1. Side effects A side effect is anything that happens in a method other than computing and/or returning a value. Example: public class hello { public int sum = 0; public char letter = ‘ ‘; public int getProduct(int a, int b) { sum = a+b; // SIDE EFFECT return (a*b); } public void setLetter(char z) { letter = z; // SIDE EFFECT }

2. precondition/postcondition Often, you will see comments before a method in which the precondition and postcondition is specified. The precondition of a method is what is true before the method executes. The postcondition of a method is what must be true after the method executes.

3. Example

4. Interfaces An abstract method is a method with a first line (also called a signature or a header), but without a body An interface is a collection of abstract methods and (sometimes) constants An abstract method can be declared using the modifier abstract, but because all methods in an interface are abstract, usually it is left off An interface is used to establish, as a “formal contract”, a set of methods that a class will implement. In simple terms: an interface is a blueprint/template that shows programmers how to write a certain type of class.

5. public interface Sample { public final double PI = 3.14; public void times2(int x); public double getPI(); } interface is a reserved word Notice: none of the methods in an interface are given a definition (body) A semicolon immediately follows each method header An interface can contain constants

6. public class Z implements Sample { public void times2(int x) { System.out.println(x*2); } public double getPI() { return PI; } Each method listed in Sample must be given a definition Z “inherits” PI from Sample

7. Open Sample, Z

8. An interface cannot be instantiated (meaning you can’t create an object of it from a client program). All methods in an interface must be public. It is nonsensical to have a private method in an interface – where would this method be called from? Remember, all methods in an interface are empty. A class formally implements an interface by – stating so in the class header (using the word implements) – providing implementations (writing code) for each abstract method in the interface If a class implements an interface, then it MUST define (provide code for) all methods that are in the interface

9. A class that implements an interface can implement other methods as well In addition to (or instead of) abstract methods, an interface can contain constants When a class implements an interface, it gains access to all its constants

10. A class can implement multiple interfaces. (Note that this is different from inheritance: a class can only extend (inherit from) ONE parent.) The interfaces are listed in the implements clause The class MUST implement all methods in all interfaces listed in the header Also, many different classes can implement the same interface. public class Mult implements interface1, interface2 { // all methods of both interfaces // MUST be defined here }

11. The Java standard class library contains many helpful interfaces The Comparable interface contains an abstract method called compareTo, which is used to compare two objects The String class implements Comparable, giving us the ability to put strings in lexicographic order: compares the letters using their ASCII (aka Unicode) values (see p. 604) Demo: compare

12. When a programmer writes a class that implements the Comparable interface, it’s up to him/her to determine what makes one object greater or less than another Not all objects are compared numerically or lexicographically

13. Confusing terminology: – A class that “connects” to an interface is said to implement the interface, by using the keyword implements – Sometimes, you will see exams and text books mention that a certain method has not been implemented – what they mean is that the method has not been given any code in its body. In other words, the method has a signature, but is otherwise empty.

14. Assignments Type these answers in a Word doc called p291-3Qs, save it in your Programs folder: p. 291-3 multiple choice #4, 6, 7, 9, 10 P. 293 true/false #9 – 10