In Java, both StringJoiner and StringBuilder classes are used for manipulating strings, but there are some key differences between them that make one more suitable than the other depending on the specific use case.

StringJoiner is a class introduced in Java 8 that is specifically designed to join strings with a delimiter in between. It provides a simple way to concatenate strings with a specified delimiter, prefix, and suffix. This makes it a good choice for cases where you need to build a string from a collection of elements, such as when constructing CSV files or SQL statements.

On the other hand, StringBuilder is a class that has been around since Java 5 and is used for general-purpose string manipulation. It provides a way to efficiently construct and modify strings by appending, inserting, or deleting characters. This makes it a good choice for situations where you need to manipulate strings in a more complex way, such as when building a string incrementally inside a loop.

In summary, if you need to concatenate strings with a delimiter, StringJoiner is the better choice, while if you need to perform more general-purpose string manipulation, StringBuilder is the way to go. However, in some cases, you may find that the two classes can be used together to achieve the desired result.