Writing methods and Java Statements

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Writing methods and Java Statements
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Java program

• import package
• // comments and / / and / javadoc here
  /
• public class Name
• // instance variables or fields
• // constructors. with no parameters is Default
• // methods

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Methods

• public/private returntype name ( / parameter
  list / )
• // statements
• public void myMethod( )
• // empty body

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Writing our own method calcVolume

• public double calcVolume( )
• // NO private in local variables final for
  constants
• final double PI 3.14159
• double volume // or george. call it anything
• volume 4 / 3 PI radius radius radius
• return volume

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Class Details Methods

• public double calcVolume( )
• // NO private in local variables
• final double PI 3.14159
• double volume
• volume 4 / 3 PI radius radius radius
• return volume

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Method vs. InstanceVariable

• Both have private or public
• Both have types
• Both have names
• instance variables have at end of
  line/methods do not
• methods have ( ) (even without parameters)
  instance variables do not
• methods have a body instance variables do not
• instance variables have memory to hold
  information methods do not

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instance variable (field) vs. Local variable

• local variables declare in a method instance
  variables outside of all methods
• local variables have the lifetime of the method
  call
• local variables and instance variables have type
  and
• local variables do NOT have private/public
  designation
• when possible, use local variables

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Writing methods Java statementsmust be within a
method

• Arithmetic Expressions
• Compound Assignment
• System.out.println
• new
• dot notation external method calls
• return
• JOptionPane

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Arithmetic

• , /, , -,
• Be careful about integer division
• Use codepad (Choose view, then codepad)
• int answer30 answer answer 4
  System.out.println("Answer is " answer)

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Compound Assignment

• assignment
• answer factor1 factor2
• answer answer newsum
• compound assignment
• answer newsum
• answer - diff
• answer product // e.g., factorial
• answer / digit // getting rid of digits
• answer digit
• single increment
• count OR count // does the same as count
  1
• --count OR count-- // does the same as count -1

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Math Problems

• int x3 double y4.0
• x y // this is ok even though different types
• x / 2
• y / 3
• x 2
• x 3
• x
• x 5
• x y // ok, but truncated
• x 2 // , --, , etc. work for doubles also

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Writing methodsMore Java statements

• Arithmetic Expressions
• Compound Assignment
• System.out.println
• new
• dot notation external method calls
• return
• JOptionPane

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System.out.println( )

• To print out messages to a terminal
• Can print strings or integers
• Use to concatenate/append. Be careful with
  numbers
• e.g.,
• System.out.println("Answer is " answer)
• System.out.println(answer answer)
• System.out.println(answer is answer answer)

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Writing methodsMore Java statements

• Arithmetic Expressions
• Compound Assignment
• System.out.println
• new
• dot notation external method calls
• return
• JOptionPane

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new, dot notation

• public void draw()
• wall new Square( )
• wall.moveVertical(80)
• wall.changeSize(100)
• wall.makeVisible()
• //rest of method from Picture class

To create a new object, use new. calls
constructor
External method call dot notation
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Most important slide of CS150 Objects, new, dot

• To declare an object
• NameOfType variableName
• To create the object, call its constructor with
  new
• variableName new NameOfType( )
• / often, caps is class constructor, lowercase
  is variable/object. 2 different things. caps is
  type of dog, lowercase, actual dog /
• name new Name( )
• To do something with the object, use variableName
  dot methodname
• variableName.DoSomething( )

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New practice on board

• Declare a variable george of type Picture. Call
  its constructor.
• Call george's draw method (use dot notation. to
  call a method within a class, no dots).

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Practice

• Assume that there is a Fraction class that has a
  write method which prints the Fraction, and two
  constructors, one that creates some default
  Fraction value and one that allows two parameters
  to be passed (e.g., Fraction(3, 4) would create
  the fraction 3/4).
• Write a UseFraction class that has two instance
  variables, both Fractions. Make one the default
  value and other 4/7. Print both of them out.

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CNU campus

• Write two classes, one a Building class and one a
  Campus class.
• Buildings have names, number of floors, square
  feet. Write getName, getFloors, getFeet methods.
• Campuses have 3 buildings and a name, a print
  method that prints school name and building info
• Write a Building class write a Campus class
• Create a campus class. Call its print method

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Writing methodsMore Java statements

• Arithmetic Expressions
• Compound Assignment
• System.out.println
• new
• dot notation external method calls
• return
• JOptionPane

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return statement

• public double calcVolume( )
• // NO private in local variables final for
  constants
• final double PI 3.14159
• double volume
• volume 4 / 3 PI radius radius radius
• return volume

type of method is return type
to return a value, use return value can be
calculation
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Other most important slideUnderstanding method
calls, object creation

• public class TestVolume // note NO ( )s for
  class
• public static void main(String args)
• Sphere sphere new Sphere( )
• double george
• george sphere.calcVolume( )
• System.out.println("value is " george)

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Trace

• on board
• using debugger

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return statement

• public double calcVolume( )
• // NO private in local variables final for
  constants
• final double PI 3.14159
• double volume
• volume 4 / 3 PI radius radius radius
• return volume

type of method is return type
to return a value, use return value can be
calculation
What is the fix?
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Casting

• Lesson 1 always test every method
• Lesson 2 dont assume answer is correct check
  it
• Fix 1 double volume 4 / 3
• Lesson 3 int / int REALLY is int.
• Fix 2 volume (double) 4/3 PI radius
  radius radius
• Casting should be used when you know the type of
  the result. int to double ok. double to int, not
  ok

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JOptionPane, Section 1.11

• To display a message
• import javax.swing.JOptionPane
• JOptionPane.showMessageDialog(
• null, // for now, always null
• "message I LOVE cpsc150!!!",
• "Window Title",
• JOptionPane.INFORMATION_MESSAGE)

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Reading information inJOptionPane, Section 2.11

• String answer JOptionPane.showInputDialog(null,
• "What is the weather today", "Weather Predictor",
  
• JOptionPane.QUESTION_MESSAGE)
• System.out.println("The weather is " answer)

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Reading information,Section 2.11converting
strings to ints

• String answer JOptionPane.showInputDialog(null,
• "Enter a number",
  "Window Title",
• JOptionPane.QUESTION_MES
  SAGE)
• System.out.println("Answer is " answer) //
  answer1 error
• int number Integer.parseInt(answer) // turns
  Strings to nbrs
• System.out.println("Answer is " number1)

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Source Code Review Terms

• import
• comments // and / / and / javadoc here
  /
• public/private
• can't see draw can see changeColor
• class
• class name
• case sensitive. convention, start with capital
• fields/instance variables
• always private, outside everything except class
• specify data type (can be primitive or object)
  and name
• primitive data types int, double, boolean, char,
  a few others
• lowercase
• can be initialized all other statements must be
  within a method
• constructor
• can have more than one. each must have different
  signature
• always the name of the class. never has a return
  value

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Review

• method (definition)
• Java statements
• return type (void if nothing returned)
• parameters
• local variable
• assignment
• , -, , /, (result is same as operands)
• , , --, , -, , /
• System.out.println
• method call
• new
• dot notation
• JOptionPane for reading and writing

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Accessor Methods

• Allows outside users to view parts of a class
• Class has control over what clients see
• Returns the value being accessed has no
  parameters
• Called getNameOfValueBeingRetrieved
• public int getNumerator( )
• return numerator

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Accessor Method on Board

• Write getX for the Circle class

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Mutator Method

• Allows clients to set a value in a class
• Class has control over how that is set
• returns a void. has parameter that is the new
  value
• called setNameOfValueBeingSet
• public void setDenominator(int d)
• if (d ! 0)
• denominator d

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Mutator Method on Board

• write setX for Circle
• Notice not called setXPosition client shouldn't
  know name of instance variable