Transcript Computer Science I Classes and Objects Professor: Evan Korth New York University Evan Korth New York University.

Computer Science I
Classes and Objects
Professor: Evan Korth
New York University
Evan Korth
New York University
Road Map
• Class modifiers
• Garbage collection
• Naming conflicts
– this
• Reference members
• Reading:
– Liang 7: chapter 7: 7.9, 7.10, 9.3, 9.4
– Liang 8: chapter 8: 8.6 – 8.9
Evan Korth
New York University
review
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What does encapsulation mean?
What is a data member?
What is a method member?
What is the difference between an object and a
class?
• What does the following line of code do?
– Integer i;
• What is i above?
• What happens if you make a class without a
constructor?
Evan Korth
New York University
Review (cont)
• What do the following modifiers mean
when applied to a data member?
– final
– static
– public
– private
• What if there is no modifier?
• What is the principle of least privilege?
Evan Korth
New York University
Review (cont)
• What data type does a set method usually return?
• What parameter does a get method usually take?
• A class has 3 objects instantiated, it also has a
static variable called x and an instance variable
called y.
– How many x values are stored in memory?
– How many y values are stored in memory?
• What is the scope of an instance variable?
• Can you call an instance method without an
instance of the class?
Evan Korth
New York University
Class modifiers
• No modifier (default) means the class is
visible in the package in which it is
declared.
• public means it is visible to everything.
• There are two others (final and
abstract) which we will discuss later in
the semester.
Evan Korth
New York University
Garbage collection
• When an object is no longer referenced by any
reference variable, that object is referred to as garbage.
• Java automatically tracks garbage objects and frees its
memory when the garbage collector runs.
• We do not have direct control over when the garbage is
collected.
• We can suggest to the compiler to collect garbage but it
is not guaranteed that it will run.
• To suggest garbage collection we make the following
method call:
– System.gc();
Evan Korth
New York University
Anonymous objects
• An object without a reference is called an
anonymous object.
• It is created, used and immediately marked
as garbage.
Evan Korth
New York University
Variable name conflicts
• It is possible to have a variable name in a
method with the same name as a data
member in a class.
• In such case, the local method variable
“hides” the data member variable.
Evan Korth
New York University
Keyword this
• The keyword this is used within a class to refer
to the specific instance of the class that is being
used.
• A variable in a class’ method that has the same
name as a field will “shadow” the field. You can
access the field using the this keyword.
• You cannot use the this keyword in static
methods. (why?)
Evan Korth
New York University
Another use for this
• this (args) in a constructor will invoke
another constructor of that class.
– If you call another constructor from a constructor, it
must be the first line in the calling constructor.
• This is useful when you overload your
constructors. In general, a constructor with fewer
parameters should call a constructor with more
parameters.
Evan Korth
New York University
Composition
• The term composition refers to the practice
of having an object as a data member within
another object.
• What is actually stored is a reference to the
member object. (therefore we can have self
referential objects)
• The default value for a reference variable is
null.
Evan Korth
New York University
Passing reference variables to
methods
• All variables in Java are passed using call by
value. However, since object variables are really
references to objects, passing an object is
simulated pass by reference.
– Objects passed to a method and modified by that
method will have the changes reflected in the calling
method.
– Primitive variables passed to a method and modified by
that method will NOT have the changes reflected in the
calling method.
Evan Korth
New York University