Fina Final R l Rev eview!!! w!!! Classes and Objects Program - PowerPoint PPT Presentation

Fina Final R l Rev eview!!! w!!! ● Classes and Objects ● Program Statements (Arithmetic Operations) ● Program Flow ● String In-depth ● java.io (Input/Output) ● java.util (Utilities) ● Exceptions

Cla lass sses – Bas es – Basic ic Ove Overv rview Cla lass sses – Bas es – Basic ic Ove Overv rview A class is a container for data values (attributes) and actions (methods). It can contain only actions (generally done with static methods only). It can also contain both in which case it usually describes an object either tangible or abstract. This blueprint of an object needs to have a special method that gives initial values to the attributes (constructor). Examples of only action classes: - Integer (contains action to convert String to int) - Math (contains a bunch of math functions: sqrt, cos, arctan, ...) Examples of Blueprint classes: - Employee (Attributes: name, position, salary, Actions: getPay) - Board (Attributes: board, Actions: userMove, full)

Cla lass sses – Bas es – Basic ic Ove Overv rview Cla lass sses – Bas es – Basic ic Ove Overv rview //import statements here import java.io.*; ... public class <className> { //attributes private <type> <name>; private int x; ... //constructors public <className> (<params>) { } public <className> (int x1) { x = x1; } ... //methods public <returnType> <methodName> (<params>) { //code to do action //return if returnType is not void } public int getX () { return x; } ... }

Cla lass sses – At es – Attr tribut butes Cla lass sses – At es – Attr tribut butes An Attribute is a variable that is stored within the class that describes a value that the object the class describes has. These are primarily defined a private since, we do not want anyone explicitly changing the value. They are defined outside any method body, but are within the class, as such any method within the class has access to it as well. This in contrast to local variables that are defined within methods...only the method which the variable is defined in has access. public class Attr { private int x; public Attr () { x = 0; } public void localDemo() { int y = 5; System.out.println(“X is “+x); //Ok since x is an attribute System.out.println(“Y is “+y); //Ok since y is local variable in the current method } public void globalDemo() { System.out.println(“X is “+x); //Ok since x is an attribute System.out.println(“Y is “+y); //ERROR! Y only exists within the localDemo method } }

Cla lass sses – At es – Attr tribut butes Cla lass sses – At es – Attr tribut butes Since attributes are variables, but defined in the class scope, they follow the same rules as local variables. They must be defined once and only once, this is done inside the class (refer to slide 3). They must be declared with the keyword private then the data type, common data types are below, they can take on any reference type (i.e. class name, see slides from week 9 (Point attribute in the Circle class)): Common Data Types: - int - double - char - boolean - String

Cla lass sses – At es – Attr tribut butes Cla lass sses – At es – Attr tribut butes Attributes can take on array types as well. Arrays are accessed using indexes ranging from [0, size-1] Generic description of Arrays below: Array Declaration: <type> [] <name>; //sets up memory for an array (no size yet) Array Assignment: <name> = new <type> [<size>]; //sets the size of the array Array Initialization <type> [] <name> = new <type> [<size>]; //creates an array of specified size //and type Array Access: <name>[<index>] //gets the value store in the array at index

Cl Classes – C – Const structo tors rs Cl Classes – C – Const structo tors rs A Constructor's job is to set-up an object initially, they should assign starting values to the attributes (e.g. In Point it set the x, y coordinates to 0, 0). Constructors can take parameters just like methods, and use them for the initialization process. Constructors are always defined as public, and have no return type. They also have the EXACT same name as the class, case and all. Then they have parenthesis for any parameters. e.g. If you have a class called Point you could have the two constructors: public Point() { x = 0; y = 0; } public Point(int px, int py) { x = px; y = py; }

Cla lass sses es – Met Methods Cla lass sses es – Met Methods Methods usually begin with the public keyword and are followed by the return type...this is the kind of information (int, double, String, ...) that the method passes back to the code that is making the method call (invoking code). These methods behave like queries, (e.g. gameOver, getPay, ...) Methods that do not return data, behave like commands, doing a specific action (e.g. userMove, displayBoard, ...), these methods specify their return type as void . Methods can have parameters, which are values that are passed from the invoking code to the method. Example: // takes an integer for the number of hours worked from the invoking code // returns the total pay for the week public double getPay (int hours){ return salary * hours; //assume salary is an attribute }

Progr ogram S m Sta tatement tements Progr ogram S m Sta tatement tements Program Statements are commands that perform a specific action, or determine program flow and are what are executed by the program. These appear only within methods. Java begins with the statements found in the main method: public static void main(String [] args) { Statements that perform a specific action always end in semi-colons. Common examples are ones that modify the value of variables, or print to the screen. Statements that control the flow of the program, usually end in braces, as these enclose code for possible paths (e.g. if a condition is true perform the statements within a set of braces)

St Statement tements – Metho s – Method Ca d Calli ling ng Statement St tements – Metho s – Method Ca d Calli ling ng You can call methods, jump to the method's body and execute the statements that are there to perform a series of steps in one line. This is why we have methods: Calling static methods (parseInt is static) <className>.<methodName>( <arguments> ); //executes methodName which is from className , and is passed //values specified as arguments Calling non-static methods (need an object created) <className> <objName> = new <className> ( <constructorArgs> ); //creates an object of type className <objName>.<methodName>( <arguments> ); //executes methodName which is from className

Statements Sta ents - Print - Printing ing Statements Sta ents - Print - Printing ing Two statements exist for printing: System.out.print( <What is to be printed> ); //this prints out the text specified and sets the cursor on //the same line so that whatever is printed next appears on //the same line System.out.print( <What is to be printed> ); //this prints out the specified text, and sets the cursor //to the next line so that the next set of text to be //printed appears on the following line.

Sta tatement tements - Ar s - Arithmeti thmetic Sta tatement tements - Ar s - Arithmeti thmetic Numeric Variables (int, double) can be modified using built-in operations: + addition - subtraction * multiplication / division % modulus = Assignment, stores what is on the right side into the left e.g. x = 5 stores the value 5 into the variable x.

Program Fl m Flow ow – if – if, els else Program Fl m Flow ow – if – if, els else The if statement uses conditionals to determine what code to run. When the condition evaluates to true then the code following the if-statement is run, using braces specifies a set of statements to run. Using an else in conjunction will result in the code following the else to be run when the condition evaluates to false . The code continues as normal after if- statement is finished if ( <condition> ) { //code to run when condition is true } else { //code to run when condition is false } Note: else part is optional

Progr ogram Flo m Flow w – Co – Cond ndit itio ions Progr ogram Flo m Flow w – Co – Cond ndit itio ions There exists some commands that can be used to compare: == checks to see if variables/values on both sides are equivalent < strictly less than <= less-than or equal > strictly greater than >= greater-than or equal != not equal || combines two conditionals with an OR statement && combines two conditionals with an AND statement

Pro Progr gram F Flo low – Lo w – Loop ops Progr Pro gram F Flo low – Lo w – Loop ops Loops repeat a section of code repeatedly until a condition is false (in the case of while and do-while loops), or a specific number of times (for- loop). The generic form of the while & do-while are described below: while ( <condition> ) { //code to run, when condition is true, will not //stop until condition is false } do { //same as while, except, code run first then //checked. } while (<condition> );