Classes and Objects in Java
This has been a long and detailed chapter. The following
list summarizes the most important points to remember.
This summary is not intended to simplify the complex
material we've covered, but it may allow you to test your
comprehension of the material now, and may help jog your
- A class is a collection of data and methods
that operate on that data.
- An object is a particular instance of a class.
- Object members (fields and methods) are accessed with a
dot between the object name and the member name.
- Instance (non-static) variables occur in each instance of a
- Class (static) variables are associated with the class.
There is one copy of a class variable regardless of the
number of instances of a class.
- Instance (non-static) methods of a class are passed an
implicit this argument that identifies the object
being operated on.
- Class (static) methods are not passed a this
argument and therefore do not have a current instance of the
class that can be used to implicitly refer to instance
variables or invoke instance methods.
- Objects are created with the new keyword, which
invokes a class constructor method with a list of arguments.
- Objects are not explicitly freed or destroyed in any way.
The Java garbage collector automatically reclaims objects
that are no longer being used.
- If the first line of a constructor method does not invoke
another constructor with a this() call, or a
superclass constructor with a super() call,
Java automatically inserts a call to the superclass
constructor that takes no arguments. This enforces
- If a class does not define a constructor, Java provides a
- A class may inherit the non-private methods and
variables of another class by "subclassing"--i.e., by
declaring that class in its extends clause.
- java.lang.Object is the default superclass for a
class. It is the root of the Java class hierarchy and has no
superclass itself. All Java classes inherit the methods
defined by Object.
- Method overloading is the practice of defining multiple
methods which have the same name but have different argument lists.
- Method overriding occurs when a class redefines a method
inherited from its superclass.
- Dynamic method lookup ensures that the correct method is
invoked for an object, even when the object is an instance
of a class that has overridden the method.
- static, private, and final methods
cannot be overridden and are not subject to dynamic method
lookup. This allows compiler optimizations such as
- From a subclass, you can explicitly invoke an overridden
method of a superclass with the super keyword.
- You can explicitly refer to a shadowed variable with the
- Data and methods may be hidden or encapsulated within a
class by specifying the private
or protected visibility modifiers.
Members declared public are visible everywhere.
Members with no visibility modifiers are visible only within
- An abstract method has no method body (i.e., no implementation).
- An abstract class contains abstract methods. The methods
must be implemented in a subclass before the subclass can be
- An interface is a collection of abstract methods and
constants (static final variables).
Declaring an interface creates a new data type.
- A class implements an interface by declaring the interface
in its implements clause and by providing a method
body for each of the abstract methods in the interface.