Static?

1
Static?
2
Static

• Not dynamic
• class Widget
• static int s
• int d // dynamic
• // or instance
• // variable

3
Instantiation

• Widget w1 new Widget()
• Widget w2 new Widget()
• Widget w3 new Widget()
• There are now 4 variables
• w1.d
• w2.d
• w3.d
• Widget.s, w1.s, w2.s, w3.s are all the same!

4
class Widget int d static int s
public static void main(String args)
Widget w1 new Widget() Widget w2 new
Widget() Widget w3 new Widget()
w1.d 101 w2.d 202 w3.d 303
Widget.s 999
System.out.println("w1.d " w1.d " w1.s "
w1.s) System.out.println("w2.d "
w2.d " w2.s " w2.s)
System.out.println("w3.d " w3.d " w3.s "
w3.s)
w1.d 101 w1.s 999 w2.d 202 w2.s 999 w3.d
303 w3.s 999
5
Why

• Think of static variables as living in the class
• Dynamic variables live in the object
• BUT note that the objects can refer to the static
  variables with no problem

6
Note

• The dynamic or instance variables were all
  accessed using their reference.
• The static (sometimes called class) variables can
  be accessed using a reference or the class name

7
A Picture
w1
class Widget static int s int d
w2
w3
8
So why static methods?

• A method that only refers to static variables and
  which could be invoked using
• class.method()
• must be marked static

9
Perhaps an example?
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class Widget int serial static int
count public Widget() count serial
count public int getSerial() return
serial public int getCount() return
count public String toString() return
"Widget " getSerial() " of
" getCount() public static void
main(String args) Widget w1 new
Widget() Widget w2 new Widget() System.out
.println(w1) System.out.println(w2) System.o
ut.println(getCount())
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class Widget int serial static int
count public Widget() count serial
count public int getSerial() return
serial public int getCount() return
count public String toString() return
"Widget " getSerial() " of
" getCount() public static void
main(String args) Widget w1 new
Widget() Widget w2 new Widget() System.out
.println(w1) System.out.println(w2) System.o
ut.println(getCount())
Can't make static reference to method int
getCount() in class Widget.
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class Widget int serial static int
count public Widget() count serial
count public int getSerial() return
serial public static int getCount() return
count public String toString() return
"Widget " getSerial() " of
" getCount() public static void
main(String args) Widget w1 new
Widget() Widget w2 new Widget() System.out
.println(w1) System.out.println(w2) System.o
ut.println(getCount())
13
class Widget int serial static int
count public Widget() count serial
count public int getSerial() return
serial public static int getCount() return
count public String toString() return
"Widget " getSerial() " of
" getCount() public static void
main(String args) Widget w1 new
Widget() Widget w2 new Widget() System.out
.println(w1) System.out.println(w2) System.o
ut.println(getCount())
Why not Widget.getCount()
14
class Widget int serial static int
count public Widget() count serial
count public static int getSerial() return
serial public static int getCount() return
count public String toString() return
"Widget " getSerial() " of
" getCount() public static void
main(String args) Widget w1 new
Widget() Widget w2 new Widget() System.out
.println(w1) System.out.println(w2) System.o
ut.println(getCount())
Can't make a static reference to nonstatic
variable serial in class Widget.
15
Questions?
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Debugging Java Code
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Debugging

• Beginning programmers typically find just getting
  a program to compile a big challenge
• Error messages are often meaningless
• Error messages are often in the wrong place
• Error messages often suggest a course of action
  which is dead wrong
• Sometimes the excitement of getting the program
  to compile leads the programmer to forget that
  the program should also work.
• What can go wrong
• Wrong answer
• Illegal operation (exceptions)

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Strategies

• Write it right the first time. It isnt easier to
  find errors later!
• The compiler only finds language errors not logic
  errors.
• Read the error message carefully. Sometimes they
  contain useful information.
• A null pointer exception is not mysterious!
• Queue q
• q null
• q.enqueue(5)

Null Pointer Exception
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Strategies

• Let the computer help you find the errors
• In every class define a constant
• public final static boolean DEBUG true
• When things arent working add lines of code like
  this
• if(DEBUG)
• System.out.println
• (methodgt location variable variable)
• Keep adding these statements until the source of
  the problem is found
• Errors are normally caused by the computer doing
  what you told it to do!
• When the code works...change DEBUG to false
• Dont remove the if(DEBUG) statements

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The Main

• A typical Java program consists of a bunch of
  class files.
• An obvious question might be, How does it all
  start up?
• No matter how many classes are available, one of
  them must be sent to the JVM to start things off.
• The mechanism is simple. Any file sent to the JVM
  must have a public static method called main.
  Thats where the action starts

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The Main for Debugging

• Starting with main has an additional benefit.
• As each class is written it can have its own main
  sometimes known as a debugging or test main.
• This test main should test all the methods in the
  class paying special attention to special or
  limiting cases.
• Once testing is complete Leave the test main in
  the source file!
• It wont have any effect and may be beneficial if
  later modifications are made to the class.

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The Main for Debugging
class Driver ... public static void
main(String args) ...
class Stack ... public static void
main(String args) ...
class Menu ... public static void main(String
args) ...
class Widget ... public static void
main(String args) ...
class Blivet ... public static void
main(String args) ...
23
Debugging

• Write code in small chunks
• Compile and test frequently (whenever possible)
• Use the test main in every class!
• Lets create a linked list class that will hold
  Student Records.
• Well start by creating a StudentRecord class.

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StudentRecord

• class StudentRecord
• private String name
• private double gpa
• private int ssn
• public StudentRecord(String n, double g, int s)
  
• setName(n)
• setGpa(g)
• setSsn(s)
• public void setName(String n) name n
• public void setGpa(double g) gpa gpa
• public void setSsn(int s) ssn s
• public String getName() return name public
  double getGpa() return gpa
• public int getSsn() return ssn

// ppp omitted to save space!
25
toString or ! toString

• You may recall that System.out.println takes as a
  parameter a String.
• So why does it work if we say
• System.out.println(5)
• Java converts it for us!
• Java also has a similar behavior when the
  argument is a reference to an object
• If we type
• StudentRecord sr
• // Make one here...
• System.out.println(sr)
• We really get
• System.out.println(sr.toString())

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So the wise programmer...

• Always define a toString() method in every class.
• THE toString() METHOD DOES NOT PRINT ANYTHING
• it merely returns a String to its caller
• If you omit the toString() method, Java will use
  a default toString() which probably wont help
  you to debug.
• What should toString() look like?

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StudentRecord toString

• // Precon fields should be initialized
• // Purpose return string representation
• // Postcon no change to StudentRecord
• public String toString()
• String retVal
• retVal getName()
• retVal getGpa()
• retVal getSsn()
• return retVal

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Now the test main!

• // Purpose test main
• public static void main(String args)
• StudentRecord sr
• sr new StudentRecord(
• "George Burdell", 4.0,
  987654321)
• System.out.println(
• "Should be George Burdell 4.0
  987654321" sr)
• sr.setName("Bill Gates")
• sr.setSsn(123456789)
• sr.setGpa(0.3)
• System.out.println(
• "Should be Bill Gates 0.3 123456789"
  sr)
• // Studentrecord

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And the test!

• C\demogtjava StudentRecord
• Should be George Burdell 4.0 987654321 George
  Burdell 0.0 987654321
• Should be Bill Gates 0.3 123456789 Bill Gates
  0.0 123456789
• C\demogt

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What's wrong?

• class StudentRecord
• private String name
• private double gpa
• private int ssn
• public StudentRecord(String n, double g, int s)
  
• setName(n)
• setGpa(g)
• setSsn(s)
• public void setName(String n) name n
• public void setGpa(double g) gpa gpa
• public void setSsn(int s) ssn s
• public String getName() return name public
  double getGpa() return gpa
• public int getSsn() return ssn

// ppp omitted to save space!
31
Corrected

• class StudentRecord
• private String name
• private double gpa
• private int ssn
• public StudentRecord(String n, double g, int s)
  
• setName(n)
• setGpa(g)
• setSsn(s)
• public void setName(String n) name n
• public void setGpa(double g) gpa g
• public void setSsn(int s) ssn s
• public String getName() return name public
  double getGpa() return gpa
• public int getSsn() return ssn

// ppp omitted to save space!
32
Under the Hood

• A detail that we need to clarify
• In Pseudocode, we defined a record
• Now we define a class
• In Pseudocode, we could make record on the heap
• Now we can make an object on the heap
• In Pseudocode, the new operator returned a a
  value we stored in a pointer variable
• Now the new command will return a value that
  well store in a reference

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Example

• Assume we have a class Widget
• Widget w1
• Widget w2
• w1 new Widget()
• w2 w1
• w1 new Widget()
• w2 null

34
Example

• Assume we have a class Widget
• Widget w1
• Widget w2
• w1 new Widget()
• w2 w1
• w1 new Widget()
• w2 null

w1
35
Example

• Assume we have a class Widget
• Widget w1
• Widget w2
• w1 new Widget()
• w2 w1
• w1 new Widget()
• w2 null

w1
w2
36
Example

• Assume we have a class Widget
• Widget w1
• Widget w2
• w1 new Widget()
• w2 w1
• w1 new Widget()
• w2 null

w1
widget object (1)
w2
37
Example

• Assume we have a class Widget
• Widget w1
• Widget w2
• w1 new Widget()
• w2 w1
• w1 new Widget()
• w2 null

w1
widget object (1)
w2
38
Example

• Assume we have a class Widget
• Widget w1
• Widget w2
• w1 new Widget()
• w2 w1
• w1 new Widget()
• w2 null

widget object (2)
w1
widget object (1)
w2
39
Example

• Assume we have a class Widget
• Widget w1
• Widget w2
• w1 new Widget()
• w2 w1
• w1 new Widget()
• w2 null

widget object (2)
w1
w2
40
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