Java Decision Control

Decision Control
Structures
Decision control structures are Java statements that
allows us to select and execute specific blocks of code
while skipping other sections.
There are two types of decision control statements in
Java. They are:
 if statements
 switch statements

The if Statement:
An if statement consists of a Boolean expression
followed by one or more statements.
Syntax:
The syntax of an if statement is:
if(Boolean_expression)
{
//Statements will execute if the Boolean expression
is true
}

If the Boolean expression evaluates to true then the
block of code inside the if statement will be executed. If
not the first set of code after the end of the if statement
(after the closing curly brace) will be executed.
Example:
public class Test {
public static void main(String args[]){
int x = 10;
if( x < 20 ){
System.out.print("This is if statement");
}
}
}
This would produce the following result:
This is if statement

An if statement can be followed by an optional
else statement, which executes when the
Boolean expression is false.
Syntax:
The syntax of an if...else is:
if(Boolean_expression){
//Executes when the Boolean expression is true
}else{
//Executes when the Boolean expression is false
}
The if...else Statement:

public class Test {
int x = 30;
if( x < 20 ){
System.out.print("This is if statement");
}else{
System.out.print("This is else statement");
}
}
}
This is else statement
Example:

An if statement can be followed by an
optional else if...else statement, which is very
useful to test various conditions using single
if...else if statement. When using if , else if , else
statements there are few points to keep in mind.
An if can have zero or one else's and it must come
after any else if's. An if can have zero to many else
if's and they must come before the else. Once an
else if succeeds, none of the remaining else if's or
else's will be tested.
The if...else if...else Statement:

The syntax of an if...else is:
if(Boolean_expression 1){
//Executes when the Boolean expression 1 is true
}else if(Boolean_expression 2){
}else if(Boolean_expression 3){
}else {
//Executes when the none of the above condition is
true.
}
Syntax:

public class Test {
int x = 30;
if( x == 10 ){
System.out.print("Value of X is 10");
}else if( x == 20 ){
}else if( x == 30 ){
}else{
System.out.print("This is else statement");
}
}
}
Value of X is 30
Example:

A switch statement allows a variable to be
tested for equality against a list of values. Each
value is called a case, and the variable being
switched on is checked for each case.
The switch Statement:

The syntax of enhanced for loop is:
switch(expression){
case value :
//Statements
break; //optional
case value :
//Statements
break; //optional
//You can have any number of case statements.
default : //Optional
//Statements
}
Syntax:

 The variable used in a switch statement can only be a byte, short,
int, or char. You can have any number of case statements within a
switch. Each case is followed by the value to be compared to and a
colon.
 The value for a case must be the same data type as the variable in
the switch and it must be a constant or a literal. When the variable
being switched on is equal to a case, the statements following that
case will execute until a break statement is reached.
 When a break statement is reached, the switch terminates, and the
flow of control jumps to the next line following the switch
statement. Not every case needs to contain a break. If no break
appears, the flow of control will fall through to subsequent cases
until a break is reached. A switch statement can have an optional
default case, which must appear at the end of the switch.
 The default case can be used for performing a task when none of
the cases is true. No break is needed in the default case.
The following rules apply to a switch
statement:

public class Test {
//char grade = args[0].charAt(0);
char grade = 'C';
switch(grade)
{
case 'A' :
System.out.println("Excellent!");
break;
case 'B' :
case 'C' :
System.out.println("Well done");
break;
case 'D' :
System.out.println("You passed");
case 'F' :
System.out.println("Better try again");
break;
default :
System.out.println("Invalid grade");
}
System.out.println("Your grade is " + grade);
}
}
Example
Compile and run above program using various command line arguments. This would produce the following result:
Well done
Your grade is a C

• Write Java program declare age variable. Then the
program will show if the person is eligible to vote. A
person who is eligible to vote must be older than or
equal to 18 years old.
• If age greater than or equal 18 It will print “You are
eligible to vote” and “You are not eligible to vote” if
it is less than 18.
Exercise 1

• CheckPassFail (if-else):
• Write a program called CheckPassFail which prints
"PASS" if the int variable "mark" is more than or equal
to 75; or prints "FAIL" otherwise. The program shall
always print “DONE” before exiting.
• 74 below Fail
• 75 -78 Fair
• 79-80 Satisfactory
• 81-85 Good
• 86-90 Very Good
• 91-95 Excellent
Exercise 2

• Write a Java program to determine whether an
variable is an even number.
• Hint: Use % to determine whether it is an even
number.
Exercise 3

Java Decision Control

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Java Decision Control