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Chapter 3
Flow of Control
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All rights reserved.
Flow of Control
As in most programming languages, flow of
control in Java refers to its branching and looping
mechanisms
Flow of
Control
Branching
Switch case
if
Looping
If… else
For
while
Do.. while
Control Structures
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Branching Statements
– Simple if statement
– if-else statement
– Nested if statements
– switch statement
4
5
Java Comparison Operators
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3-6
Simple if Statement
Syntax:
if (expression)
statement ;
if (hour < 12)
System.out.println("Good morning.");
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Example: Simple if Statement
import java.util.Scanner;
public class Result{
public static void main(String[] args){
Scanner keyboard=new Scanner(System.in);
int grade= keyboard.nextInt();
if ( grade > = 60 )
System.out.println ( " passed " );
}
}
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Example: Simple if Statement
Write a Java program prints the absolute value of
a number.
import java.util.Scanner;
class absolute {
public static void main(String [] args) {
Scanner kb = new Scanner(System.in);
System.out.print(" Enter a number: ");
int x = kb.nextInt();
int y = x;
if( y < 0)
y = -y;
System.out.print(" The absolute value of " + x + " is " + y);
}
}
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Branching with an if-else Statement
Two-Way Selection
Syntax:
if (expression)
statement1;
else
statement2;
The Expression must be enclosed in parentheses
If the Expression is true, then the Statement1 is executed
If the Expression is false, then the Statement2 is executed
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Example: Even/Odd number
import java.util.Scanner;
class EvenOdd{
public static void main(String [] args) {
Scanner kb = new Scanner(System.in);
System.out.print(" Enter a number: ");
int num= kb.nextInt();
if(num %2 = = 0)
System.out.println ( " The Number is Even" );
else
System.out.println ( "The Number is Odd" );
}
}
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if ( grade > = 60 )
System.out.println (" passed " );
else
System.out.println (" failed " );
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Compound (Block of) Statements
Syntax:
{
statement1
statement2
.
.
.
statementn
}
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Compound Statements
•
•
You have to use braces if the <then> or <else> block has multiple statements.
if only one statement is there braces are optional but it is advisable to always use
them to enhance readability
if (testScore < 60)
{
System.out.println("You did not pass");
System.out.println("Try harder next time");
Then Block
}
else
{
System.out.println("You did pass");
System.out.println("Keep up the good work");
}
Else Block
Multiway if-else Statement
...
if (Boolean_Expression)
Statement_1
else if (Boolean_Expression)
Statement_2
else if (Boolean_Expression_n)
Statement_n
else
Statement_For_All_Other_Possibilities
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if - else- if
if (score >= 90) {
Test Score
System.out.println("Grade is A"); 90  score
} else if (score >= 80) {
80  score  90
System.out.println("Grade is B"); 70  score  80
} else if (score >= 70) {
System.out.println("Grade is C");
} else if (score >= 60) {
System.out.println("Grade is D");
} else {
System.out.println("Grade is F");
}
60  score  70
score  60
Grade
A
B
C
D
F
The switch Statement
• The switch statement is multiway branching
– When a switch statement is evaluated, one of a number
of different branches is executed
– The choice of which branch to execute is determined by a
controlling expression enclosed in parentheses after the
keyword switch
• The controlling expression must evaluate to a char, int, short,
or byte
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The switch Statement
• Each branch statement in a switch statement starts with the
reserved word case, followed by a constant called a case
label, followed by a colon, and then a sequence of statements
– Each case label must be of the same type as the controlling expression
– Each sequence of statements may be followed by a break statement
( break;)
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switch Structures
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19
switch (expression)
{
case value 1: statements 1;
break;
case value 2: statements 2;
break;
...
case value n: statements n ;
break;
default: statements
}
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switch With No break Statements
public class SwitchNoBreak {
public static void main( String s[] ) {
char ch = 'a';
int x = 0;
ch ==
'a' ?
switch ( ch ) {
false
case 'a':
x = 10;
ch ==
case 'b':
'b' ?
x = 20;
case 'c':
false
x = 30;
ch ==
}
'c' ?
System.out.println( "x = " + x );
}
true
x = 10;
true
x = 20;
true
x = 30;
false
}
x = 30
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switch With break Statements
public class SwitchWithBreak {
public static void main(String s[]) {
char ch = 'a';
int x = 0;
switch ( ch ) {
case 'a':
x = 10;
break;
case 'b':
x = 20;
break;
case 'c':
x = 30;
break;
}
System.out.println( "x = " + x
ch ==
'a' ?
false
ch ==
'b' ?
true
x = 10;
break;
true
x = 20;
false
ch ==
'c' ?
false
break;
true
x = 30;
break;
);
}
}
x = 10
‫احمد يوسف‬/‫د‬
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import java.util.Scanner;
class Arithmatic{
public static void main(String [ ] args){
Scanner keyboard=new Scanner(System.in);
System.out.print("Enter First number"); int x= keyboard.nextInt();
System.out.print("Enter Second number“); int y= keyboard.nextInt();
System.out.print("Enter 1:Add\n 2:Sub\n 3:Mul.\n 4: Div.“);
int menu = keyboard.nextInt();
switch (menu)
{
case 1 : System.out.println(“x+y = " + (x+y) ); break ;
case 2 : System.out.println(“ x – y =" + (x - y)) ; break ;
case 3 : System.out.println("x * y ="+ (x * y)) ; break ;
case 4 : System.out.println("x / y ="+ (x / y)) ;
break ;
}
}
}
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The Conditional Operator
• The conditional operator is a notational variant on certain forms of the
if-else statement
– Also called the ternary operator or arithmetic if
– The following examples are equivalent:
if (n1 > n2)
max = n1;
else
max = n2;
vs.
max = (n1 > n2) ? n1 : n2;
– If the Boolean expression is true, then the expression evaluates to the value of
the first expression (n1), otherwise it evaluates to the value of the second
expression (n2)
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Pitfall: Using == with Strings
• The equality comparison operator (==) can correctly test two
values of a primitive type
• In order to test two strings to see if they have equal values,
use the method equals, or equalsIgnoreCase
string1.equals(string2)
string1.equalsIgnoreCase(string2)
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Building Boolean Expressions
• When two Boolean expressions are combined using the "and" (&&)
operator, the entire expression is true provided both expressions are true
– Otherwise the expression is false
• When two Boolean expressions are combined using the "or" (||)
operator, the entire expression is true as long as one of the expressions is
true
– The expression is false only if both expressions are false
• Any Boolean expression can be negated using the ! operator
– Place the expression in parentheses and place the ! operator in front of it
• Unlike mathematical notation, strings of inequalities must be joined by &&
– Use (min < result) && (result < max) rather than
min < result < max
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Truth Tables
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