2-1 Types of data  Java has two types of data  primitive types that store one value char boolean byte int long float double  reference types to store many values with one name  any of the hundreds of classes that come with Java String BufferedReader URLAddress JButton String BufferedReader URLAddress JButton  any domain types for specific applications BankAccount Student Jukebox CD BankAccount Student Jukebox CD  Classes that provide utility in a wide variety of applications Scanner String arrays JFrame Scanner String arrays JFrame

2-2 What's the difference  Primitive type variables that store the actual value of the variable int n = 0; // n stores 0 int n = 0; // n stores 0  Reference type variables store a special value for finding the object at runtime  the object is a bunch of bits on the heap (special part of memory that gets allocated at runtime)

2-3 Methods  Abstraction:  allows programmer to concentrate on essentials  how does a String's substring?  How does System.out.println send output to the console?  how does a JButton know it's been clicked?  What algorithm does the Math.sqrt method use?  However, programmers often create new methods  We will need to work out the details and know who to write the methods

2-4 Methods   A Java class typically contains many methods, each of which has a method heading   Method headings specify the number and types of arguments required  There are two major components to a method:  the method heading   the block (a set of curly braces with the code that completes the method's responsibility)

2-5 Method Headings  Sun uses method headings (and other things) to document methods so we know how to use them public String substring( int beginIndex, int endIndex ) Returns a new string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex-1. Thus the length of the substring is endIndex-beginIndex. Examples: "hamburger".substring(4, 8) returns "urge" "smiles".substring(1, 5) returns "mile" Parameters: beginIndex - the beginning index, inclusive. endIndex - the ending index, exclusive. Returns: the specified substring.

2-6 What method headings tell us  Method headings provide the information needed to use it they show us how to send a message to instances of the class public String substring(int beginIndex, int endIndex) public String substring(int beginIndex, int endIndex)  What does the method evaluate to? String  What is the method name? substring  How many arguments are required? 2  What type of arguments are required? int  Example message String fullName = "Murphy, John"; String fullName = "Murphy, John"; String lastName = fullName.substring( 0, 6 ); String lastName = fullName.substring( 0, 6 );

2-7 Parameters = Arguments  The substring method requires two arguments in order to specify the portion of the string to return.  When the message is sent, the argument 0 is assigned to the parameter named beginIndex and the argument 6 to the parameter name endIndex fullName.substring( 0, 6 ); fullName.substring( 0, 6 ); public String substring( int beginIndex, int endIndex ) public String substring( int beginIndex, int endIndex ) Implementation of the method is not shown here Implementation of the method is not shown here

2-8 Arguments / Parameters  When a message is sent  The first argument is assigned to the first parameter,  second argument gets assigned to the second parameter,...  If you do not supply the correct number and type of arguments, you get compiletime errors  Examples of errors fullName.substring( "wrong type" ); fullName.substring( "wrong type" ); fullName.substring( 0, 6, fullName.length() ); fullName.substring( 0, 6, fullName.length() ); fullName.substring( ); fullName.substring( ); fullName.substring( 0.0, 6.0 ); fullName.substring( 0.0, 6.0 );

2-9 Method Headings  General form of a Java method heading public return-type method-name ( parameter-1, parameter-2, parameter-n,... ) parameter-2, parameter-n,... )  public says a method is known where objects are constructed  return-type may be any primitive type, any class, or void  A void method returns nothing  therefore, a void method can not be assigned to anything  also, you can not place a call to a void method in println  parameters, which are optional, specify the number and type of arguments required in the message

2-10 Parameters  General form of a parameter between ( and ) in method headings class-name identifier class-name identifier-or- primitive-type identifier primitive-type identifier  Example method headings: public int length( ) // class String public int length( ) // class String public int equals( String anotherString ) // class String public int equals( String anotherString ) // class String public void withdraw( double amount ) // class BankAccount public void withdraw( double amount ) // class BankAccount public double readDouble( ) // class TextReader public double readDouble( ) // class TextReader public void move( int spaces ) // class Grid public void move( int spaces ) // class Grid

2-11 The Method Body  The method body is Java code enclosed within a block { }  Curly braces contain the same things you saw in main  variable declarations and initializations: int creditOne = 0;  input and output messages: readInt, System.out.println  construct objects: Grid g= new Grid(8,6,1,1,Grid.EAST)  messages: g.turnLeft()  Method bodies have access to the state of the object  The "state" of an object is implemented as variables declared inside the class rather than a method body.