Java.lang.Void class is a placeholder that holds a reference to a class object if it represents a void keyword. It is an uninstantiable placeholder. Well, uninstantiable means that this class has a private constructor and no other constructor that we can access from outside. 

Methods of lang.void class is all inherited from Object class in Java:

• getClass(): Returns the runtime class of the argument Object.
• hashCode(): Returns a hash code value for the object.
• equals(): Checks whether the two objects are equal or not.
• clone(): Returns a copy of the object.
• toString(): Returns string representation of the object.
• notify(): Wakes up a single thread that is waiting on this object’s monitor.
• notifyAll(): Wakes up all threads that is waiting on this object’s monitor.
• wait(): Makes current thread to wait until previous thread invokes either notify() or notifyAll() methods.
• finalize(): Called By the garbage collector on an object when garbage collection determines that there are no more references to the object.

Reference: 

Java Oracle docs

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