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1. Introduction

The String class in Java provides a wide variety of methods for manipulating and working with strings.
These methods allow you to perform common tasks such as:

Searching and comparing strings
Modifying string content
Extracting portions of a string
Converting strings to different formats

In this lecture, we’ll go over the most commonly used String methods and demonstrate how they work.

2. Common String Methods

Java's String class has many useful methods that can make string handling easy and efficient. Some of the most commonly used methods are:

length()
charAt()
substring()
equals()
compareTo()
contains()
toUpperCase() / toLowerCase()
replace()
trim()

Each method has a specific use case, and understanding them is essential for effective string manipulation.

3. `length()`: Get the Length of a String

The length() method returns the number of characters in a string.

String str = "Hello";
int length = str.length();  // Returns 5
System.out.println(length);  // Output: 5

This method is useful when you need to determine the size of a string.

4. `charAt()`: Get a Character at a Specific Index

The charAt() method returns the character at the specified index (zero-based index) in a string.

String str = "Hello";
char ch = str.charAt(1);  // Returns 'e'
System.out.println(ch);    // Output: e

The index must be within the valid range, i.e., 0 to length() - 1. Attempting to access an invalid index results in an IndexOutOfBoundsException.

5. `substring()`: Extract a Part of a String

The substring() method extracts a part of a string, starting from a specified index to the end or another index.

Syntax:

String substring(int startIndex)
String substring(int startIndex, int endIndex)

startIndex: Starting position (inclusive).
endIndex: Ending position (exclusive).

Example:

String str = "Hello, World!";
String sub1 = str.substring(0, 5);  // "Hello"
String sub2 = str.substring(7);     // "World!"
System.out.println(sub1);  // Output: Hello
System.out.println(sub2);  // Output: World!

substring() is useful for extracting portions of a string based on specific indices.

6. `equals()`: Compare Two Strings

The equals() method compares two strings for exact content equality.

String str1 = "Hello";
String str2 = "Hello";
String str3 = "World";

System.out.println(str1.equals(str2));  // Output: true
System.out.println(str1.equals(str3));  // Output: false

Returns true if the strings have the same content.
Returns false if they have different content.

Remember: == checks for reference equality, whereas equals() checks for content equality.

7. `compareTo()`: Compare Strings Lexicographically

The compareTo() method compares two strings lexicographically (alphabetical order). It returns an integer:

0 if the strings are equal
A negative value if the first string is lexicographically less than the second
A positive value if the first string is lexicographically greater than the second

Example:

String str1 = "Hello";
String str2 = "World";
System.out.println(str1.compareTo(str2));  // Output: -15

Here, "Hello" is lexicographically smaller than "World", so the result is negative.

8. `contains()`: Check if a String Contains a Substring

The contains() method checks if a string contains a specific sequence of characters (substring).

String str = "Hello, World!";
boolean contains = str.contains("World");  // Returns true
System.out.println(contains);  // Output: true

Returns true if the substring is found.
Returns false if the substring is not found.

This method is case-sensitive.

9. `toUpperCase()` / `toLowerCase()`: Change Case

The toUpperCase() and toLowerCase() methods are used to convert a string to all uppercase or lowercase letters, respectively.

String str = "Hello, World!";
String upper = str.toUpperCase();  // "HELLO, WORLD!"
String lower = str.toLowerCase();  // "hello, world!"
System.out.println(upper);  // Output: HELLO, WORLD!
System.out.println(lower);  // Output: hello, world!

These methods are commonly used for case-insensitive comparisons or when you want to normalize the case of the string.

10. `replace()`: Replace Characters or Substrings

The replace() method is used to replace characters or substrings in a string with new ones.

Syntax:

String replace(char oldChar, char newChar)
String replace(CharSequence oldSeq, CharSequence newSeq)

Example:

String str = "Hello, World!";
String newStr = str.replace('o', 'a');  // "Hella, Warld!"
System.out.println(newStr);  // Output: Hella, Warld!

You can also replace substrings:

String newStr2 = str.replace("World", "Java");  // "Hello, Java!"
System.out.println(newStr2);  // Output: Hello, Java!

11. `trim()`: Remove Leading and Trailing Spaces

The trim() method removes leading and trailing whitespace from a string.

String str = "   Hello, World!   ";
String trimmedStr = str.trim();
System.out.println(trimmedStr);  // Output: "Hello, World!"

It does not remove spaces in the middle of the string.
Useful for cleaning user input or any string that might have unwanted spaces.

12. Summary

String methods in Java provide a variety of functionalities to manipulate, modify, and analyze strings.
Key methods include length(), charAt(), substring(), equals(), replace(), and toUpperCase().
Immutability of strings ensures that each modification creates a new object, avoiding side effects.
For efficient string manipulation, consider using StringBuilder or StringBuffer when many modifications are required.

Written By: Shiva Srivastava

String Methods

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