What are the differences between String and StringBuilder classes in Java, and in which situations is each recommended?

In Java, the classes String and StringBuilder are used for working with strings, but they have different characteristics and use cases.

• String — this is an immutable class. Every time the string is modified, a new object is created. This is convenient for strings that do not change and ensures safety in a multithreaded environment.

• StringBuilder — a mutable class for working with strings. It allows efficient modification of the string content without creating new objects.

When to use:

• Use String if the string will not change or changes are minimal. This simplifies the code and increases safety.
• Use StringBuilder when you need to modify the string frequently (for example, in loops, when concatenating a large number of strings) to avoid creating many temporary objects and to improve performance.

Example:

String s = "Hello";
s += " World";  // Creates a new String object

StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");  // Modifies the existing object