster
Use case	Fixed text	Building dynamic text

Real-World Analogy

Think of a String like a printed book. Once the book is printed, you cannot erase or change its pages. If you want a "new version", the publisher has to print a completely new book (new object).

StringBuilder is like a whiteboard. You can write, erase, and rewrite as many times as you want without creating a new whiteboard each time.

Code Example

public class StringsDemo {
    public static void main(String[] args) {

        String name = "  Hello, Java World!  ";

        // Basic methods
        System.out.println("Length: " + name.trim().length());          // 19
        System.out.println("Upper: " + name.trim().toUpperCase());       // HELLO, JAVA WORLD!
        System.out.println("Lower: " + name.trim().toLowerCase());       // hello, java world!
        System.out.println("Trimmed: '" + name.trim() + "'");            // 'Hello, Java World!'

        // charAt and substring
        String word = "Programming";
        System.out.println("Char at 0: " + word.charAt(0));             // P
        System.out.println("Substring(0,7): " + word.substring(0, 7));  // Program

        // indexOf and contains
        System.out.println("Index of 'gram': " + word.indexOf("gram")); // 3
        System.out.println("Contains 'ming': " + word.contains("ming")); // true

        // replace and split
        String csv = "apple,banana,cherry";
        String[] fruits = csv.split(",");
        for (String fruit : fruits) {
            System.out.println(fruit);  // apple / banana / cherry
        }

        String replaced = csv.replace("banana", "mango");
        System.out.println(replaced);   // apple,mango,cherry

        // == vs equals
        String s1 = "Java";
        String s2 = "Java";
        String s3 = new String("Java");

        System.out.println(s1 == s2);         // true  (same pool reference)
        System.out.println(s1 == s3);         // false (different object in heap)
        System.out.println(s1.equals(s3));    // true  (same content)

        // StringBuilder performance
        StringBuilder sb = new StringBuilder();
        for (int i = 1; i <= 5; i++) {
            sb.append("Item").append(i).append(" ");
        }
        System.out.println(sb.toString());    // Item1 Item2 Item3 Item4 Item5
    }
}

Output

Length: 19
Upper: HELLO, JAVA WORLD!
Lower: hello, java world!
Trimmed: 'Hello, Java World!'
Char at 0: P
Substring(0,7): Program
Index of 'gram': 3
Contains 'ming': true
apple
banana
cherry
apple,mango,cherry
true
false
true
Item1 Item2 Item3 Item4 Item5

Common Mistakes

❌ Mistake: Using == to compare String values → ✅ Fix: Always use .equals() or .equalsIgnoreCase()
❌ Mistake: Concatenating Strings in a loop with + (very slow) → ✅ Fix: Use StringBuilder.append() inside loops
❌ Mistake: Calling .length() on a null String → ✅ Fix: Always null-check first: if (str != null)
❌ Mistake: substring(2, 5) thinking end is inclusive → ✅ Fix: The end index is exclusive — substring(2, 5) returns characters at index 2, 3, 4

Best Practices

Always use .equals() (not ==) when comparing String content
Use StringBuilder when you need to build or modify Strings repeatedly
Use trim() when reading user input to remove accidental spaces
Prefer equalsIgnoreCase() for user input comparisons (avoids case issues)
Use isEmpty() or isBlank() (Java 11+) instead of .length() == 0 for readability

Interview Questions

Q: Why is String immutable in Java?
A: String is immutable for three main reasons: (1) Security — Strings are used for file paths, network URLs, database credentials. Making them immutable prevents tampering. (2) String Pool — Immutability allows multiple references to safely share the same pool object. (3) Thread Safety — Immutable objects are inherently thread-safe without synchronization.

Q: What is the String constant pool?
A: The String constant pool is a special region in Java heap memory where String literals are stored. When you create a String literal, Java checks the pool first. If the same value exists, it reuses the reference rather than creating a new object. This saves memory. Strings created with new String("...") bypass the pool.

Q: What is the difference between == and .equals() for Strings?
A: == compares object references (memory addresses) while .equals() compares the actual character content. Two different String objects can have the same content but different references, so == would return false while .equals() returns true.

Q: When should you use StringBuilder over String?
A: Use StringBuilder when you need to perform many modifications to a String, especially in loops. String concatenation with + creates a new object each time, which is O(n²) in a loop. StringBuilder is O(n) because it modifies the same buffer in place.

Quick Revision

✔ Strings are immutable — changes create new objects, not modify existing ones
✔ The String pool reuses String literals to save memory
✔ Always use .equals() for content comparison, never ==
✔ substring(start, end) — end index is EXCLUSIVE
✔ StringBuilder is faster for repeated concatenations

StringBuilder and StringBuffer
Java Memory Model (Heap & Stack)
Wrapper Classes and Autoboxing

Next Lesson

02 - StringBuilder and StringBuffer

Topic Summary​

What You'll Learn​

Prerequisites​

Explanation​

What Is a String?​

Immutability — What Does It Mean?​

The String Pool​

== vs .equals() — The Most Common Mistake​

Key String Methods​

1. length() — Count Characters​

2. charAt(index) — Get Character at Position​

3. substring(start) / substring(start, end) — Extract Part​

4. indexOf(str) — Find Position​

5. contains(str) — Check If Present​

6. replace(old, new) — Replace Characters​

7. toUpperCase() / toLowerCase() — Change Case​

8. trim() — Remove Extra Spaces​

9. split(delimiter) — Break Into Array​

10. equals(str) / equalsIgnoreCase(str) — Compare​

StringBuilder vs String — Performance​

Real-World Analogy​

Code Example​

Output​

Common Mistakes​

Best Practices​

Interview Questions​

Quick Revision​

Related Topics​

Next Lesson​