In Java, what is the difference among String, StringBuffer and StringBuilder classes?

Introduction / Context:
String handling is one of the most common tasks in Java. The core Java library provides three related classes for this purpose: String, StringBuffer and StringBuilder. Although they all deal with sequences of characters, they have different performance characteristics and thread safety guarantees. This question asks you to summarize the key differences between these three classes, which is important when choosing the right type in real applications.

Given Data / Assumptions:
- We work in standard Java, using java.lang.String, java.lang.StringBuffer and java.lang.StringBuilder.
- We care about mutability, synchronization and typical use cases such as building and concatenating strings.
- We assume basic knowledge of multithreading and why synchronization can impact performance.
- The question focuses on conceptual differences, not on detailed method lists.

Concept / Approach:
String is immutable: once created, its contents cannot change. Any apparent modification, such as concatenation, results in the creation of a new String object. StringBuffer and StringBuilder, on the other hand, are mutable, so their contents can be changed in place, which is more efficient for repeated modifications. The difference between StringBuffer and StringBuilder is that StringBuffer methods are synchronized, making it safer to share between threads at the cost of some overhead, while StringBuilder is not synchronized and is intended for faster use in single threaded contexts.

Step-by-Step Solution:
Step 1: Recall that String is immutable and every modification creates a new object. Step 2: Remember that StringBuffer was introduced as a mutable sequence of characters with synchronized methods. Step 3: Note that StringBuilder was added later as a faster, unsynchronized alternative for single threaded code. Step 4: Consider performance: repeated concatenations using String inside loops can be slow, so StringBuffer or StringBuilder are preferred. Step 5: Compare the options and select the one that correctly states the immutability and synchronization characteristics of all three classes.

Verification / Alternative check:
Imagine concatenating inside a loop: using a plain String with s = s + "x" repeatedly allocates many new String objects. Using a StringBuilder and calling append("x") modifies the same buffer, which is efficient. In a multithreaded context where multiple threads append to the same buffer, StringBuffer can be safer due to synchronized methods. None of the classes are limited to specific data types like numbers or dates. This reasoning confirms that the description in option A matches real Java behavior.

Why Other Options Are Wrong:
Option B incorrectly states that all three classes are immutable and behave the same, which ignores the design of StringBuffer and StringBuilder as mutable builders.
Option C invents roles for each class based on data type (numbers, dates, characters), which is not how these classes are defined or used.
Option D reverses the mutability story, claiming that String changes while the other two are immutable, which is the opposite of Java reality.

Common Pitfalls:
A common mistake is to use String in heavy concatenation scenarios, resulting in performance issues. Another is to use StringBuffer everywhere for safety, even when only a single thread is involved and StringBuilder would give better speed. Some developers also forget that even though String is immutable, references to it can still be reassigned. Understanding the differences among these three classes helps you choose the most efficient and safe option for each situation.

Final Answer:
The correct summary is String is immutable, StringBuffer is mutable and synchronized, and StringBuilder is mutable but not synchronized for single threaded use.