Lecture Summary 2a

Control Flow: Sequence

• A sequence of statements are executed sequencially.
  S1; S2; ...; Sn;

  Example:

  temp = x*x + y*y;
  d = Math.sqrt(temp); x = 5;
  y = 11;
  z = x + y;
  System.out.println(z);
   

• A block consists of a sequence of statements or local variable declarations enclosed by { and }
  { S1; S2; ...; Sn; }

  Example:

  {
    double temp, d;
    temp = x*x + y*y;
    d = Math.sqrt(temp);
  }

Control Flow: Selection

• If statement: Execute a block of code (Action) if a condition is true. The block of code is executed at most once.
  

  if (condition) {
    statements
  }

  Example:

  if (x % 2 == 0) {
    System.out.println("The number is even.");
  }

  Example: Age.java
   

• If-else statement:
  

  if (condition) {
    statement1;
  } else {
    statement2;
  }

  if (condition1) {
    statement1;
  } else if (condition2) {
    statement2;
  } else if (condition3) {
    statement3;
  } else {
    statement4;
  }

  Example: Wages.java

  • Nested if Statements

    Example: MinOfThree.java         MinOfThree1.java   (Version avoiding nesting)
     

• Switch statement:

  switch (expression) {
  case value1:
    statement1;
    break;
  case value2:
    statement2;
    break;
  case value3:
    statement3;
    break;
  default:
    statement4;
  }

  Example: GradeReport.java)

• Conditional Operator
  The conditional operator is a ternary operator. The symbol is ? with the following syntax:
  (condition) ? exp1 : exp2 ;

  Example:   (x >= 0) ? x : -x ;
  
  The conditional expression returns one of the two values. Usualy we use the value returned by the expression in an assignment statement:
  

  Example:   int larger = (x > y) ? x : y ;

Control Flow: Repetition

• While loop: Repeat a block of code while a condition is true. There is no limit to the number of times that the block can be executed.

  while (condition) {
    statements;
  }

  When the body of a while statement, or of a for statement consists of a single statement, no braces are necessary.

  Example:

  int i = 1;
  while (i <= 20) {
    System.out.println(i);
    i++;
  }
   

• For loop:

  for (initialization; condition; incrementation) {
    statements;
  }

  Example:

  for (int i = 1; i <= 20; i++) {
    System.out.println(i);
  }
  
  

• Do-while loop:

  do {
    statements;
  } while (condition);         // Do NOT forget the semi-colon

  Example:

  int i = 1;
  do {
    System.out.println(i);
    i++;
  } while (i <= 20);
   

• Break and continue
  • A break statement in a loop terminates the loop.
  • A continue statement in a loop terminates the current iteration of the loop, and continue  with the next iteration of the loop.

  Example:

  int i = 0;
  while (i <= 20) {
    i++;
    if (i % 5 == 0) break;
    System.out.println(i);  //prints 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21,
                                          //one number per line
  }

  Example:

  int i = 0;
  while (i <= 20) {
    i++;
    if (i % 5 == 0) continue;
    System.out.println(i);    // print 1 2 3 4 , one number per line
  }

Static Methods

• Some of the methods of a class can be declared static.
• These methods can be called directly without instances of the class.

  
  ClassName.methodName(parameters, ...)

Using Keyboard Class

• Summary of Keyboard class
• The source code of the Keyboard class is on the CD-ROM.  You can also get it here [ Keyboard.java].
• Assume that your working directory is Homework , create a sub-directory named cs1 (the last character is 'one' not 'el'). Save Keyboard.java in the cs1 sub-directory. Put your own Java source files directly under Homework . Let's use Echo.java (p.86) in the textbook as an example. Save Echo.java under Homework. The directory structure should be something like this:
  
     Homework directory
       |
       +- Echo.java and other Java source files
       |
       +- cs1 subdirectory
           |
           +- Keyboard.java
   
  • Change directory to Homework
  • Compile: javac Echo.java
    Run: java Echo

If this does not work do the following:

• Install the cs1.jar file from your text CD/ROM into two locations on your computer. This file contains the Keyboard class originated by the authors of the text. The cs1.jar file is located in the Keyboard folder of the CD/ROM. Use Windows Explorer to copy it to these folders: c:\jdk1.3\jre\lib\ext (or whatever drive you have installed the JDK on) C:\Program Files\JavaSoft\JRE\1.3\lib\ext (MUST be here in c:!) Now try to compile and run the program Echo on page 86 of the text.

Other examples using the Keyboard class:   Average.java   Quadratic.java

Here are now the Echo and Quadratic classes without using Keyboard class   EchoStd.java   Quadratic1.java

Applets (cont.).

• More about paint method
• HelloFromVenus Applet

This applet displays the greeting message from Venus graphically. It consists of a text message and an image of the planet Venus. This program illustrates the structure of Java applets, and various graphics capabilities such as text display and font usage (size, style, and color).  

• Get the source file (HelloFromVenus.java)
• Get the Venus image (Venus.gif) 
• Compile:
javac HelloFromVenus.java
• Run the HelloFromVenus applet using the appletviewer:
appletviewer HelloFromVenusDemo.html
• Run the HelloFromVenus applet in your browser



• Applets using for-loop       Bullseye.java       Bullseye.html       Bullseye1.html