In Java, `String`, `StringBuilder`, and `StringBuffer` are classes used for handling strings, but they have different characteristics and use cases:

1. **String**:
– **Immutability**: `String` objects are immutable, meaning once a `String` object is created, it cannot be changed. Any modification creates a new `String` object.
– **Performance**: Because of immutability, concatenation operations involving `String` can be inefficient as they create multiple intermediate objects.
– **Usage**: Best used when the string value is constant and will not be modified.

2. **StringBuilder**:
– **Mutability**: `StringBuilder` objects are mutable, meaning they can be modified after creation without creating new objects.
– **Performance**: More efficient than `String` for concatenation and other modifying operations due to in-place modifications.
– **Thread Safety**: Not thread-safe. Should be used when thread safety is not a concern.
– **Usage**: Best used in a single-threaded environment where string modifications are frequent.

3. **StringBuffer**:
– **Mutability**: Like `StringBuilder`, `StringBuffer` objects are mutable.
– **Performance**: Similar to `StringBuilder` in terms of efficiency for modification operations.
– **Thread Safety**: Thread-safe. All methods in `StringBuffer` are synchronized, which makes it safe to use in a multi-threaded environment.
– **Usage**: Should be used when working with strings in a multi-threaded context to ensure thread safety.

To summarize:

– **Use `String`** when you have a constant string that won’t change.
– **Use `StringBuilder`** for high-performance string manipulations in a single-threaded environment.
– **Use `StringBuffer`** for string manipulations in a multi-threaded environment to ensure thread safety.