Lecture Summary 2

More on expressions

• Relational expression.
  • Use equality operators or relational operators, which all return boolean results. Both operands must be of numerical types.
     

    ==	equal to
    !=	not equal to
    <	less than
    >	greater than
    <=	less than or equal to
    >=	greater than or equal to

  
  
• Postfix/prefix operators
   

  		value of x  before	value of  expression	value of x  after
  Postfix increment	x++	1	1	2
  Postfix decrement	x--	1	1	0
  Prefix increment	++x	1	2	2
  Prefix decrement	--x	1	0	0



• Logical expression.
   

  		a == T    b == T	a == T    b == F	a == F    b == T	a == F    b == F
  !a	logical negation	F	F	T	T
  a | b	logical-or	T	T	T	F
  a & b	logical-and	T	F	F	F
  a || b	conditional-or	T (*)	T (*)	T	F
  a && b	conditional-and	T	F	F (*)	F(*)

  
  * b not evaluated
  • For logical-and and logical-or, both operand are always evaluated.
  • For conditional-or, the second operand is evaluated only if the first operand evaluates to  false.
  • For conditional-and, the second operand is evaluated only if the first operand evaluates to true.
     

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++;
  }

  Example:

  int i = 20;
  while (i >= 0) {
    System.out.println(i);
    i--;
  }
   

• For loop:

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

  Example:

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

  Example:

  for (int i = 20; i >= 0; 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);

  Example:

  int i = 20;
  do {
    System.out.println(i);
    i--;
  } while (i >= 0);
   

• 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);  // print 1 2 3 4 , one number per line
  }

  Example:

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

Another example using the Keyboard class:  Average.java
Here is now an example of Echo class without using Keyboard class   EchoStd.java

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