CSc2310 tutoring session, week 8 Fall, 2012

1. CSc2310 tutoring session, week 8Fall, 2012 • Class Members vs Instance Members Haidong(Haydon) Xue 5:30pm—8:30pm 11/27/2012 and 11/28/2012

2. CSc2310 Tutoring • Time: 5:30pm-8:30pm • Tutor: HaidongXue • Website: http://www.cs.gsu.edu/~hxue1/csc2310_Tutoring/ There are 2 sections: • Review • Class Members vs Instance Members 2. Q&A • Answer your questions about java programming

3. Class Members vs Instance Members Relations about “members”

4. Class Members vs Instance Members • Grammar • To define a “instance member” • Inside a class, define a variable or a method • To define a “class member” • Inside a class, put a static modifier before the data type, and define the variable or the method

5. Example: public class ClassVariableAndInstanceVariable{ private String instance_v; // each object has one private static intclass_v=0; // all objects share the same one public ClassVariableAndInstanceVariable(String s ){ instance_v = s; class_v++; } public void instance_method(){ System.out.println(instance_v+ "'s instance method is called"); System.out.println("There are " + class_v + " objects of this ClassVariableAndInstanceVariable"); } public static void class_method(){ System.out.println("There are " + class_v + " objects of this ClassVariableAndInstanceVariable"); } } Instance methods can access all types of members Static methods can only access static members

6. Class Members vs Instance Members • How to use static members (class members) and instance members? • Instance Members • From an object! • Class Members • From a class!

7. Example: public class ClassVariableAndInstanceVariableTester{ public static void main(String[] args){ ClassVariableAndInstanceVariableo1 = new ClassVariableAndInstanceVariable("o1"); ClassVariableAndInstanceVariable.class_method(); ClassVariableAndInstanceVariableo2 = new ClassVariableAndInstanceVariable("o2"); ClassVariableAndInstanceVariable.class_method(); ClassVariableAndInstanceVariableo3 = new ClassVariableAndInstanceVariable("o3"); ClassVariableAndInstanceVariable.class_method(); o1.instance_method(); o2.instance_method(); o3.instance_method(); } } Output: There are 1 objects of this ClassVariableAndInstanceVariable There are 2 objects of this ClassVariableAndInstanceVariable There are 3 objects of this ClassVariableAndInstanceVariable o1's instance method is called o2's instance method is called o3's instance method is called

8. An Examplein you class – CourseAverage2 publicclassDisplayText2{ privatestatic Graphics g; // Class variable publicstaticvoid main(String[] args){ // Create drawable frame DrawableFramedf = newDrawableFrame("Display Text"); df.show(); df.setSize(210, 85); // Obtain graphics context g= df.getGraphicsContext(); // Display "Monospaced Bold" displayFont(Color.red, "Monospaced", Font.BOLD, "Monospaced Bold", 25); // Display "SansSerif Italic" displayFont(Color.green, "SansSerif", Font.ITALIC, "SansSerif Italic", 50); // Display "Serif Plain" displayFont(Color.blue, "Serif", Font.PLAIN, "Serif Plain", 75); // Repaint frame df.repaint(); } privatestaticvoiddisplayFont(Color c, String fontName, intfontStyle, String message, int y){ g.setColor(c); g.setFont(newFont(fontName, fontStyle, 20)); g.drawString(message, 15, y); } } It illustrates how to use static members, but it is not a very good example since not too much advantage is obtained by doing so.

9. An Examplein you class – DisplayText2 Using static members to finish the task privatestaticdoublereadProgramScores() { doubleprogramTotal = 0.0; for (int i = 1; i <= NUM_PROGRAMS; i++) programTotal += readDouble("Enter Program " + i + " score: "); returnprogramTotal; } privatestaticdoublereadQuizScores() { doublequizTotal = 0.0; for (inti = 1; i <= NUM_QUIZZES; i++) quizTotal += readDouble("Enter Quiz " + i + " score: "); returnquizTotal; } privatestaticdoublereadDouble(String prompt) { SimpleIO.prompt(prompt); String userInput = SimpleIO.readLine(); returnConvert.toDouble(userInput); } // Constants privatestaticfinalintNUM_PROGRAMS = 8; privatestaticfinalintNUM_QUIZZES = 5; privatestaticfinalintMAX_PROG_SCORE = 20; privatestaticfinalintMAX_QUIZ_SCORE = 10; privatestaticfinaldoublePROGRAM_WEIGHT = .30; privatestaticfinaldoubleQUIZ_WEIGHT = .10; privatestaticfinaldoubleTEST_WEIGHT = .15; privatestaticfinaldoubleFINAL_EXAM_WEIGHT = .30;

10. Now you know how to use static members. However, here is the warning from Haydon: Minimize the number of them, i.e. use them only when there is really a huge advantage. The shared variables (all static variables are shared by all objects) will exponentially increase the time of debugging .

11. Please let me know your questions. I will be here till 8:30pm