A variable declared using class type can store what to an object

An object stores its state in fields (variables in some programming languages) and exposes its behavior through methods (functions in some programming languages). Methods operate on an object’s internal state and serve as the primary mechanism for object-to-object communication.

The this keyword refers to the current object in a method or constructor. The most common use of the this keyword is to eliminate the confusion between class attributes and parameters with the same name (because a class attribute is shadowed by a method or constructor parameter).

Generally Object should only be used when dealing with a collection of elements of disparate or unknown type . This then usually is followed by instanceof and cast statements. Many APIs return Object when then can provide disparate types and some of this is a holdover from Java 4 and older prior to generics.

Since Java is strongly typed, you cannot do a whole lot with Object . This is because methods, operators, etc. available to use depend on the static type of the variable. println can accept Object because it only needs to call the toString method.