2/5/00SEM107 © Kamin & Reddy Review -1 Class 19 - Review r This lecture contains a selection of slides from previous lectures, giving the “high points” of the material we have covered. r This selection is intended merely as an “outline” of the topics. All material covered in lecture may be included in the exam. (Exception: you will not be asked to draw a Hilbert curve.)

2/5/00SEM107 © Kamin & Reddy Review -2 Applications vs. applets r Applications: stand-alone programs, usually invoked from command line.

2/5/00SEM107 © Kamin & Reddy Review -3 Strings Strings have many instance methods provided in the Java API, e.g.  int length ()  int indexOf (String)  String substring(int, int)  String toUpperCase() r... lots more...

2/5/00SEM107 © Kamin & Reddy Review -4 Review  Class methods: call using class name and method name, e.g. Math.sin(...) In documentation, begins with word static.  Instance methods: call using value and method name, e.g. String s ; … s.substring(0,4) In documentation, does not begin with word static.

2/5/00SEM107 © Kamin & Reddy Review -5 Summary r Classes introduce types. Values with these types are called objects.  Create objects using new (exception: String ’s) r Classes have  Class methods: classname. methodname (arg’s)  Class variables: classname. variablename

2/5/00SEM107 © Kamin & Reddy Review -6 Summary (cont.) r Objects have: m Instance methods: expression. methodname (arg’s)  Instance variables: expression. variablename  Variables declared inside main (or other methods) are called local variables. These are different from class variables and instance variables, and are referred to simply by their names.

2/5/00SEM107 © Kamin & Reddy Review -7 Methods r Method = named collection of statements  Can be class method ( static ) or instance method m Has return type  Method call is statement if return type is void  Method call is expression if return type is not void

2/5/00SEM107 © Kamin & Reddy Review -8 Conditional statements A conditional statement has the form if ( condition ) statement1 else statement2 It executes statement1 if the condition is true, statement2 if the condition is false.

2/5/00SEM107 © Kamin & Reddy Review -9 Conditional expressions r A conditional expression has the form: condition ? expression1 : expression2 r It evaluates the condition and then either evaluates expression1 or expression2, depending whether condition is true or false r Note: the type of expression1 and expression2 must be the same; that type is the type of the conditional expression

2/5/00SEM107 © Kamin & Reddy Review -10 Recursive methods As of now, we lack the ability to do repetitive actions. Can solve them using recursive methods: A recursive method is one that calls itself. A recursive method is one that calls itself.

2/5/00SEM107 © Kamin & Reddy Review -11 Recursive methods in Java Recursive method f with argument x has the form static typename f ( typename x) { if (x is simple enough ) solve directly else use f( value “smaller than” x) to calculate f(x) }

2/5/00SEM107 © Kamin & Reddy Review -12 Thinking recursively Key ideas: m Know exactly what the method is supposed to do. m Assume method works for smaller values. Then just figure out how to use it to compute method for larger values. m Don’t forget base cases - small values that can be calculated directly.

2/5/00SEM107 © Kamin & Reddy Review -13 Lists Lists are a simple data structure in which data are stored in a row. We will talk first about lists of integers: x 0, x 1,..., x n-1 (n >= 0) Elements can be added and removed only at the beginning (x 0 ).

2/5/00SEM107 © Kamin & Reddy Review -14 Recursion on lists Writing recursive methods on lists follows same principle as for integers: m To compute f(L), assume f(L’) can be calculated for lists L’ smaller than L, and use f(L’) to calculate f(L). m Some lists are small enough for f to be calculated directly

2/5/00SEM107 © Kamin & Reddy Review -15 Lines We will provide a class called Line, with several operations:  Construct a line from (x0,y0) to (x1,y1) by: new Line(x0,y0,x1,y1)  Give line L, find coordinates of starting and ending points using instance methods x0(), y0(), x1(), y1().  Lines can be printed by System.out.println.

2/5/00SEM107 © Kamin & Reddy Review -16 Line lists The class LL has the same operations as IL, but contains operations on lists of Line objects:  LineList cons (Line ln, LineList L) - construct list containing i at front m nil

2/5/00SEM107 © Kamin & Reddy Review -17 Drawing lines at an angle To draw a line of length m at an angle theta from the x-axis:  m y = m sin  x = m cos  (But remember, this is upside-down...)

2/5/00SEM107 © Kamin & Reddy Review -18 Drawing “stars” (cont.) static LineList star1 (int order, int radius, double theta, double angleIncr) { if (order == 0) return LL.nil; else return LL.cons( new Line(0, 0, (int)Math.round(radius*Math.cos(theta)), (int)Math.round(radius*Math.sin(theta))), star1(order-1, radius, theta+angleIncr, angleIncr)); } (Detail: Math.cos and Math.sin take arguments in radians, instead of degrees.)

2/5/00SEM107 © Kamin & Reddy Review -19 Rotation Rotation is more complicated. Consider rotating one point around the origin by angle  : 1. Calculate m and  m =  x 2 +y 2  = tan -1 (y/x) 2.  =  +  3. (x’,y’) = point of length m, at angle  (x,y) (x’,y’)   

2/5/00SEM107 © Kamin & Reddy Review -20 Rotation (cont.) We’ll rotate shapes (i.e. LineList’s) about the origin by rotating each line: static LineList rotateShapeAboutOrigin (LineList L, double theta) { if (LL.empty(L)) return LL.nil; else return LL.cons(rotateLine(LL.hd(L), theta), rotateShapeAboutOrigin(LL.tl(L), theta)); }

2/5/00SEM107 © Kamin & Reddy Review -21 HTML r Web pages are text documents with special formatting commands, called tags. r The formatting language is called HTML. r Can view raw HTML by selecting “view source” in browser.

2/5/00SEM107 © Kamin & Reddy Review -22 The APPLET tag  Place all class files in a subdirectory, say appletcode. <APPLET CODE=“ appletname.class” CODEBASE=“appletcode” HEIGHT = 400 WIDTH = 300> mandatory stuff in here will be displayed if browser is not Java-equipped

2/5/00SEM107 © Kamin & Reddy Review -23 Writing applets The general form of an applet is: import java.applet.*; import java.awt.*; public class appletname extends Applet { public void init () {... } public void paint (Graphics g) {... }... other methods... } Usually present

2/5/00SEM107 © Kamin & Reddy Review -24 init and paint methods Most applets have these two methods (at least):  init initializes instance variables (see below).  paint performs graphics operations to draw shapes in the applet’s window r Called when applet is first displayed, and whenever it is re-displayed.  The Graphics objects is used to do the drawing

2/5/00SEM107 © Kamin & Reddy Review -25 Drawing operations The Graphics class defines numerous instance methods:  drawLine(int x0, int y0, int x1, int y1) - draw a line from (x0,y0) to (x1,y1)  drawRect(int x0, int y0, int width, int height) - draw a width x height box, with upper left corner at (x0,y0)  fillRect(int x0, int y0, int width, int height) - same as drawRect, but filled

2/5/00SEM107 © Kamin & Reddy Review -26 Example: drawing a LineList r When the paint method is called with a Graphics object, it can call other methods. The Graphics object can be used to draw in the applet window. r Draw a LineList in a Graphics object: static void drawLineList (LineList L, Graphics g) { if (!LL.empty(L)) { Line ln = LL.hd(L); g.drawLine(ln.x0(), ln.y0(), ln.x1(), ln.y1()); drawLineList(LL.tl(L), g); }

2/5/00SEM107 © Kamin & Reddy Review -27 Color  The Color class (in the java.awt package) has values that denote different colors.  Symbolic constants Color.black, Color.red, etc., are predefined.  Arbitrary colors can be created by giving RGB values in the range when constructing a Color value: m Color col1 = new Color(100,220,155); m Color darkred = new Color(100,50,50);

2/5/00SEM107 © Kamin & Reddy Review -28 Instance variables r Variables declared inside a method (the kind we’ve used so far) are either parameters or local variables.  Variables declared inside a class but outside of any method are either class variables or instance variables. (They are class variables if they are declared with the static keyword; we will consider only instance variables for now.)

2/5/00SEM107 © Kamin & Reddy Review -29 Components Some sample components:  Label: Place text in applet (alternative to drawString ): new Label(“A label”); m Button: Place button, possibly with label: new Button(); new Button(“Press me”); m TextField: Create field of given width to input text: new TextField(10);

2/5/00SEM107 © Kamin & Reddy Review -30 Placing GUI components in an applet  Normally, declare an instance variable of the component type ( Button, Checkbox, TextField, etc.)  In init m Initialize variable.  Use add method to place it in applet.  paint does not have to draw components - they are automatically drawn

2/5/00SEM107 © Kamin & Reddy Review -31 Example: A component-filled applet public class BusyApplet extends Applet { Label L = new Label(“A label”); Button b = new Button(); Button bl = new Button(“Press me”); TextField t = new TextField(10); CheckBox cb = new Checkbox(); public void init () { add(L); add(b); add(bl); add(t); add(cb); }

2/5/00SEM107 © Kamin & Reddy Review -32 Input in applets r Users enter text in TextField components r Reading the value in a TextField involves four changes to what we’ve seen: import java.awt.*; import java.applet.*; import java.awt.event.*; public class appletname extends Applet implements ActionListener { 1 2

2/5/00SEM107 © Kamin & Reddy Review -33 Input in applets (cont.) TextField t = new TextField(4); public void init () {... add(t); t.addActionListener(this); } public void actionPerformed (ActionEvent e) {... t.getText()... repaint(); } 3 4