Class and Method Definitions

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Class and Method Definitions
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UML Class Diagram
Automobile
- fuel double - speed double - license String
increaseSpeed(double howHardPress) void stop(double howHardPress) void
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Class Files and Separate Compilation

• Each Java class definition should be in a file by
  itself.
• The name of the file should be the same as the
  name of the class, and the file name should end
  in .java.
• You can compile a Java class before you have a
  program in which to use it.
• The compiled byte-code for the class will be
  stored in a file of the same name but ending in
  .class.

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Class Files and Separate Compilation (contd)

• Later you can compile a program file with a main
  part that uses classes that you have already
  compiled.
• Every program with a main part has a class name
  at the start of the file this is the name you
  need to use for the file that holds the program.
• As long as all the classes you use in a program
  are in the same directory as the program file,
  you dont need to worry about directories.

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Example of a Class Definition

• import java.util.
• Public class Species
• public String name
• public int population
• public double growthRate
• public void readInput()
• Scanner keyboard new Scanner(System.in)
• System.out.println(What is the species
  name?
• name keyboard.nextLine()
• System.out.println(What is the population of
  the species?)
• population keyboard.nextInt()
• while (population lt 0)
• System.out.println(Population cannot be
  negative.)
• System.out.println(Reenter population)
• population keyboard.nextInt()

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Example of a Class Definition (contd)

• public void writeOutput()
• System.out.println(Name name)
• System.out.println(Population
  population)
• System.out.println(Growth rate growthRate
  )
• public int populationIn10()
• double populationAmount population
• int count 10
• while ((count gt 0) (populationAmount gt 0))
• populationamount (populationAmount
• (growthRate/100)populationAmount)
• count--
• if (populationAmount gt 0)

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Instance Variables

• The class name in the previous example is
  Species.
• The class is designed to hold records of
  endangered species.
• Each object in the class has three pieces of
  data a name, a population size, and a growth
  rate.
• Objects of this class have three methods
  readInput, writeOutput, and populationIn10.

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Instance Variables (contd)

• The data items and methods are called members of
  the object.
• The data items are sometimes called fields, but
  we will refer to them as instance variables.
• In the previous example the instance variables
  were name, population, and growthRate.
• The word public simply means that there are no
  restrictions on how these variables are used.
• An object is a complex item with instance
  variables inside it.

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Example of a Program that Uses the Class Species

• public class SpeciesDemoProgram
• public static void main(String args)
• Species speciesOfTheMonth new Species()
• int futurePopulation
• System.out.println(Enter data on the Species
  of the Month)
• speciesOfTheMonth.readInput()
• speciesOfTheMonth.writeOutput()
• futurePopulation speciesOfTheMonth.populationI
  n10()
• System.out.println(In ten years the population
  will be
• futurePopulation)
• speciesOfTheMonth.name Klingon ox
• speciesOfTheMonth.population 10
• speciesOfTheMonth.growthRate 15
• System.out.println(The new Species of the
  Month)
• speciesOfTheMonth.writeOutput()
• System.out.println(In ten years the population
  will be
• speciesOfTheMonth.populationIn10())

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Example of a Program that Uses the Class Species
(contd)

• You can refer to one of the instance variables of
  speciesOfThemonth by writing the object name
  followed by a dot and then followed by the
  instance variables name,
  e.g., speciesOfTheMonth.name
• Each instance variable has a type, and the type
  of the previous example is String.

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Example of a Program that Uses the Class Species
(contd)

• The new is used to create a new object of the
  class Species.
• You can think of it as creating the instance
  variables of the object.

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Using Methods

• All methods perform some action or actions.
• Some return a value, e.g., nextInt.
• Some do not, e.g., println.
• A method defined in a class is usually invoked
  using an object of that class. This object is
  known as the calling object, e.g.,
  keyboard.nextInt()

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Using Methods (contd)

• You invoke a method by writing the calling object
  followed by a dot, then the name of the method,
  and finally a set of parentheses that may or may
  not have information to pass to the method.
• If the method invocation returns a value, then
  you can use the method invocation anyplace that
  you are allowed to write a value of the type
  returned by the method, e.g.,
• futurePopulation speciesOfTheMonth.populationIn
  10()

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Using Methods (contd)

• If the method invocation does not return a value,
  then you place a semicolon after the method
  invocation and that produces a Java statement,
  e.g.,
• speciesOfTheMonth.readInput()

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Void Method Definitions

• Each method definition is given inside the
  definition of the class to which it belongs.
• The definition of a method that does not return a
  value starts with the keywords public void.
• The word void means that nothing is returned.
• The keyword public may be replaced with other
  words that restrict the use of the method.

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Void Method Definitions (contd)

• The first part of the method definition is called
  the heading.
• The statements that follow, enclosed in braces
  are referred to as the body.
• When a void method is invoked it is as if the
  method invocation were replaced with the body of
  the method where the instance variable names are
  replaced with those specific to the calling
  object, e.g., name is replaced by
  speciesOfTheMonth.name

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Void Method Definitions (contd)

• A program definition is just a class definition
  that has a method named main.
• When you run a program you are simply invoking
  the void method that is name main.
• The extra words static and String args will be
  explained later.

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Methods that Return a Value

• Methods that return a value are defined the same
  way as those that dont with the following
  exceptions
• Instead of the word void the type of the value
  returned by the method is given, e.g.,
• public int populationIn10()
• The body of the method must contain one or more
  return statements to return a value, e.g.,
• return (int)populationAmount

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Naming Methods

• Choose clear meaningful names.
• Its usually a good idea to use verbs to name
  methods that perform some action but do not
  return values, e.g. println.
• Nouns are usually the best choice for methods
  that return a value, e.g., populationIn10.
• By convention, methods begin with lowercase
  letters, and class names begin with uppercase

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Use of return in void Methods

• A return statement can also be used in methods
  that do not return values just to cause the
  method to terminate and return control to the
  method it was invoked from.
• In this case you simply say return with nothing
  following the return.

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The this Parameter

• When giving a method definition, you can use the
  keyword this as a name for the calling object.
• For example, in the definition the writeOutput()
  method of the Species class we wrote,
• System.out.println(name name)
• We could have written,
• System.out.println(name this.name)

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The this Parameter (contd)

• You can think of this as just a place holder for
  the calling object to be given when the method is
  invoked.
• this is usually omitted although there are some
  situations where it is needed.

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Local Variables

• Methods usually declarations for variables used
  within them beyond those declared in the class.
• These are local to the method in which they are
  declared and cannot be used outside the method.
• Two methods may have variables of the same name
  but they are treated as separate variables.

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Blocks

• A block is any set of Java statements enclosed
  within braces ().
• Sometimes blocks are referred to as compound
  statements.
• If you declare a variable within a block, it is
  not valid outside the block.
• In Java you cannot use the same name for
  different variables in different blocks even
  though it is valid only in the block where it is
  declared.

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Parameters of a Primitive Type

• The method populationIn10 for the class Species
  returns the projected population of a species 10
  years in the future.
• If we wanted to generalize this method to
  calculate the projected population for any number
  of years in the future we could pass number of
  years as a parameter to the method.

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Revised Method for Projecting Population

• public int projectedPopulation(int years)
• double populationAmount population
• int count years
• while ((count gt 0) (populationAmount gt 0))
• populationamount (populationAmount
• (growthRate/100)populationAmount)
• count--
• if (populationAmount gt 0)
• return (int)populationAmount
• else
• return 0

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Example of Using a Method with a Parameter

• public class SpeciesDemoProgram2
• public static void main(String args)
• Species speciesOfTheMonth new Species()
• int futurePopulation
• System.out.println(Enter data on the Species
  of the Month)
• speciesOfTheMonth.readInput()
• speciesOfTheMonth.writeOutput()
• futurePopulation speciesOfTheMonth.projectedPo
  pulation(10)
• System.out.println(In ten years the population
  will be
• futurePopulation)
• speciesOfTheMonth.name Klingon ox
• speciesOfTheMonth.population 10
• speciesOfTheMonth.growthRate 15
• System.out.println(The new Species of the
  Month)
• speciesOfTheMonth.writeOutput()
• System.out.println(In ten years the population
  will be
• speciesOfTheMonth.projectedPopulation(10))

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Formal Parameters and Arguments

• In the projectedPopulation(int years) method
  definition the word years is called a formal
  parameter or simply a parameter.
• It is a stand-in for a value that will be plugged
  in when the method is invoked.
• The item that is plugged in is called an argument
  ( or sometimes an actual parameter).

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Formal Parameters and Arguments (contd)

• In the invocation
• futurePopulation speciesOfTheMonth.projectedPop
  ulation(10)
• the value 10 is an argument.
• It is important to note that only the value of
  the argument is used in this substitution
  process. If the argument is a variable only the
  value of the variable is plugged in, not the name
  of the variable. This is called call-by-value.

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Correspondence Between Formal Parameters and
Arguments

• Formal parameters are given in parentheses after
  the method name at the beginning of a method
  definition.
• In a method invocation, arguments are given in
  parentheses after the method name.
• There must be exactly the same number of
  arguments as formal parameters, and they must be
  given in the same order.