A friend class can access private and protected members of other class in which it is declared as friend. It is sometimes useful to allow a particular class to access private members of other class. For example a LinkedList class may be allowed to access private members of Node.
Friend Function Like friend class, a friend function can be given special grant to access private and protected members. A friend function can be:
a) A method of another class
b) A global function
Important points about friend functions and classes:
1) Friends should be used only for limited purpose. too many functions or external classes are declared as friends of a class with protected or private data, it lessens the value of encapsulation of separate classes in object-oriented programming.
2) Friendship is not mutual. If a class A is friend of B, then B doesn’t become friend of A automatically.
3) Friendship is not inherited (See this for more details)
4) The concept of friends is not there in Java.