Object creation

How objects are created

• The line Vehicle minivan = new Vehicle(); declares an object of type Vehicle.

• The declaration does two things:
  Declares a variable called minivan of class Vehicle.
  Creates a physical copy of the object and assigns the minivan a reference to it using the new operator.

• The new operator dynamically allocates memory for an object and returns a reference to it.

• The reference is, more or less, the address of the object in memory allocated by new.

• The reference is then stored in a variable.

• In Java, all objects of a class must be dynamically allocated.

• The two steps of the instruction can be rewritten to show each step individually.

Vehicle minivan; // declare a reference to the object.
minivan = new Vehicle(); // allocates a Vehicle object.

• The first line declares minivan as a reference to an object of type Vehicle.

• minivan is a variable that can reference an object, but is not an object.

• For now, minivan does not reference an object.

• The next line creates a new Vehicle object and assigns the minivan a reference to it.

• Now minivan is linked to an object.

Reference variables and assignment

• Object reference variables act differently than primitive type variables in assignment operations.

• In primitive type variables, the left variable receives a copy of the value of the right variable.

• In object reference variables, the variable on the left references the same object as the variable on the right.

• This may cause unexpected results.

• Example:
  Vehicle car1 = new Vehicle();
  Vehicle car2 = car1;

• car1 and car2 reference the same object.

• Changes made through car1 or car2 affect the same object.

• When:
  car1.mpg = 26;
  System.out.println(car1.mpg);
  System.out.println(car2.mpg);

• Exit 26.

• car1 and car2 reference the same object, but are not linked in other ways.

• Subsequent assignments to car2 do not affect car1.

• Example:
  Vehicle car1 = new Vehicle();
  Vehicle car2 = car1;
  Vehicle car3 = new Vehicle();
  car2 = car3;

• car2 now references the same object as car3.

• The object referenced by car1 remains unchanged.

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