Answer 1

String is immutable - it cannot change its internal state. Each method you invoke on a String creates a new object and returns it. For the reason, String does not allow appending.

On the other hand, StringBuilder is mutable - When you call append(..) it alters the internal char array, rather than creating a new String object. Let us take this code as an example :

StringBuilder str = new StringBuilder();
for (int i = 0; i < 50; i ++) {
str.append(i);
}

// This appends value of i to str.

This is not possible with a String object!