AP Computer Science Chapter 10 Implementing and Using Classes Study Guide

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1 AP Computer Science Chapter 10 Implementing and Using Classes Study Guide 1. A class that uses a given class X is called a client of X. 2. Private features of a class can be directly accessed only within the class s own code. Public features can be accessed in client classes (with an appropriate name-dot prefix). 3. In OOP, instance variables are almost always private. If necessary, the developer provides a special accessor method that returns the value of a particular private field. The developer may also define methods that update private fields. These are called mutator methods. 4. An exception to the above convention is sometimes instance variables that are constants are declared public: Math.PI or Color.RED. Helper methods that are only used within a class are declared private. 5. A constructor is a short procedure for creating objects of a class. Constructors always have the same name as a class. Unlike methods, constructors do not return any values and therefore do not need a return type, even void. 6. If a class has more than one constructor, then they must differ in the number or types of parameters. 7. Methods within the same class that have the same name but different numbers or types of parameters are called overloaded methods. 8. If a class doesn t have any constructors, the compiler supplies a default, no-args constructor. It allocates memory for the object and initializes its fields to default values. But, if you define at least one constructor for your class, then the default no-args constructor is not supplied. 9. A constructor can call another constructor of the same class using the keyword this or refer to itself using the keyword this.

2 Point(int x, int y) { this.x=x; this.y=y; } Point() { this(0, 0); } 10. Objects are created using the new operator. When you declare a variable of a class type, this variable holds a reference to an object of the corresponding type. It is crucial to initialize the reference before using it by using the new operator. 11. The assignment operator, when applied to references copies only the value of the reference (that is the object s address) and not the object itself.

3 12. When you define a method you need to give it a name, specify the types of its parameters, and assign them names so that you can refer to them in the method s code. You also specify the type of its return value. 13. Defining a tostring method inside of a class overrides the default tostring method that is defined in the Object class (from which all classes inherit from). System.out.println looks for a tostring method when called to print an object. tostring returns a reasonable representation of an object as a String. 14. A static method belongs to the class as a whole and doesn t deal with any instance variables. Such a method can be called by simply using the class s name and a dot as a prefix to the method s name as in: x = Math.random(). Non-static (instance) methods we call it for a particular object: obj.run(); 15. Parameters of primitive data types are always passed by value. Objects are always passed by reference. 16. Immutable classes do not provide public methods that allow the instance variables to be changed. 17. A return statement tells the method that value to return to the caller and immediately quits the method s code and passes control back to the caller. 18. If a method returns a type of some class, the method returns a reference to an object of that class. 19. Non-static fields are called instance variables and can be thought of as the private memory of an object. Static fields are also called class variables and they are shared by all objects of the class. 20. Memory for static fields is reserved before any objects of the class has been created. All objects have access to this class field. This allows objects of the same class to share constants or keep track of info across objects.

4 21. Static methods do not access/manipulate instance variables because they do not belong to a particular class. They can access static fields or simply provide a service that has nothing to do with an object. Static methods are referred to by the class name: Math.max() 22. Instance methods can access both static and non-static fields and call static and non-static methods. Review Questions 1. Are you allowed to define a class with no constructors? 2. Can a class have two different no-args constructors? 3. What is the common Java style for method names? 4. Are parameters of primitive data types passed to methods by value or by reference? 5. Can a method have more than one return statement?

5 6. Can a method have no return statements? 7. Can a method create a new object and return it to the calling method? 8. Describe the difference between static and instance variables. 9. What is the syntax for referring to a public static field outside the class? 10. Can non-static methods access static fields? 11. Can static methods access instance variables? 12. Can static methods have local variables?

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