This document discusses using variables and constants in Java, including: - Declaring and initializing different variable types like int, float, and String - Assigning values to variables and constants - Using methods like getText() and setText() to work with text fields and text areas - Adding buttons and adding listeners to buttons to detect click events - Getting the system date using the Calendar class

1. Chapter 2 Using Variables and Constants

3. Applet Panel with Text Field, Text Area, and a Button

4. Constructor Methods Constructor method executes automatically

8. Java Primitive Data Types Unicode character—a code designed for internationalization of applications char code 0 single character chr char -128 to 127 0 small integers byt byte true or false false true or false bln boolean Possible Values Default Initial Value Contents Naming Prefix Data Type

9. Java Primitive Data Types Continued -32,768 to 32,767 0 short integer int short -9,223,372.036,854,775,808L to 9,223,372,036,854,775,807L 0 long integer lng long -2,147,483,648 to 2,147,483,647 0 integer data int int 7 digit precision 0.0 single precision floating point number flt float 15 digit precision 0.0 double precision floating point number dbl double

10. Valid and Invalid Variable Names Accepted by Java but is a terrible identifier, since the name is not meaningful. valid X No periods allowed. invalid dblBig.Number Only letters, letters, and underscore characters allowed. No special characters. invalid fltBig# Must begin with a letter or underscore. invalid 2Times valid blnIsFinished valid chrLetter valid int_Person_Count No spaces allowed in a name. invalid int Person Count Reason Valid or Invalid Identifier

14. Selected Java Data Type Wrapper Classes Integer Int Integer floating point numeric Flt Float True or False Bln Boolean character data str String Contents Naming Prefix DataType

15. Scope and Lifetime of Variables New copy each time the method is called. Until the method ends. Visible only inside the method where it is declared. Inside a method. Local One copy for each object instantiated from the class. As long as this specific instance of the class exists. Visible in all methods of the class. Inside a class but not within a method. Instance One copy for the class. As long as any instance of the class exists. Visible in all methods of the class. Inside a class but not within a method. Keyword static . Class Memory allocated Lifetime Scope (Visibility) Location of Declaration Type

16. The TextField Component—Constructor Formats TextField() TextField(int maximumNumberCharacters) TextField(String initialValue) TextField(String initialValue, int maximumNumberCharacters)

17. The TextField Component—Examples TextField txtName = new TextField(20); TextField txtQuantity = new TextField(5); TextField txtZipCode = new TextField("91789"); TextField txtZipCode = new TextField("91789",9);

18. The TextArea Component—Constructor Formats TextArea() TextArea(int numberRows, int maximumNumberCharacters) TextArea(String initialValue) TextArea(String initialValue, int numberRows, int maxNumberCharacters) TextArea(String initialValue, int numberRows, int maxNumberCharacters, int Scroll)

19. The TextArea Component—Examples TextArea txaInvoice = new TextArea(20,40); TextArea txaGrades = new TextArea(15,20); TextArea txaGrade = new TextArea("Name Average",15, 20); TextArea txaGrade = new TextArea("Name Average", 15, 20, TextArea.SCROLLBARS_BOTH)

20. Selected Methods of the TextField and TextArea Components Returns only the text that is selected in the text component. getSelectedText() Selects (highlights) the contents of the text component. selectAll() Adds text to the end of the contents, only available with TextArea. append() Assigns the value to the text component. setText(String value ) Returns the contents of the text component. getText() Purpose Method

21. The getText Method—General Format componentName .getText()

22. The getText Method —Example strName = txtName.getText();

23. The setText Method—General Format componentName.setText(string to be displayed)

24. The setText Method—Examples lblMessage.setText("Hello World"); txtGreeting.setText(strName); txtAnswer.setText("Hello " + txtName.getText()); txtName.setText(""); //Clear the text field

25. Concatenation “ +” means concatenation of two items

27. The append Method—General Format componentName .append( string );

28. The Append Method—Examples txaInvoice.append("Sold To: " + strName +""); txaInvoice.append("Address: " + strStreet + ""); txaInvoice.append(" City: " + strCity);

29. Use Unnamed Labels for Prompts Prompts

33. Partial List of Calendar Integer Constants Minute of hour MINUTE Hour (12 hour time) HOUR BC or AD ERA Starts with 0 for Sunday DAY_OF_WEEK Julian date DAY_OF_YEAR Day starting with 1 DAY_OF_MONTH Month starting with 0 MONTH 4 digit year YEAR Returns Constant

34. The Button Component—Constructor Formats Button() Button(String label );

35. The Button Component—Examples Button btnBlank = new Button() Button btnClear = new Button ("OK"); Button btnDisplay = new Button("Display");

42. The showStatus Method—General Format showStatus(String);

43. The showStatus Method—Examples showStatus("Click here to Calculate"); showStatus("Invalid entry");