In
any programming language decision structures are necessary.
Decision
structures allow your programs to make
decisions based upon user input or the results of a process.
Java contains if/else and switch structures, as in C++.
They follow the general principles of logic, and in addition to
employing relational and mathematical operators, they
also employ logical operators. Logical operators allow more than one
relational expression to be evaluated at a time,
allowing for a greater complexity of decisions as bits can be "AND'd" and "OR'd"
together :)
if statement - enable
testing for a condition and branching to different parts of code depending
on the result. The syntax is as following:
if(expression)
statement;
else clause
- makes code more easily readable, program follows one branch of code if the
condition is true and another branch of code if it is false. Syntax is as
follows:
if(expression)
statement;
else
statement;
Example 1:
import
javax.swing.*;
public class
DecideStuff
{
public
void main(String[] args)
{
int FirstNumber = 0;
int SecondNumber = 0;
FirstNumber =
Integer.parseInt
(JOptionPane.showInputDialog
(null,
"Enter a larger number:"));
SecondNumber =
Integer.parseInt
(JOptionPane.showInputDialog
(null,
"Enter a smaller number:"));
if(firstNumber > secondNumber)
{
JOptionPane.showMessageDialog
(null,
"The 1st number is greater.");
}
else
{
JOptionPane.showMessageDialog
(null,
"The 2nd number is greater.");
}
System.exit(0);
}//close main() function
}//close DecideStuff class |
Notice: "JOptionPane.showInputDialog"
returns a string and may be used to ask a question and take input from the
user. "JOptionPane.showMessageDialog" may be used for output, it does not
take input. Braces may be used to clarify a decision
structure, and are imperative if there is more than one line of code beneath
an if or an else.
Output =
Please
enter a big number: 10
Please enter a smaller number: 12
Oops. The second is bigger!
Example 2:
import
javax.swing.*;
import
java.io.*;
public
class
DecideStuff
{
public
void main(String[] args)
{
int FirstNumber = 0;
int SecondNumber =
0;
String
choice = "";
LineNumberReader cin = new
LineNumberReader
(new
InputStreamReader(System.in));
System.out.print("\nEnter a larger number: ");
try { choice = cin.readLine();
}
catch(IOException e) {
System.err.println("Error.");
FirstNumber =
Integer.parseInt(choice);
System.out.print("\nEnter a smaller number: ");
try { choice = cin.readLine();
}
catch(IOException e) {
System.err.println("Error.");
SecondNumber =
Integer.parseInt(choice);
if(firstNumber >= secondNumber)
{
System.out.println("The 1st number is greater.");
if(firstNumber
== secondNumber)
{
System.out.println("\nThey
are equal.");
}
else
{
System.out.println("\nThey
are not equal.");
}
}
else
{
System.out.println("The 2nd number is greater.");
}
System.exit(0);
}//close main() function
}//close DecideStuff class |
Notice: The
console is used for input and output, not JOptionPanes. To do this, we had
to create a LineReader object. This object
must be placed in try/catch blocks where input is received. The input
received from the LineNumberReader object, like the JOptionPane objects
above, is a string. Therefore, we must also pass the value to "Integer.parseInt()". We use "System.out.print"
and "System.out.println" to display output to the console. In
addition, our if/else structures are nested.
switch(case) statement - A switch
statement is a decision structure possessing a
default and several cases.
Switch
structures allow
you to branch off on any of a number of different
values. "if" and "else...if" combinations can
become confusing when nested too deeply.
The
switch statement evaluates "expression" and compares
the result to each of the case values. The
evaluation is ONLY for equality. Relational
operators (<>) and Boolean expressions can not be
used here. If one of the case values matches the
expression, execution jumps to those statements and
continues to the end of the switch block unless a
break statement is encountered. If nothing at all
matches, execution branches to the "default"
statement. If there is no default statement and
there is no matching value, execution falls through
the switch statement and the statement ends. You
should always include a default case in switch
statements, if only to render an error message.
Note: If
there is no break statement at the end of a case
statement, execution will fall through to the next
case. This is sometimes necessary or what you
want as in the MFC, but many times it is
an error. If you do decide to let execution fall
through, you might consider placing a comment
indicating your intention and that you simply didn't
just forget the "break" statement.
Example:
public
class
DecideStuff
{
public
void main(String[] args)
{
int Number = 0;
String
choice = "";
LineNumberReader cin = new
LineNumberReader
(new
InputStreamReader(System.in));
System.out.print("\nEnter a smaller number: ");
try { choice = cin.readLine();
}
catch(IOException e) {
System.err.println("Error.");
Number =
Integer.parseInt(choice);
switch(Number)
{
case 0
: RunProgram = 0;
break;
case 1
: cout << "You chose 1";
break;
case 2
:
cout << "You chose 2";
break;
case
3 :
cout << "You chose 3";
break;
case 4
:
cout << "You chose 4";
break;
case 5
:
cout << "You chose 5";
break;
case 6
:
cout << "You chose 6";
break;
case 7
:
cout << "You chose 7";
break;
default
: cout << "Sorry, invalid choice.";
break;
}
//close switch statement
System.exit(0);
}//close main() function
}//close DecideStuff class |
Notice: The
switch statement is easier to follow than a bunch of nested if/else
statements. Still, there are some situations that demand if/else statements.
You can not switch on a string for each case, but you may compare string
with "equals" and if/else. In a similar fashion, you can not use complex
operational expressions and comparisons with switch statements the way
you can with if/else. You may pass a char into a switch statement also:
public
class
DecideStuff
{
public
void main(String[] args)
{
char Option = 0;
String
choice = "";
LineNumberReader cin = new
LineNumberReader
(new
InputStreamReader(System.in));
System.out.print("\nEnter a smaller number: ");
try { choice = cin.readLine();
}
catch(IOException e) {
System.err.println("Error.");
Option =
choice.charAt(0);
switch(Option)
{
case
'a'
: RunProgram = 0;
break;
case
'b'
: cout << "You chose b";
break;
case
'c'
: cout << "You chose c";
break;
case
'd' : cout << "You chose
d";
break;
case
'e'
: cout << "You chose f";
break;
case
'f'
: cout << "You chose f";
break;
case
'g'
: cout << "You chose g";
break;
case
'h'
: cout << "You chose h";
break;
default
: cout << "Sorry, invalid choice.";
break;
}
//close switch statement
System.exit(0);
}//close main() function
}//close DecideStuff class |
Notice: The
LineNumberReader object returns a string from the user, so we must use a new
function, "charAt()", to grab the
very 1st character of the string. This char can be passed to the switch and
each case evaluated (with single quotes, not double).
Alternatively for a char, there is a "System.in.read()"
function.
|
Operator |
Symbol |
Example |
|
AND |
&& |
expression1
&&
expression2 |
|
OR |
|| |
expression1 ||
expression2 |
|
NOT
|
! |
!expression |
logical AND = Evaluates two
expressions. If both expressions are true the AND statement is true. If
one or both expressions are false the AND statement is false. Example:
if ( (x ==
5) && (y == 5))
would evaluate true if both x and y are 5.
logical OR =
If either expression is true, the OR statement is true. Example:
if ( (x ==
5) || (y == 5)) would
evaluate true if either x OR y is equal to 5.
logical NOT= Evaluates true if
the expression being tested is FALSE. If the expression being tested is
false, the value of the test is true:
if ( !(x
== 5) ) is true only if x
is not equal to 5. You could also write:
if (x != 5).
Relational
Precedence - Relational operators and logical
operators each return 1 (true) or 0 (false). They
have a precedence order that determines which
relations are evaluated first. Example:
if( x > 5 && y >
5 || z > 5)
is better
rewritten with parentheses:
if( (x > 5) &&
(y > 5 || z > 5) )
More about truth
and falsehood:
if (x)
x=0;
Can be read as
"If x is true (has a non-zero value), set it to 0".
It would be clearer written as:
if (x != 0)
x = 0;
So these two
statements are also equivalent;
if (!x)
//if x is
false (zero)
if (x == 0)
//if
x is zero
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