Java Reference
In-Depth Information
2.
A constructor is used to construct an instance of a class. Unlike properties and meth-
ods, the constructors of a superclass are not inherited in the subclass. They can be
invoked only from the constructors of the subclasses, using the keyword
super
.
3.
A constructor may invoke an overloaded constructor or its superclass's constructor.
The call must be the first statement in the constructor. If none of them is invoked
explicitly, the compiler puts
super()
as the first statement in the constructor, which
invokes the superclass's no-arg constructor.
4.
To
override
a method, the method must be defined in the subclass using the same sig-
nature and the same return type as in its superclass.
5.
An instance method can be overridden only if it is accessible. Thus, a private method
cannot be overridden because it is not accessible outside its own class. If a method
defined in a subclass is private in its superclass, the two methods are completely
unrelated.
6.
Like an instance method, a static method can be inherited. However, a static method
cannot be overridden. If a static method defined in the superclass is redefined in a
subclass, the method defined in the superclass is hidden.
7.
Every class in Java is descended from the
java.lang.Object
class. If no super-
class is specified when a class is defined, its superclass is
Object
.
8.
If a method's parameter type is a superclass (e.g.,
Object
), you may pass an object to
this method of any of the parameter's subclasses (e.g.,
Circle
or
String
). This is
known as polymorphism.
9.
It is always possible to cast an instance of a subclass to a variable of a superclass,
because an instance of a subclass is
always
an instance of its superclass. When
casting an instance of a superclass to a variable of its subclass, explicit casting
must be used to confirm your intention to the compiler with the (
SubclassName
)
cast notation.
10.
A class defines a type. A type defined by a subclass is called a
subtype
and a type
defined by its superclass is called a
supertype
.
11.
When invoking an instance method from a reference variable, the
actual type of
the
variable decides which implementation of the method is used
at runtime
. This is
known as dynamic binding.
12.
You can use
obj instanceof AClass
to test whether an object is an instance of a
class.
13.
You can use the
ArrayList
class to create an object to store a list of objects.
14.
You can use the
protected
modifier to prevent the data and methods from being
accessed by nonsubclasses from a different package.
15.
You can use the
final
modifier to indicate that a class is final and cannot be
extended and to indicate that a method is final and cannot be overridden.
