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

• 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.

 
• Conditional expression

condition ? exp1 : exp2

Example:

(x >= 0) ? x : -x
 

• 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

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.");
}
 

• If-else statement.

 

 
 
 

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

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

Example: Day2.java (compare to Day.java)
 

• Switch statement.

 

 
 
 

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

Example: Day3.java (compare to Day.java)
 

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);

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);
}

Example:

int i = 0;
while (i <= 20) {
  i++;
  if (i % 5 == 0) continue;
  System.out.println(i);
}

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