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 EFFECTreturn (a*b);

}

public void setLetter(char z){

letter = z; // SIDE EFFECT}

}

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.

Example

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.

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 inan interface are given

a definition (body)

A semicolon immediatelyfollows each method header

An interface can contain constants

public class Z implements Sample{ public void times2(int x) { System.out.println(x*2); }

public double getPI() {

return PI; }

}

Each method listedin Sample must begiven a definition

Z “inherits” PI from Sample

• Open Sample, Z

• 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

• 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

• 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}

• 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

• 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

• 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.

TL;DR interfaces are confusing.

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