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String vs StringBuffer in Java: Key Differences You Need to Know

String vs StringBuffer in Java: Key Differences You Need to Know

Q: 1. What is the difference between String and StringBuffer?

The primary difference is that String is immutable, meaning its value cannot be changed once created. On the other hand, StringBuffer is mutable, allowing its content to be modified without creating new objects.

Q: 2. How does thread-safety differ between String and StringBuffer?

String is inherently thread-safe since its values cannot be altered, while StringBuffer is thread-safe because it is synchronized, ensuring safe use in multi-threaded environments.

Q: 3. When is it better to use String instead of StringBuffer?

Use String when dealing with constant or infrequently modified text, as it provides simplicity and avoids unnecessary memory usage from modifications.

Q: 4. Why would you choose StringBuffer over String?

Choose StringBuffer for scenarios where frequent string manipulations are required, as it is more memory-efficient and faster in such cases due to its mutable nature.

Q: 5. Can String and StringBuffer be used interchangeably?

No, they cannot be directly used interchangeably. String is immutable, and operations on it create new objects, while StringBuffer is mutable and modifies the same object. You can convert StringBuffer to String using the toString() method.

Q: 6. Which one performs better in memory usage, String or StringBuffer?

StringBuffer performs better in memory usage for repeated string modifications, as it avoids the creation of new objects each time, unlike String, which creates a new object on every modification.

Q: 7. Is StringBuffer always thread-safe?

Yes, StringBuffer is thread-safe because it uses synchronization to ensure that only one thread can modify the buffer at a time, preventing concurrent modification issues.

Q: 8. Can I modify a String after it is created?

No, String is immutable, so once a String is created, its value cannot be modified. Any operation on a String that appears to modify it will actually create a new object.

Q: 9. What happens when I concatenate strings using String?

When you concatenate strings using String, a new String object is created every time, which can lead to performance issues in cases of frequent concatenation.

Q: 10. Does StringBuffer use the string pool?

No, StringBuffer does not use the string pool. String uses a string pool to optimize memory usage by reusing identical string literals.

By Mukesh Kumar

Updated on Feb 05, 2025 | 8 min read | 1.4k views

Table of Contents

In Java, handling and manipulating strings is a fundamental aspect of programming. Strings are widely used in various applications, from simple text processing to complex data manipulation. Java provides two primary ways to work with strings: String and StringBuffer. While both are used to store and process character sequences, they differ significantly in terms of immutability and mutability.

A String in Java is immutable, meaning once created, its value cannot be changed. This makes it suitable for scenarios where security and consistency are essential. On the other hand, StringBuffer is mutable, allowing modifications without creating new objects, making it more memory-efficient for operations involving frequent string manipulations.

This blog will explain the key difference between String and StringBuffer in Java, helping you understand when to use each based on performance, memory usage, and specific programming requirements.

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What Is a String in Java?

In Java, a String is a sequence of characters that is widely used for text processing. It is an immutable object, meaning once a String is created, its value cannot be changed. Instead of modifying an existing String, any operation that alters its content results in the creation of a new String object in memory.

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Immutability in String

When a String is modified (concatenation, replacement, etc.), Java creates a new String object rather than altering the existing one. This ensures security, thread safety, and consistent behavior but can lead to performance overhead due to excessive memory usage when handling multiple modifications.

Example of String Immutability

// Save this file as StringExample.java
public class Main {
    public static void main(String[] args) {
        // Creating a String
        String str1 = "Hello";
        
        // Assigning str1 to another reference variable
        String str2 = str1; // str2 also refers to "Hello"

        // Modifying str1 by concatenation
        str1 = str1 + " World"; // A new String object is created

        // Printing values of str1 and str2
        System.out.println("str1: " + str1); // Output: str1: Hello World
        System.out.println("str2: " + str2); // Output: str2: Hello

        // Checking if both references point to the same object
        System.out.println("Are str1 and str2 pointing to the same object? " + (str1 == str2));
    }
}

Output:

str1: Hello World  
str2: Hello  
Are str1 and str2 pointing to the same object? false

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Why this happens:

Initially, both str1 and str2 point to the same string "Hello".
When we change str1 by adding " World" to it, a new string "Hello World" is created.
str1 now points to "Hello World", but str2 still points to the original "Hello".
Because str1 and str2 point to different strings now, str1 == str2 is false.

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What Is a StringBuffer in Java?

In Java, StringBuffer is a class that represents a mutable sequence of characters. Unlike String, which is immutable (its value cannot be changed once created), a StringBuffer allows you to modify its content without creating new objects every time you perform an operation. This mutability makes StringBuffer particularly useful when performing frequent string manipulations, such as appending, inserting, or deleting characters, without incurring the performance cost of creating new objects.

Mutability of StringBuffer

A StringBuffer object can be changed after it's created. For example, you can append, delete, or modify its content, and the same object will be updated without creating a new one.

Benefits of Mutability

Efficiency: StringBuffer is more efficient than String when performing operations like concatenation, especially in loops. Since the string's content can be modified in place, no new objects are created, avoiding unnecessary memory usage and overhead.
Performance: For scenarios that involve extensive string modifications, such as building large strings or manipulating text in loops, StringBuffer can significantly reduce the time complexity compared to using String.

Here’s a simple example of how StringBuffer works:

public class Main {
    public static void main(String[] args) {
        // Creating a StringBuffer
        StringBuffer sb = new StringBuffer("Hello");

        // Modifying the StringBuffer by appending text
        sb.append(" World");

        // Printing the updated StringBuffer
        System.out.println(sb); // Output: Hello World

        // Modifying further by inserting text
        sb.insert(5, ",");
        System.out.println(sb);
    }
}

Output:

Hello World
Hello, World

Why this happens:

sb.append(" World");
- Adds " World" to the string, so sb becomes "Hello World".
System.out.println(sb);
- Prints the current value of sb: "Hello World".
sb.insert(5, ",");
- Inserts a comma , after "Hello", so sb becomes "Hello, World".
System.out.println(sb);
- Prints the updated value of sb: "Hello, World".

Difference Between String and StringBuffer in Java

In Java, both String and StringBuffer are used to represent a sequence of characters. However, they differ in several important aspects, including mutability, performance, memory consumption, and usage. This table highlights the key differences between String and StringBuffer to help you understand when and why to use each.

Feature	String	StringBuffer
Immutability vs. Mutability	Immutable, meaning once created, its value cannot be changed. Any modification creates a new object.	Mutable, meaning its value can be changed after creation without creating a new object.
Performance	Slower when concatenating or modifying strings repeatedly (e.g., in loops) because each modification creates a new object.	More efficient for repeated string manipulation due to mutability and no creation of new objects.
Memory Consumption	Consumes more memory because it creates a new object each time it's modified.	Consumes less memory as it modifies the existing object without creating new ones.
Thread Safety	Not thread-safe because it’s immutable. Thread safety isn't a concern as the string cannot be modified.	Thread-safe due to synchronization, suitable for use in multi-threaded environments.
Usage	Best for situations where the string value doesn’t change often (e.g., constant strings).	Best for situations where strings are modified frequently (e.g., loops or building large strings dynamically).
Constructor	Can be initialized using literals or constructors (e.g., new String("text")).	Requires explicit initialization with a capacity or the default constructor (e.g., new StringBuffer("text")).
Methods	Provides methods like substring(), toUpperCase(), and others but doesn’t directly modify the string.	Provides methods like append(), insert(), and delete() to directly alter the string’s value.
String Pooling	Uses string pooling to save memory by reusing identical string literals.	Does not use string pooling, as it creates new objects for modifications.
Garbage Collection	Old String objects are eligible for garbage collection when no longer referenced.	As it’s mutable, the StringBuffer object stays in memory until discarded or unreferenced.
Default Capacity	No concept of capacity—memory is allocated based on the string length.	Has a default capacity of 16 characters, growing dynamically (doubling its size when it exceeds its capacity).

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Similarities Between String and StringBuffer in Java

Although String and StringBuffer differ in their mutability and use cases, they do share several similarities. Both classes are designed to handle sequences of characters, and while their underlying behavior is different, they offer comparable methods for working with text data. Here are some key similarities between them:

1. Both Represent Sequences of Characters

Both String and StringBuffer are used in Java to store and manipulate sequences of characters. While String is immutable, and StringBuffer is mutable, both serve the common purpose of handling textual data.

2. Methods for Character Manipulation

Even though String and StringBuffer have different characteristics (immutability vs. mutability), they share some common methods for character manipulation. Here are a few methods both classes provide:

charAt(index): Returns the character at the specified index.
length(): Returns the length of the string or buffer (number of characters).
toString(): Converts the StringBuffer object into a String. The reverse is also possible, though String objects can be easily manipulated into StringBuffer objects.

3. Support for Regular Expressions

While String provides extensive support for regular expressions through methods like matches(), replaceAll(), and split(), StringBuffer does not have built-in regex functionality. However, since StringBuffer can be easily converted to String, you can still apply regular expressions indirectly.

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Conclusion

The difference between String and StringBuffer lies primarily in their mutability. String is immutable, meaning its value cannot be changed after creation, which makes it ideal for scenarios where text remains constant. In contrast, StringBuffer is mutable, allowing modifications to its content without creating new objects, making it more efficient for frequent string manipulations.

Use String when dealing with immutable data, like constant strings or cases where text doesn’t change often. Opt for StringBuffer when you need to frequently modify strings, such as in loops or when dynamically building large strings, as it offers better performance and memory efficiency. The right choice between String and StringBuffer depends on the specific needs of your program and its performance requirements.

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