String

String Class in Java

Overview

The String class in Java represents a sequence of characters. It is one of the most commonly used classes and is immutable, meaning once created, its value cannot be changed.

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Characteristics of String Class

Feature	Details
Package	java.lang
Immutable	Strings are immutable.
Default Value	null for uninitialized string references.

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Key Features of the String Class

• Immutability: Once created, the content of a String object cannot be changed.
• Efficient Memory Management: Strings are stored in a special memory area called the "string pool."
• Supports Multiple Constructors: Allows creating strings from character arrays, byte arrays, or directly using string literals.
• Utility Methods: Provides numerous methods for manipulating and examining strings.

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Operations on String Types

String Operations

Operation	Symbol	Example
Concatenation	+	str1 + str2

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Static Methods

Method	Description	Return Type	Example
valueOf(Object obj)	Returns the string representation of the specified object.	String	String.valueOf(42); // "42"
format(String format, Object... args)	Returns a formatted string using the specified format and arguments.	String	String.format("Hello, %s!", "World");
join(CharSequence delimiter, CharSequence... elements)	Joins the given elements with the specified delimiter.	String	String.join("-", "2025", "01", "09");
copyValueOf(char[] data)	Returns a string that represents the sequence of characters in the array.	String	String.copyValueOf(new char[] {'H', 'i'});
copyValueOf(char[] data, int offset, int count)	Returns a sub string that represents the sequence of characters in the array.	String	String.copyValueOf(new char[] {'H', 'i'}, 1, 4);

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Instance Methods

Method	Description	Return Type	Example
length()	Returns the length of the string.	int	str.length();
charAt(int index)	Returns the character at the specified index.	char	str.charAt(0);
substring(int beginIndex, int endIndex)	Returns a substring between the specified indices.	String	str.substring(0, 4);
equals(Object obj)	Checks if two strings are equal.	boolean	str.equals("Hello");
toLowerCase()	Converts all characters in the string to lowercase.	String	str.toLowerCase();
toUpperCase()	Converts all characters in the string to uppercase.	String	str.toUpperCase();
replace(char oldChar, char newChar)	Replaces all occurrences of a character with another character.	String	str.replace('H', 'h');
indexOf(String str)	Returns the index of the first occurrence of the specified substring.	int	str.indexOf("H");
trim()	Removes leading and trailing whitespace from the string.	String	str.trim();
split(String regex)	Splits the string into an array of substrings based on the specified regex.	String[]	str.split(" ");
contains(CharSequence s)	Checks if the string contains the specified sequence.	boolean	str.contains("ell");
replaceAll(String regex, String replacement)	Replaces each substring matching the regular expression with the given replacement.	String	str.replaceAll("world", "everyone");
isEmpty()	Checks if the string is empty.	boolean	str.isEmpty();
matches(String regex)	Checks if the string matches the specified regular expression.	boolean	str.matches("\\d+");
concat(String str)	Concatenates the specified string to the current string.	String	str.concat(" world");

For more: Comprehensive List of Instabce Methods

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Example Code

public class StringExample {
    public static void main(String[] args) {
        // Create a string
        String str = "Hello, World!";

        // Get the length of the string
        System.out.println("Length: " + str.length());

        // Get a character at a specific index
        System.out.println("Character at index 0: " + str.charAt(0));

        // Get a substring
        System.out.println("Substring: " + str.substring(0, 5));

        // Check equality
        System.out.println("Equals 'Hello, World!': " + str.equals("Hello, World!"));

        // Convert to lowercase
        System.out.println("Lowercase: " + str.toLowerCase());

        // Replace a character
        System.out.println("Replace 'H' with 'h': " + str.replace('H', 'h'));

        // Split the string
        String[] words = str.split(", ");
        System.out.println("First word: " + words[0]);

        // Trim whitespace
        String padded = "   Hello   ";
        System.out.println("Trimmed: '" + padded.trim() + "'");
    }
}

For more: StringDataType.java