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

3. Evan Korth New York University Road Map Dealing with multiple files modifiers Static variables Reading –Liang 4: chapter 6: 6.5 – 6.8 –Liang 5: chapter 6: 6.6, 6.7, 6.10, 6.11 –Liang 6: chapter 7.5 – 7.8, 7.11, 7.14

4. Evan Korth New York University review What is meant by the term encapsulation? What are the default values for data members? What does it mean to instantiate an object? What does this statement do? Integer i;

5. Evan Korth New York University Review Given: Integer i; What does the following statement do? i = new Integer(100); Generally, what should a constructor do?

6. Evan Korth New York University Modifiers Java provides us with several keywords used to modify the accessibility of variables, methods and classes. –Visibility modifiers public private protected (None) –others static final abstract

7. Evan Korth New York University Principle of least privilege You should pick the modifier that allows the least privilege for other classes while allowing your code to do what it needs to do. This helps reduce debugging time by localizing potential problem areas.

8. Evan Korth New York University Data member modifiers No modifier (default) means the data is visible in the package in which it is declared. public means the data is visible to everything. private means the data is visible only within the class in which it is defined. –Trying to access private data from another class will result in a compile time error. final means the variable cannot be changed. There are two other modifiers applicable to variables: –static : We will discuss in a moment –protected : We will discuss later in the semester.

9. Evan Korth New York University Accessor methods When a data member is declared to be private, we still need a way to refer to that data. A method used to change or retrieve a private data item is referred to as an accessor method. Two kinds of accessor methods are the get method and the set method.

10. Evan Korth New York University Get methods A method that is used to retrieve the value of a data object is referred to as a get method. Also known as a getter. Get method header should look like this: public returnType getPropertyName () It may just return a data field or it may calculate the value. Remember information hiding.

11. Evan Korth New York University Predicate methods A get method that returns a Boolean value should have a header like this: public boolean isProperty () It can simply return a Boolean data field or it can use a Boolean formula to calculate it’s data. Remember, information hiding!

12. Evan Korth New York University Set methods Methods used to set or change the value of a data method are referred to as set methods. Also known as setters and mutators. Header of set method will look like this: public void setProp (propType)

13. Evan Korth New York University Data modifiers (cont) A data member can be either an instance variable or a static variable (also known as a class variable).

14. Evan Korth New York University Static variable (AKA class variable) A static variable has only one value no matter how many objects are instantiated from a class. The value is shared by all instances of the class. A static variable does not need an instance of the class in order to be accessed. You can access a static variable either with ClassName.varName (better style), or objectReference.varName notation. For static variables, every object of the class refers to the same memory location. Static variables can be accessed by static methods OR instance methods. The memory for a static variable is allocated when the class is loaded into memory.

15. Evan Korth New York University Instance variables An instance variable has a unique value for each object of that class. –This does not mean that two objects cannot have the same value; it does mean that those values will be stored separately in memory. You can access an instance variable only with objectReference.varName notation. No memory is allocated until an object is instantiated. Can be accessed by instance methods only (we will talk about instance methods in just a moment). (i.e. not by static methods)

16. Evan Korth New York University Scope of data members Whether a data member is a class variable or an instance variable, it’s scope is the entire class. It does not matter where in the class, the variable is declared. Remember, if they are not initialized, data members are assigned a default value.

17. Evan Korth New York University Local method variables Do not automatically get initialized. –Using them without initializing them is a compilation error. Cannot have visibility modifiers. The scope of a local method variable starts where it is declared. It ends at the end of the block where it was declared.

18. Evan Korth New York University Method members Methods are used to define the behaviors of an object. They can be overloaded. –Having more than one method in a class with the same name is referred to as method overloading. –Each of the methods must have a different method signature. That is, they must have different argument lists.

19. Evan Korth New York University Method modifiers No modifier means the method is visible in the package in which it is declared. public means the method is visible to everything. private means the method is visible only within the class in which it is defined. –Trying to call a private method from another class will result in a compile time error. static means it is a static method. Static methods can use other modifiers as well. There are three others ( final, protected and abstract ) which we will discuss later in the semester. There are still others which we will not discuss this semester.

20. Evan Korth New York University Static methods (AKA class methods) Can be called without an instance of the method. All the methods in the Math class are static methods which is why we can call them without a Math object. In fact, we cannot instantiate an object of the Math class. You can call a static method either with ClassName.method (args) (better style), or objectReference.method (args) notation.

21. Evan Korth New York University Instance methods Can only be called after an object is instantiated. You can call an instance method only with the objectReference.method (args) notation. An instance method acts on the specific instance for which it has been called.