CECS 220 Java Test Review

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CECS 220 Java Test Review
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About me

• Matthew Thomas
• Senior
• CECS
• From Utica, KY

3
Variable Names

• May Contain a-z, A-Z, 0-9, _, , characters from
  other languages.
• May not start with 0-9.
• Legal
• MyVariable
• myVariable
• MYVARIABLE
• x
• _my_variable
• myvariable
• a?d???
• This_is_an_insanely_long_variable_name_that_never_
  ends
• Not Legal
• My Variable
• 9pins
• ac
• include
• OReilly
• Oreilly__Associates
• _at_hotmail

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Function, or procedure?

• void calculate()
•   int value 5 4
•   return
• int calculate()
•   int value 5 4
•   return value
• void calculate(int a, int b)
•   if (a 5 b 4)
•     System.out.println(20)
•     return
•  
•   System.out.println(a b)

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Testing

• JUnit vs not using an automated framework
• Reference utilities.CalculatorTest
  vs utilities.testing.TestCalculator

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To be or not to be static?

• It's a matter of belonging. Static belongs to
  the class, not static belonging to an instance
  of the class.
• class Vehicle
•   // Do these variables apply to vehicles in
  general, or to a specific one?
•   private int numWheels 4
•   private static int numPeople 5
• class Motorcycle
•   private static int numWheels 2
•   private int numPeople 2

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To be or not to be static?

• Same applies to functions and procedures
• Should eat() be a function of apples in general,
  or a specific one?
• Should getNumberOfLegs() be a function of cats in
  general, or a specific one?

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Packages

• For organization
• package toolspublic class Calculator  public
  static int multiply(int a, int b) return a b
  
• package toolspublic class AnotherTool  public
  static void function()
• package stuffpublic class Calculator  public
  static void doSomething()

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Packages

• package com.example.toolspublic class
  Calculator   public static int multiply(int a,
  int b)     return a b 
• com.examples.tools.Calculator.multiply(4, 5)
• import com.examples.tools.Calculator...Calculat
  or.multiply(4, 5)

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Class, instances of class

• JFrame f new JFrame()f.add(...)JFrame.add(..
  .) ???
• Double num new Double(5.5)double value
  num.doubleValue()double value2
  Double.doubleValue() ???
• double value Double.parseDouble("5.5")double
  value2 value.parseDouble("5.5") ???

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this

• class Foo   private static final int DEFAULT
  5  private int myNumber  public Foo()    
  this(DEFAULT) // ??    public Foo(int number)
      myNumber number 

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this

• class Foo   private static final int DEFAULT
  5  private int number  public Foo()    
  this(DEFAULT)     public Foo(int number)  
    this.number number // ?? 

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Extending classes

• class Animal   private string name "animal" 
  public Animal(string name)     this.name
  name    public string getName()     return
  myName 
• class Dog extends Animal   private string name
  "dog"  public Dog(string name)    
  super(name) // ?? 
• System.out.println(new Dog("Caesar").getName())
  // ???

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Scope

• For a gun? For a clean mouth?
• // Which variable is which??class Foo  
  private double variable  public Foo(double
  variable)     this.variable variable //
  "this"??  // variable variable // Why is this
  incorrect?    public boolean compare(double
  variable)     if (this.variable variable)
        return true        return false 

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Scope

• Where do variables live? Inside curly braces
• class Foo   private double variable1 1.0 
  void function1()     double variable2 2.0 
    variable2 variable1    void function2()
      double variable3 3.0    variable1
  variable3    variable3 variable2 // ??? 

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Scope

• Where do variables live?
• void foo()   int a 5  int b 6 a b
    b a // ???

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If

• These are identical
• int a ...if (a lt 0)   ...if (a 0)  
  ...if (a gt 0)   ...
• int a ...if (a lt 0)   ... else if (a
  0)   ... else   ...

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For

• These are identical
• int array ...
• for (int i 0 i lt array.length i)   int
  element arrayi  System.out.println(element)
  
• for (int element array)   System.out.println(e
  lement)

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Casting

• Bronze? For an arm? Throwing?
• Object o (Object)(new Integer(5))
• Animal a new Dog() // Dog extends Animal
• This is "casting up the class hierarchy"

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Casting

• What about "casting down the class hierarchy"?
• HondaCivic h new Car() // extends Car/ Wrong,
  and will fail at runtime! All HondaCivics have
  all the parts of a general car because HondaCivic
  extends Car, but there might be things (i.e.
  fields) that the HondaCivic class has in
  addition. Would the computer just make up things
  to fill these gaps? /
• HondaCivic h (HondaCivic)((Car)(new
  HondaCivic(1994, "blue")))/ This works though,
  and won't fail at runtime. The "real type" stays
  the same even though the "apparent type" changes
  /
• Protect with "if (c instanceof HondaCivic) ..."

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Wrapper classes

• ArrayListltintgt aListOfNumbers ... // ???
• ArrayListltIntegergt aListOfNumbers ...
• Remember "all classes are subclasses of Object"?
  Actually, the primitives aren't without
  wrappers, this causes problems for things like
  ArrayLists that only work for subclasses of Object

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Debugging

• It's why there are best practices and good habits
  . You'll learn them over time
• Maybe most common
• class Foo   int DEFAULT 5  private int
  value DEFAULT  public Foo(int value)    
  value value    this.value value    value
  this.value    public boolean isSameAs(int
  otherValue)     return value otherValue 
    public boolean isSameAs2(int otherValue)  
    return value otherValue    public void
  theProcedureIForgotToComplete()  
• System.out.println(new Foo(4).isSameAs(5)) //
  Prints "true"???
• System.out.println(new Foo(4).isSameAs2(10)) //
  Prints "true"???
• new Foo(4).theProcedureIForgotToComplete() //
  Won't work???

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Arrays

• 2D array
• "Student ID" and "Student Name" aren't actually a
  part of it, that's just what we're arbitrarily
  labeling the columns
• Student ID   Student Name1            Bob2    
         Sue3            Ann

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Arrays

• In Java, a 2D array is actually a 1D array of
  arrays
• int, array new int, 0, 2, 4, 1, 3, 5
• 0, 1 2  3 4  5
• 3D arrays are an array of arrays of arrays. Etc
• You can have "jagged" arrays in Javaint,
  array new int, 0, 0, 1, 2string
  months new string "January", "Feb"

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GUI and ActionListeners

• See project set option in main() to
  DisplayOptions.ShowForms and run

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Exceptions

• class Int   public static int Parse(string s)
      if (the string can't be turned into an int)
        throw new Exception("Invalid string") 
        return (the string as an int) 
• try   int i Int.Parse("5.5") catch
  (Exception e)   System.out.println(e.getMessage(
  ))

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Making Exceptions

• Just extend from the Throwable class
• class MeaningLessException extends Throwable  
  ...
• if (...)   throw new MeaningLessException(...)
  

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vs Error

• Errors represent something more critical,
  something "unrecoverable"

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Garbage collection

• PPT 3 Slide 16

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Questions?