AP Computer Science A Unit 5 Writing Classes

Unit 5: Writing Classes Anatomy of a Class: All members with public access can be accessed or modified inside or outside of the class where they are defined. When you need many methods to have access to the same variable, you make that variable an instance variable. The scope of an instance variable is the entire class where the variable is defined. Ex: Public class Run { private int one, two; private int answer; public void jog() { answer = one + two; } Public int getAnswer() { return answer; } } Always keep in mind: • All instance variables should be private • All constructors should be public • Methods may be public or private Constructors: They are very similar to methods. They also have the same name as the class and have no return type – no void, int, etc. Make sure to initialize all instance variables. Steps to create a constructor heading: 1. public (access type) 2. Name (always has same name as class) 3. () and must have own set of curly braces There are default constructors and initialization constructors. Default constructors have no parameter list. When a default constructor is called, all instance variables / data fields are set to a zero value. If no constructors are provided for a class, Java will provide a default constructor that will initialize all data to a zero value. Initialization constructors have a parameter list and will receive parameters when called. The number of parameters and types passed in must match up with the parameter list following the method name. Documentation with Comments: In addition to single and multi-line comments, Java has a third comment syntax, which generates Javadoc documentation. These comments are enclosed by /** comment */, and are used to document the description, purpose, and conditions associated with a class, instance variable, constructor, or method. A precondition is a condition that must be true immediately prior to a method being called. If it is not true, the method should not be expected to work as described and may cause an error to occur. Accessor Methods: Accessor methods are methods that retrieve or grant access to the properties of an object, but do not make any changes. Ex: public String printA() { return sideA; //sideA is an instance variable } Mutator Methods: Mutator/Modifier methods are methods that change the properties of an object. Ex: public void setSides(int a, int b, int c) { sideA=a; sideB=b; sideC=c; } Writing Methods: A method is a block of code that exists within a class and has access to the instance variables. The syntax for writing a method is as shown: visibility returnType methodName(parameters) { // method body } Visibility can be either public or private. Most methods are public, but some situations might use private to keep the accessibility of a method limited to its class. The return type specifies what type of value, the method will return. If the method will not return any value, the keyword void is used in place of the return type. Remember: A method is called an accessor if it returns something, but does not modify data. To return a value from a method, a return statement is used. Also note: A return statement will immediately terminate the execution of the method. Any code that follows it will never be reached, and if it is within a loop there will be no more iterations. Static Variables and Methods: For example: AplusBug cs = new AplusBug(); AplusBug dude = new AplusBug(); A reference variable is used to store the location of an Object. In most situations, a reference stores the actual memory address of an Object. In the example cs and dude store the location / memory address of two new AplusBug Objects. A variable is a storage location for a specific type of value. Static variables and methods are associated with a class, rather than with instances of the class, and are declared by including the static keyword. Ex: private static int count; public static double getValue() { … } Static methods can be either public or private, but they only have access to static variables that cannot read or change any instance variables. In particular, they do not have access to the this keyword (We will talk about this keyword later in this unit). Scope and Access: In variable scope when a variable is defined within a set of braces, that variable can only be accessed inside those braces. As shown below: { int fun = 99; } The variable has a scope limited to those braces. When you need only one method to have access to a variable, you should make that variable a local variable. The scope of a local variable is limited to the method where it is defined. Example including instance and local variables: public class Can { private int soup; //instance variable public void food() { int soup= 99; //local variable } public void print() { System.out.println(soup); } public static void main(String args[]) { Can test = new Can(); test.change(); test.print(); } } This Keyword: The keyword this can be used in a class to refer to the current calling object. For example, in the following Class Person, when we create an object p1 and call the constructor or p1.setEmail(), the word this refers to p1. And when we make the same method calls with object p2, this refers to p2. public class Person { // instance variables private String name; private String email; private String phoneNumber; // constructor public Person(String theName) { this.name = theName; } // accessor methods - getters public String getName() { return this.name;} public String getEmail() { return this.email;} public String getPhoneNumber() { return this.phoneNumber;} // mutator methods - setters public void setName(String theName) { this.name = theName;} public void setEmail(String theEmail) {this.email = theEmail;} public void setPhoneNumber(String thePhoneNumber) { this.phoneNumber = thePhoneNumber;} public String toString() { return this.name + " " + this.email + " " + this.phoneNumber; } // main method for testing public static void main(String[] args) { Person p1 = new Person("Sana"); System.out.println(p1); Person p2 = new Person("Jean"); p2.setEmail("jean@gmail.com"); p2.setPhoneNumber("404 899-9955"); System.out.println(p2); } }