Java Object super class (Manohar Academy) смотреть онлайн

In this lesson we learn about Object super class.

Object is the super class of all classes and it is at the root of inheritance hierarchy. Every class directly or indirectly extends Object super class.

As a result every class will inherit following methods from Object super class.
String toString();
boolean equals(Object obj);
int hashcode();

Object clone();
void finalize();
Class getClass();

void wait();
void wait(long timeout);
void wait(long timeout, int nanos);
void notify();
void notifyAll();

Wait, notify and notifyAll are used in concurrent programming.
Clone is used to create the object clones.
getClass returns the class of the object.
finalize method will be called on every object before it is claimed by garbage collector.

toString is used whenever a customized message should be printed.

equals and hashcode methods should be carefully overridden when required.

equals method contract:
It is reflexive: for any non-null reference value x, x.equals(x) should return true.
It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
For any non-null reference value x, x.equals(null) should return false.

Hashcode method contract:
when ever equals method is overriddent the hashcode should also be overridden.
if x.equals(y) returns true then x.hashcode() == y.hascode() should be true
two equal objects should return same hashcode