16-Jul-15 Access

2 Overview Questions covered in this talk: How do we access fields and methods? Why have access restrictions? What can have access restrictions? How do we provide or restrict access?

3 Instance and class variables You can declare variables within a class These variables are called instance variables, or fields Every object of that class has its own copy of those fields The fields describe something about the object You can also declare static variables within a class There is only one of each static variable A static variable is also called a class variable The static variable describes something about the class as a whole

4 Method variables You can declare variables within a method or within a constructor These are called method variables, not fields Method variables are basically used for computation Method variables are strictly temporary, and are used only within that method When a method returns (completes), all its variables are discarded

5 Example: a “Rabbit” class class Rabbit { static int population; // class variable (counts Rabbits) double hunger; //instance variable double fear; // instance variable double courage = 0.75; // instance variable void eat() { double temp; // method variable temp = courage * hunger; if (temp > fear) { System.out.println(“Eating!”); hunger = hunger - 1; } } }

6 Statements You can declare variables inside a class or inside a method or a constructor You can put statements (executable code) only within methods and constructors, not inside a class Declarations with initializations are still declarations, not statements

7 Statements must be in methods (or in constructors) class Rabbit { double hunger; // OK--declaration double fear = 5.0; // OK--still a declaration hunger = 5.0; // illegal--assignment statement Rabbit ( ) { hunger = 5.0; // OK—statement in a constructor } void eat ( ) { hunger = hunger - 1; // OK—statement in a method } }

8 Access from inside a class Inside a class, you can access other fields and methods inside the class just by naming them Example:  class Person {  int age;  void birthday( ) { age = age + 1; }  void growOlder( ) { birthday( ); } } Equivalently, you can use the keyword this :  void birthday( ) { this.age = this.age + 1; }  void growOlder( ) { this.birthday( ); }

9 Accessing from outside a class, 1 Outside a class (from some other class) you access instance variables and methods by Naming the object you want to talk to Putting a dot Naming the variable or method Example:  // if NOT in class Person, say: if (john.age < 75) john.birthday(); Inside the class, the keyword this means “this object”: if (this.age < 75) this.birthday(); // "this" may mean john

10 Accessing from outside a class, 2 Outside a class (from some other class) you access class variables and methods by Naming the class you want to talk to Putting a dot Naming the variable or method Examples: Person.population = Person.population + 1; x = Math.abs(y);

11 Responsibility In Java, objects are considered to be active They have behaviors They are responsible for their own data Data (variables) must be kept consistent Example: population should never be negative In order for a class or object to be responsible for its own data, it must keep control of that data

12 Loss of control Suppose a Rabbit object, bugsBunny, has a variable named hunger Inside the class, this method is fine: void eat ( ) { hunger = hunger - 1; } From outside the class, the following is legal: bugsBunny.hunger = bugsBunny.hunger - 1; But should we be allowed to “reach inside” a rabbit? The class needs to protect itself from errors in other classes (and from malicious behavior)

13 private variables and methods If you declare a variable or method to be private, that variable or method can only be accessed from within the class private methods also make sense, e.g. digest() If you declare a variable or method to be public, then any code anywhere can access it Typically, a class or object has both Methods for use by the rest of the program Methods and variables that it alone should control

14 Levels of access private -- access only from within the class “package” -- access from within the class, or from any class in the same directory (“folder”) This is the default; there is no package keyword protected -- access from within the class, or from within any subclass, or from any other class in the same directory public -- access from anywhere at all

15 Levels of access, II To make a variable or method visible Only within this class: private From this class and its subclasses: not possible From this class and its subclasses, and any other class in this directory: “package” (default) From this subclass and its subclasses, and any other classes in this directory: protected From anywhere: public

16 Getters and setters One way to control access is via getters and setters: class Rabbit { private double hunger; // getter public double getHunger() { return hunger; } // setter public void setHunger(double hunger) { this.hunger = hunger; } This seems silly, but it’s much safer and more flexible

17 Immutable objects Suppose a Planet has a mass, and you want to be able to see its mass but not change it: class Planet { private long mass; // Constructor Planet(long mass) { this.mass = mass; } //getter long getMass() { return mass; } // Notice there is no setter! }

18 The End