All classes that we create are mutable class. No special technique is used. Don't get confused. Mutable is said to compare it with immutable classes. That's it.
The Object class, in the java.lang package, sits at the top of the class hierarchy tree. Every class is a descendant, direct or indirect, of the Object class. Every class you use or write inherits...
No. In Java a class can extend only one class. that means, a class can have only a single parent. but if you need multiple inheritance you can implement as many interfaces as you want.