Java: How to Program Methods Summary

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Java How to ProgramMethodsSummary

• Yingcai Xiao

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• How to Declare/Define a Method (Chapter 3)
• A method is just a named block of code that can
  be reused/called/invoked again and again.
• A method must be define inside a class.
• General format
• public return-type method-name (parameter list)
  // header of the method
• // body of the method
• Examples Figures 3.7-3.8

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• How to Declare/define a Method (Chapter 3)
• public class GradeBook
• private String courseName // course name for
  this GradeBook
• // method to set the course name
• public void setCourseName( String name ) //
  return type void no return value
• courseName name // store the course name
• // return back to the calling method even
  though there is no return value.
• // method to retrieve the course name
• public String getCourseName()
• return courseName
• // end method getCourseName
• // display a welcome message to the GradeBook
  user
• public void displayMessage()
• // calls getCourseName to get the name of
  the course this GradeBook represents
• System.out.printf( "Welcome to the grade
  book for\ns!\n", getCourseName() )
• // end method displayMessage
• // end class GradeBook

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• How to use/call a Method (Chapter 3)
• import java.util.Scanner // program uses Scanner
• public class GradeBookTest
• // main method begins program execution
• public static void main( String args )
• // create Scanner to obtain input from
  command window
• Scanner input new Scanner( System.in )
• // create a GradeBook object and assign it
  to myGradeBook
• GradeBook myGradeBook new GradeBook()
• // display initial value of courseName
• System.out.printf( "Initial course name is
  s\n\n", myGradeBook.getCourseName() )
• // prompt for and read course name
• System.out.println( "Please enter the
  course name" )
• String theName input.nextLine() // read
  a line of text
• myGradeBook.setCourseName( theName ) //
  set the course name
• System.out.println() // outputs a blank
  line
• // display welcome message after specifying
  course name
• myGradeBook.displayMessage()
• // end main

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• How to use/call a Method (Chapter 3)
• You need to create an object of a class in order
  to use its (non-static) methods.
• General format
• object.method(argument list) // argument list
  must match parameter list in type
• String theName input.nextLine() // read a
  line of text
• myGradeBook.setCourseName( theName ) //
  set the course name
• General format
• For methods returning a value, it can be
  assigned to a variable.
• String cName myGradeBook.getCourseName()
• System.out.printf( "Initial course name
  is s\n\n",cName)
• When calling a method of a class inside another
  method of the same class, no object needs to be
  create, just call the method.
• public void displayMessage()
• System.out.printf( "Welcome to the grade book
  for\ns!\n", getCourseName() )
• // end method displayMessage

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• How to use/call a Method (Chapter 3 and chapter 6)

• Three ways to call a method
• Using a method name by itself to call another
  method of the same class
• Using a variable that contains a reference to an
  object, followed by a dot (.) and the method name
  to call a method of the referenced object
• Using the class name and a dot (.) to call a
  static method of a class

• Three ways to return control to the statement
  that calls a method
• When the program flow reaches the method-ending
  right brace
• When the following statement executes
• return
• When the method returns a result with a statement
  like
• return expression

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• Static
• static methods
• public class MyApp
• public static void main(String args)
• .
• No need to create an object to use static methods
  in a class.
• Math.max(1,2)
• Can only invoke other static methods inside a
  static method.

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• Summary on Static
• static fields
• Class vriables one copy shared by all objects of
  the class
• Example, object counter
• class MyObject
• public int i
• public static int c
• MyObject() i c
• public class MyApp
• public static void main(String args)
• MyObject obj1 new MyObject()
• MyObject obj2 new MyObject()
• MyObject obj3 new MyObject()
• System.out.printf(i is d, obj3.i)
• System.out.printf(Number of object has been
  created d, obj3.c)

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• Final
• Final fields
• Vales can only be initialized at initialization
  and can not be changed.
• final float PI 3.14
• Static and final
• Math.PI
• final its value cant be changed
• static you dont need to create a Math object
  to use it and there is only one copy for the Math
  class.

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• Parameters and Arguments
• Number and types of arguments in a method call
  should match the number and types of the
  parameters of called method.
• Called method
• public double maximum( double x, double y,
  double z )
• Calling method
• double max maximum (1.2, 3.4, 3.5)
• The signature of a method / function consists of
  the name, number of parameters, type of each
  parameter and the order of them. (Note return
  type is not part of the signature.)
• Any part of the signature differs, you get a
  different method / function.
• The signature of the calling statement should
  match the signature of the called method.

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• Parameters and Arguments
• Argument promotion
• Converting an arguments value, if possible, to
  the type that the method expects to receive in
  its corresponding parameter.
• In cases where information may be lost due to
  conversion, the Java compiler requires you to use
  a cast operator to explicitly force the
  conversion to occurotherwise a compilation error
  occurs.

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• Random Number Generator
• Scale and shit can be used to control the range
  of the random numbers being generated.
• face 1 randomNumbers.nextInt( 6 )
• number shiftingValue randomNumbers.nextInt(sca
  lingFactor)

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• Enumeration
• An enumeration in its simplest form declares a
  set of constants represented by identifiers.
• Special kind of class that is introduced by the
  keyword enum and a type name.
• Definition Example
• private enum Status CONTINUE, WON, LOST
• Usage Example
• Status gameStatus // can contain CONTINUE, WON
  or LOST
• gameStatus Status.WON // write to
• if(gameStatus Status.WON) . // read
  from

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• Scope
• The scope of a declaration is the portion of the
  program that can refer to the declared entity by
  its name.
• The scope of an identifier (variable or method
  name) is the inner most block it is defined.
• The scope of a parameter in the parameter list of
  a method is the method.
• The scope of a loop variable defined in the for
  loop header is the body of the loop.
• A method or fields scope is the entire body of
  the class.
• The scope of a local-variable declaration is from
  the point at which the declaration appears to the
  end of that block.
• Example
• Figure 6.12

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• Method overloading
• Methods of the same name declared in the same
  class
• Must have different sets of parameters
• Example
• Figure 6.15