Static?. Static Not dynamic class Widget { static int s; int d; // dynamic // or instance // variable }

Static?

Static

• Not dynamic class Widget

{

static int s;

int d; // dynamic

// or instance

// variable

}

Instantiation

Widget w1 = 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!

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 = 999w2.d = 202 w2.s = 999w3.d = 303 w3.s = 999

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

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

A Picture

class Widget { static int s; int d;}

Widget Object static int s; int d;



w1

w2

w3

So why static methods?

• A method that only refers to static variables and which could be invoked using:

• class.method()

• must be marked static

Perhaps an example?

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.out.println(getCount());

}}



}public int getSerial() {return serial;}public int getCount() {return count;}public String toString() {




}}

Can't make static referenceto method int getCount() in class Widget.



}public int getSerial() {return serial;}public static int getCount() {return count;}public String toString() {




}}



}public int getSerial() {return serial;}public static int getCount() {return count;}public String toString() {




}} Why not Widget.getCount()



}public static int getSerial() {return serial;}public static int getCount() {return count;}public String toString() {




}}

Can't make a static reference to nonstatic variable serial in class Widget.

Questions?

Debugging Java Code

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)

Strategies

• Write it right the first time. It isn’t 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 PointerException

Strategies

• Let the computer help you find the errors• In every class define a constant public final static boolean DEBUG = true;• When things aren’t working add lines of code like this:

if(DEBUG) {System.out.println

(“method> 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• Don’t remove the if(DEBUG) statements

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. That’s where the action starts

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 won’t have any effect and may be beneficial if later modifications are made to the class.

The Main for Debuggingclass 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[]) { ... }}

Debugging

• Write code in small “chunks”

• Compile and test frequently (whenever possible)

• Use the test main in every class!

• Let’s create a linked list class that will hold Student Records.

• We’ll start by creating a StudentRecord class.

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; }

// pppomitted tosave space!


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 getSystem.out.println(sr.toString());

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 won’t help you to debug.

• What should toString() look like?

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;

}

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

And the test!

C:\demo>java 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:\demo>

What's wrong?



private double gpa;

private int ssn;


setName(n);

setGpa(g);

setSsn(s);

}


public void setGpa(double g) { gpa = gpa; }






Corrected



private double gpa;

private int ssn;


setName(n);

setGpa(g);

setSsn(s);

}


public void setGpa(double g) { gpa = g; }






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 we’ll store in a reference

Example

Assume we have a class Widget

Widget w1;

Widget w2;

w1 = new Widget();

w2 = w1;

w1 = new Widget();

w2 = null;

Example


Widget w1;

Widget w2;

w1 = new Widget();

w2 = w1;

w1 = new Widget();

w2 = null;

w1

Example


Widget w1;

Widget w2;

w1 = new Widget();

w2 = w1;

w1 = new Widget();

w2 = null;

w1

w2

Example


Widget w1;

Widget w2;

w1 = new Widget();

w2 = w1;

w1 = new Widget();

w2 = null;

w1

w2

widget object (1)

Example


Widget w1;

Widget w2;

w1 = new Widget();

w2 = w1;

w1 = new Widget();

w2 = null;

w1

w2

widget object (1)

Example


Widget w1;

Widget w2;

w1 = new Widget();

w2 = w1;

w1 = new Widget();

w2 = null;

w1

w2

widget object (1)

widget object (2)

Example


Widget w1;

Widget w2;

w1 = new Widget();

w2 = w1;

w1 = new Widget();

w2 = null;

w1

w2

widget object (2)

Documents

Static?. Static Not dynamic class Widget { static int s; int d; // dynamic // or instance // variable }