Strings in Java & String Methods with Immutable Nature

Strings are one of the most commonly used data types in Java. They allow you to store and manipulate sequences of characters, such as names, messages, or any text data. Understanding strings and their methods is crucial for writing efficient Java programs.

Strings are one of the most commonly used data types in Java. They allow you to store and manipulate sequences of characters, such as names, messages, or any text data.

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What is a String in Java?

• A String in Java is an object that represents a sequence of characters.
• Strings are immutable, meaning once created, their value cannot be changed.
• Strings belong to the java.lang package, so you don’t need to import anything to use them.

Example:

String name = "John";
System.out.println("Name: " + name);

Output:

Name: John

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String Declaration and Initialization

There are two main ways to create strings:

1. Using String Literal

String message = "Hello Java";

2. Using new Keyword

String message = new String("Hello Java");

Note : String literals are stored in the String Pool to save memory. Using new creates a new object in the heap every time.

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String Characteristics

Feature	Description
Immutable	Strings cannot be modified after creation
Indexed	Characters in a string are indexed from 0
Object	Strings are objects, not primitive data types
Methods	Strings have many built-in methods for manipulation

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Common String Methods

Method	Description	Example
length()	Returns the number of characters	"Java".length() → 4
charAt(int index)	Returns character at specified index	"Java".charAt(2) → 'v'
concat(String s)	Concatenates another string	"Java".concat(" Programming") → "Java Programming"
toUpperCase()	Converts to uppercase	"java".toUpperCase() → "JAVA"
toLowerCase()	Converts to lowercase	"JAVA".toLowerCase() → "java"
equals(String s)	Compares two strings	"Java".equals("java") → false
equalsIgnoreCase(String s)	Compares ignoring case	"Java".equalsIgnoreCase("java") → true
substring(int start, int end)	Extracts substring	"Java".substring(1,3) → "av"
trim()	Removes leading and trailing spaces	" Java ".trim() → "Java"
replace(char old, char new)	Replaces character	"Java".replace('a','o') → "Jovo"

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Immutable Nature of Strings

• Immutable: Once a string is created, it cannot be modified.
• Any operation on a string returns a new string object.

Example:

String str = "Java";
String newStr = str.concat(" Programming");

System.out.println(str);    // Java
System.out.println(newStr); // Java Programming

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Why Strings Are Immutable

1. Security: Strings are used in sensitive operations (e.g., passwords, file paths).
2. Thread-Safety: Immutable objects can be shared safely in multithreaded environments.
3. Memory Efficiency: String Pool reuses immutable objects.
4. Performance: Cached hashcode improves performance in collections like HashMap.

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String Pool in Java

• The String Pool is a special memory area for storing string literals.
• Strings created using literals go into the pool.
• Strings created with new do not go into the pool (stored in heap).

Example:

String s1 = "Java";
String s2 = "Java";
String s3 = new String("Java");

System.out.println(s1 == s2); // true, same object in pool
System.out.println(s1 == s3); // false, different objects

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Practical Example

public class StringExample {
    public static void main(String[] args) {
        String firstName = "John";
        String lastName = "Doe";

        // Concatenation
        String fullName = firstName + " " + lastName;
        System.out.println("Full Name: " + fullName);

        // Length
        System.out.println("Length: " + fullName.length());

        // Substring
        System.out.println("First Name: " + fullName.substring(0, 4));

        // Uppercase
        System.out.println("Uppercase: " + fullName.toUpperCase());
    }
}

Output:

Full Name: John Doe
Length: 8
First Name: John
Uppercase: JOHN DOE