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

Evan Korth New York University

• Class modifiers • Garbage collection

Road Map

• Naming conflicts – this • Reference members • Reading: – Liang 5: chapter 6: 6.9, 6.12

– Liang 6: chapter 7: 7.10, 7.12, 7.13

– Liang 7: chapter 7: 7.9, 7.10, 9.3, 9.4

Evan Korth New York University

review

• 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