Lecture 5.pdf
- 1. Lecture 5 Wrappers and GUIs
- 3. Wrappers
- 4. Wrapper Classes ▪ Wrapper classes are a way of using primitive data types as objects. ▪ Each primitive data type will have a corresponding wrapper class. ▪ The wrapper class gives us a way to convert primitive data types into objects and convert objects into primitive data types.
- 5. Wrapper Classes Primitive DataType Wrapper Class byte Byte short Short int Integer long Long float Float double Double boolean Boolean char Character
- 6. Wrapper Classes ▪ Usage - we can use the wrapper classes in the same way that we can the primitive versions. Integer i = 1; Integer j = new Integer(5); Integer k; k = i + j; System.out.println(k); int i = 1; int j = 5; int k; k = i + j; System.out.println(k);
- 7. Wrapper Classes ▪ Usage - we can use the wrapper classes in the same way that we can the primitive versions. Boolean b = true; if (b) System.out.println("The Boolean object is true."); boolean b = true; if (b) System.out.println("The boolean variable is true.");
- 8. Wrapper Classes ▪ Usage - we can use the wrapper classes in the same way that we can the primitive versions. Double d; d = ‐1.5; d = Math.abs(d); double d; d = ‐1.5; d = Math.abs(d);
- 9. Wrapper Classes ▪ Usage - we can use the wrapper classes in the same way that we can the primitive versions. Character c1 = 'A'; System.out.println(c1); char c1 = 'A'; System.out.println(c1);
- 10. Wrapper Classes ▪ Because wrappers are objects, they have methods ▪ Example: These are some objects methods of the Integer class – Constructors: Integer(int i), Integer(String s) Integer j = new Integer(6); Integer j = new Integer("5");
- 11. Wrapper Classes ▪ Because wrappers are objects, they have methods ▪ Example: These are some objects methods of the Integer class – String toString() – converts the integer to a string of the same number String s = j.toString();
- 12. Wrapper Classes ▪ Because wrappers are objects, they have methods ▪ Example: These are some objects methods of the Integer class – double doubleValue(), long longValue() – converts Integer to the specified primitive type double d = j.doubleValue();
- 13. Wrapper Classes ▪ Because wrappers are objects, they have methods ▪ Example: These are some objects methods of the Integer class – Boolean equals(Integer) – returns true if this Integer is equal to the passed Integer. Integer i = new Integer(5); Integer j = new Integer(6); Boolean b = i.equals(j); //will be false
- 14. Wrapper Classes ▪ Because wrappers are objects, they have methods ▪ Example: Class methods of the Integer class – int max(int, int); int i = 5; int k = 6; int k = Integer.max(i,j);
- 15. Wrapper Classes ▪ Because wrappers are objects, they have methods ▪ Example: Class methods of the Integer class – int parseInt(String); String s = "4"; int l = Integer.parseInt(s);
- 16. Wrapper Classes ▪ The other classes, Float, Double, Boolean, have similar class methods and object methods.
- 17. Wrapper Classes ▪ Example: two different ways of reading numbers from user: Scanner scan = new Scanner(System.in); int i; System.out.println(“Enter a number: "); i = scan.nextInt(); Returns an int
- 18. Wrapper Classes ▪ Example: two different ways of reading numbers from user: Scanner scan = new Scanner(System.in); int i; System.out.println("Please enter a number: "); i = Integer.parseInt(scan.next()); Returns a String Returns an int
- 19. Wrapper Classes ▪ Example: two different ways of converting int to Integer : ▪ Explicitly - using the Integer.valueOf(i) class method ▪ Autoboxing – compiler implicitly uses the Integer.valueOf(i) class method int i = 5; Integer k = Integer.valueOf(i); int i = 5; Integer j = i; ▪ Converts explicitly ▪ Autoboxing
- 20. Wrapper Classes ▪ Example: two different ways of converting Integer to int : ▪ Explicitly - using the intValue() object method ▪ Unboxing - compiler implicitly uses the intValue() object method. Integer i = new Integer(5); int k = i.intValue(); Integer i = new Integer(5); int j = i; ▪ Converts explicitly ▪ Unboxing
- 21. Wrapper Classes Exercise: Write code that: ▪ creates an integer, ▪ then explicitly wraps it into an Integer, ▪ converts this Integer into a Double, ▪ then unwraps this Double to a double
- 22. Wrapper Classes int i = 5; Integer i2 = Integer.valueOf(i); Double d = Double.valueOf(i2.doubleValue()); double d2 = d.doubleValue();
- 23. Wrapper Classes Why do we want to go through all this hassle of using Wrapper classes? Java is always pass by value.
- 24. Wrapper Classes ▪ Recall that in C: – & operator returns the address of the variable. int main() { int i = 4; printf("%dn", i); printf("%pn", &i); system("PAUSE"); return 0; } 4 00F7F8AC Press any key to continue . . .
- 25. Wrapper Classes ▪ Recall that in C: – Can also create a variable through its pointer. int main() { int *i; i = malloc(sizeof(int)); *i = 5; printf("%pn", i); printf("%dn", *i); system("PAUSE"); return 0; } 012860B8 5 Press any key to continue . . .
- 26. Wrapper Classes ▪ However, when we pass objects by value, object identifiers are references themselves. ▪ So it passes references by their values, which is implicitly passing by reference. Why do we want to go through all this hassle of using Wrapper classes? Java is always pass by value.
- 27. Wrapper Classes ▪ Therefore if we want to pass int, float, double variables … by reference, we can’t… ▪ But we can create object wrappers (which are objects that do the same thing as the primitive variables)… ▪ …therefore can pass by reference. Primitive Non-primitive
- 28. GUIS
- 29. GUIS ▪ So far, our programs have looked like this… ▪ We want to write programs that have a more natural interface, rather than the text-only interface of the console.
- 30. GUIS ▪ The elements that make up a window interface are objects. – Frame objects – Button objects – TextField objects – Label objects – Menu objects – Checkbox objects – List objects – Radio objects
- 31. GUIS ▪ These objects are part of the javax.swing package, so this must be included. ▪ There are also bits and pieces that are from the java.awt package.
- 32. GUIS ▪ Two main types of windows: – General purpose frame – JFrame object – Special purpose frame – JDialog object
- 33. Simple GUI I/O with JOptionPane JFrame class – JFrame creates a simple window. ▪ Create a JFrame object. ▪ This object has methods that can be used to set its title, its size and visibility: JFrame jF = new JFrame(); jF.setTitle("GUIs are awesome!"); jF.setSize(400,300); jF.setVisible(true);
- 34. Simple GUI I/O with JOptionPane JFrame class – JFrame creates a simple window. ▪ Also a method to set its position (relative to the top left corner) jF.setLocation(200,250); 200 250
- 35. Simple GUI I/O with JOptionPane JFrame class – JFrame creates a simple window. ▪ We make sure that our program ends when the window is closed using the setDefaultCloseOperation() method ▪ It is passed a class constant: JFrame.EXIT_ON_CLOSE jF.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); Class constant of the JFrame class
- 36. Simple GUI I/O with JOptionPane JFrame class – JFrame creates a simple window. ▪ The main part of the window is called the content pane – it contains everything but the title, menu bars and border. ▪ In order to do stuff, e.g. add buttons to it, we need it as object. ▪ But we are not going to create a new object, we are going to get the object that is already there: Container CP = jF.getContentPane(); Container is a class, part of the java.awt package – ignore this for now *
- 37. Simple GUI I/O with JOptionPane JFrame class – JFrame creates a simple window. ▪ Now that we have the Container object, we can e.g. change its color: ▪ Color is a class in java.awt package and Color.BLUE is a class constant. Container CP = jF.getContentPane(); CP.setBackground(Color.BLUE);
- 38. Simple GUI I/O with JOptionPane JFrame class – JFrame creates a simple window. ▪ One of the main uses of constants is that they give a significant label that can be used, without having to worry about what it is ▪ Color is a class in java.awt package used to represent colours. It has class constants to represent common colours, like Color.BLUE Container CP = jF.getContentPane(); CP.setBackground(Color.BLUE);
- 39. Simple GUI I/O with JOptionPane JFrame class – JFrame creates a simple window. ▪ We can combine these two lines: ▪ Use the returned content pane directly Container CP = jF.getContentPane(); CP.setBackground(Color.BLUE); jF.getContentPane() Returns the content pane Returns the content pane .setBackground(Color.BLUE);
- 40. Simple GUI I/O with JOptionPane JOptionPane – allows for simply GUI-based input and output ▪ Creates special purpose frames ▪ showMessageDialog() – simple message. ▪ First argument specifies where it is centred. JOptionPane.showMessageDialog(jF, "Java is so much fun!"); JOptionPane.showMessageDialog(null, "Java is so much fun!");
- 43. Simple GUI I/O with JOptionPane JOptionPane – allows for simply GUI-based input and output ▪ Creates special purpose frames ▪ showInputDialog() – simple message with box for user to enter text. ▪ First argument specifies where it is centred. String s = JOptionPane.showInputDialog(jF, “Enter your age");
- 44. Simple GUI I/O with JOptionPane JOptionPane – allows for simply GUI-based input and output ▪ showInputDialog() always returns a String. ▪ If we want numerical data from the user, we must convert it – using wrapper classes String s = JOptionPane.showInputDialog(jF, “Enter your age?"); int i = Integer.parseInt(s);
- 45. Simple GUI I/O with JOptionPane