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#java

String Wrapper Class

A String is a sequence of characters. In Java, String is a class.

String name = "Moxit";

Important: Strings are immutable. Once created, they cannot be changed.

1. Creating a String

String literal

String greeting = "Hello World";

From a character array

char[] chars = {'H', 'e', 'l', 'l', 'o'};

String text = new String(chars);

System.out.println(text);

Output:

Hello

2. length()

Returns the number of characters.

String text = "Hello";

System.out.println(text.length());

Output:

5

3. charAt()

Returns the character at a specific index.

Index starts from 0.

String text = "Hello";

System.out.println(text.charAt(1));

Output:

e

H e l l o
0 1 2 3 4

4. Concatenation

Using +

String first = "Hello";
String second = "World";

String result = first + " " + second;

System.out.println(result);

Output:

Hello World

Using concat()

String result = "Hello".concat(" World");

System.out.println(result);

5. Text Blocks

Useful for multi-line strings. Available from Java 15.

String html = """
        <html>
            <body>Hello</body>
        </html>
        """;

System.out.println(html);

6. String.format()

Creates a formatted string.

String name = "John";
int age = 22;

String result = String.format("Name: %s, Age: %d", name, age);

System.out.println(result);

Output:

Name: John, Age: 22

Common format specifiers:

%s → String
%d → integer
%f → floating-point

7. String → Number

Use parseXXX().

int age = Integer.parseInt("22");
double price = Double.parseDouble("99.5");

System.out.println(age);
System.out.println(price);

Output:

22
99.5

8. Number → String

String.valueOf()

int number = 100;

String text = String.valueOf(number);

System.out.println(text);

toString()

int number = 100;

String text = Integer.toString(number);

9. substring()

Gets part of a string.

String text = "Hello World";

String result = text.substring(6, 11);

System.out.println(result);

Output:

World

Remember:

substring(start, end) → end is exclusive.

You can also use:

String result = text.substring(6);

This gets everything from index 6 to the end.

10. split()

Splits a string into an array.

String text = "Java,Python,JavaScript";

String[] languages = text.split(",");

for (String language : languages) {
    System.out.println(language);
}

Output:

Java
Python
JavaScript

11. trim()

Removes whitespace from the beginning and end.

String text = "   Hello   ";

System.out.println(text.trim());

Output:

Hello

For modern Unicode-aware whitespace handling, strip() is generally preferred over trim().

12. toLowerCase() and toUpperCase()

String text = "Hello World";

System.out.println(text.toLowerCase());
System.out.println(text.toUpperCase());

Output:

hello world
HELLO WORLD

13. indexOf()

Finds the first occurrence.

String text = "Hello World";

System.out.println(text.indexOf("o"));

Output:

4

Returns -1 if not found.

14. lastIndexOf()

Finds the last occurrence.

String text = "Hello World";

System.out.println(text.lastIndexOf("o"));

Output:

7

15. contains()

Checks whether a string contains another string.

String text = "Hello World";

System.out.println(text.contains("World"));

Output:

true

16. replace()

Replaces characters or strings.

Character replacement

String text = "Hello";

String result = text.replace('l', 'x');

System.out.println(result);

Output:

Hexxo

String replacement

String text = "I like Java";

String result = text.replace("Java", "Python");

System.out.println(result);

Output:

I like Python

17. subSequence()

Returns a part of the string as a CharSequence.

String text = "Hello World";

CharSequence result = text.subSequence(0, 5);

System.out.println(result);

Output:

Hello

18. Important: Strings Are Immutable

String methods do not change the original string.

String text = "Hello";

text.toUpperCase();

System.out.println(text);

Output:

Hello

You need to store the returned String:

text = text.toUpperCase();

System.out.println(text);

Output:

HELLO

Think of it as:

String
  ↓
Cannot be changed
  ↓
Methods create a new String

Quick Revision

Method Purpose
length() Gets length
charAt() Gets character
+ / concat() Joins strings
String.format() Formats string
parseInt() String → int
parseDouble() String → double
String.valueOf() Value → String
substring() Gets part of string
split() String → String array
trim() Removes surrounding basic whitespace
strip() Removes Unicode-aware surrounding whitespace
toLowerCase() Converts to lowercase
toUpperCase() Converts to uppercase
indexOf() Finds first occurrence
lastIndexOf() Finds last occurrence
contains() Checks if text exists
replace() Replaces text/characters
subSequence() Gets part as CharSequence

Most important for DSA

Focus on:

length()
charAt()
substring()
indexOf()
contains()
split()
replace()
toLowerCase()
toUpperCase()

One rule to remember

String is immutable, so string operations return a new String instead of modifying the original.