3.1 Introduction

Control structures are used for repetition, which permit Java programs to perform actions repeatedly. In this chapter you will learn about the control statements, and you’ll be motivated for the need of repetition. You will also write and run a few programs based on each of  the control statements.

3.2 The control statements

Programming languages uses control statements to cause the flow of execution to advance and branch based on changes to the state of the program. Java’s program control statements are:

• Selection statements: if, if-else-if, switch
• Iteration: while, do-while, for
• Jump: break, continue and return

Let us understand each of them.

3.2.1 if statement

The general form of if statement is:

if(condition) statement;

If the condition is evaluated to true then the enclosed statement is to be executed otherwise not. The control goes to next instruction following the if statement.

Consider the following code fragment:

int i=12;
if(i>10) System.out.println("i >10 is true");  //output will be i >10 is true

We can also add multiple statements  in { } when a condition evaluates to true as below.

int i=2;

if (i<=2){

i=i-1;

System.out.println(i);  //prints 1

}

3.2.2 if-else statement

The simple if statement does not  specify what is to be done if condition evaluates to false. The if-else statement specifies what is to be done when the condition evaluates to both true and false.

The general form is:

if(condition)
{
statements
}
else {
statements
}

Program. 

if(i<10)
{
system.out.println("i <10);
}
else{
system.out.println(“i >10);
}

3.2.3 if-else if-else

If we have multiple conditions , the we use if-else if-else. The general form of if-else if-else is:

if(condition)
{
Statements block1
}
else if{
statements block2
}
else{
statements block3
}

Program 3.1.   Program to demonstrate conditions. Print the day of the week using if-else if-else

// conditionsDemo.java
// click here for code


public class conditionsDemo
{
public static void main (String args[])
{
 int i;
 i = 4; // i is the week day number
 if (i == 0)
 {
  System.out.println ("it is Sunday");
 }
else if (i == 1)
{
 System.out.println ("it is Monday");
}
else if (i == 2)
{
 System.out.println ("it is Tuesday");
}
else if (i == 3)
{
 System.out.println ("it is Wednesday");
}
else if (i == 4)
{
 System.out.println ("it is Thursday");
}
else if (i == 5)
{
 System.out.println ("it is Friday");
}
else if (i == 6)
{
 System.out.println ("it is Saturday");
}
}
}
//Output:
//it is Thursday

3.2.4 Nested if

Nested if’s are allowed in Java. Nested if means an if statement within an if statement.

3.2.5 Complex conditions in if

We can also have complex conditions in if and else. The following program demonstrates the use of complex conditions.

Program 3.2.  Program to print season for given month using else-if.

//seasonDemo.java
//click here for code
class seasonDemo
{
  public static void main (String args[])
  {
    int month = 5;
    String season;
    if (month == 12 || month == 1 || month == 2)
      season = "winter";
    else if (month == 3 || month == 4 || month == 5)
      season = "spring";
    else if (month == 6 || month == 7 || month == 8)
      season = "summer";
    else if (month == 9 || month == 10 || month == 11)
      season = "autumn";
    else
       season = "invalid month";
     System.out.println ("the season is " + season);
  }
 }

 //output
 //the season is spring

3.2.4 Conditional operator

The conditional/ternary operator of C is valid in Java too.

The general form is:

Expression ? true-result : false-result

If expression evaluates to true the statement  true-result gets evaluated otherwise false-result gets evaluated.

Program 3.3. Using the ternary operator

//ternaryDemo.java
//click here for code
public class ternaryDemo
{
public static void main (String[] args)
{
int i, result1, result2;
i = 1;
result1 = (i == 1) ? 1 : 0;  // evaluates to true
System.out.println ("result is: " + result1);
result2 = (i == 2) ? 1 : 0;    //evaluates to false
System.out.println ("result is: " + result2);
}
}

// output: 
//result is: 1
//result is: 0

3.2.5 The Switch statement

This is a multi way branch statement.

The general form is:

Switch(expression){

       case value 1;

       //statements

       break;

       case value 2;

       //statements

       break;

       // remaining case statements

       case value n;

       //statements

       break;

       default:

       //default sequence

   }

If break is missing, the next case is also evaluated. If break is present the control comes out of the switch block.

Program 3.4 Repeat conditionsDemo.java with switch statement

// condition-switch.java
//Program to print day of the week using switch statement
//click here for code
public class condition-switch
{
public static void main(String args[])
{
int i;
i=4;
switch(i)
{
case 0:
System.out.println("it is sunday");
break;
case 1:
System.out.println("it is Monday");
break;
case 2:
System.out.println("it is Tuesday");
break;
case 3:
System.out.println("it is Wednesday");
break;
case 4:
System.out.println("it is Thursday");
break;
case 5:
System.out.println("it is Friday");
break;
case 6:
System.out.println("it is Saturday");
break;
default:
System.out.println("invalid");
break;
}
}
}
//output
it is Thursday

Program 3.5  Demonstrating switch statement for printing the season.

// season-switch.java

// click here for code

class season-switch
{
public static void main(String args[])
{
int month=5;
String season;
switch(month)
{
case 12:
case 1:
case 2:
season="winter";
break;
case 3:
case 4:
case 5:
season="winter";
break;
case 6:
case 7:
case 8:
season="winter";
break;
case 9:
case 10:
case 11:
season="winter";
break;
default:
season="invalid month";
}
System.out.println("the season is "+season);
}
}
/* Output:
the season is winter
*/

3.2.6 while loop

The while loop helps in executing a set of statements as long as the condition is true.

The general form of while loop is

while(condition){

//body of loop

}

The body of the loop is executed as long as the conditional expression is true. When condition becomes false, control passes to the next line of code immediately following the loop.

Program 3.6. Program to add ‘n’ numbers using while

// add-while.java
// click here for code
class add-while
{
public static void main(String args[])
{
 int n=10,sum=0;
 while(n>0)
 { // this loop executes until n becomes equal to 0
  sum=sum+n;
  n--;
 }
  System.out.println("sum="+sum);
 }
}
/*Output:
sum=55
*/

3.2.7 do-while loop

The do-while loop is slightly different from while loop. It executes the body of the loop at least once and then checks the condition.

General form of do-while loop is

do{

//body of the loop
}

while(condition);

Program 3.7: To add first ‘n’ numbers using do-while

// add-do-while.java

// click here for code

class add-do-while
{
public static void main(String args[])
{
int n=10,sum=0,i=0;
do
{
sum=sum+i;
i++;
}

while(i<=n); // condition is checked after the body of the loop is executed
System.out.println("sum="+=sum);
}
}
/* Output:
sum=55
*/

3.2.8 for loop

The statements in the body of the for loop are executed repeatedly until the condition is true on a given variable which is initialised and is modified as per the expression given in iteration.

The general form is

for(initialization; condition; iteration) 
{
//body
}

In the for loop, initialization is done first before the control gets into the body of the for loop. Next the condition is evaluated and if it is true then the statements in the body of the loop are executed. After completing the execution of all statements in the body of the for loop, the expression in iteration is evaluated. The above steps of checking condition and iteration expression evaluation are done repeatedly until the condition becomes false.

 

//factorial.java
//click here for code
class factorial
{
 public static void main(String args[])
 {
 int n=5, fact=1,i;
 for (i=1;i<=5;i++)
   fact=fact*i;  // this statement is iterated as long as i<=5
 System.out.println("fact="+fact);
 }
}

/* Output:
fact=120
*/

The usage of for loop is similar to its usage in C or C++. Nesting of for loops is allowed in Java.

Program 3.9 Program to list all ASCII characters

//asciiChars.java
//click here for code
public class asciiChars
{
public static void main(String args[])
{
for(char c=0; c<128; c++)

if(c!=26)
System.out.println("vale:"+(int)c+"character:"+c););

}
}
/*
Output:
....
value:33 character:!
value:34 character:"
value:35 character:#
value:36 character:$
value:37 character:%
value:38 character:&
value:39 character:'
value:40 character:(
value:41 character:)
value:42 character:*
value:43 character:+
value:44 character:,
value:45 character:-
value:46 character:.
value:47 character:/
value:48 character:0
value:49 character:1
value:50 character:2
value:51 character:3
value:52 character:4
value:53 character:5
value:54 character:6
value:55 character:7
value:56 character:8
value:57 character:9
value:58 character::
value:59 character:;
value:60 character:<
value:61 character:=
value:62 character:>
value:63 character:?
value:64 character:@
value:65 character:A
.....
*/

3.2.9 Simple break statement

The break statement makes the control exit from that particular loop where it is executed.

Program 3.10 Find whether a given alphabet is vowel or consonant

//vowelsConsonants.java
//click here for code
public class vowelsConsonants
{
 public static void main(String args[])
 {
 for(int i=0;i<10;i++)
 {
 char c=(char)(Math.random()*26+'a'); // random character
 System.out.print(c+":");
 switch(c)
 {
 case 'a':case 'e':case 'i':case 'o':case 'u':
 System.out.println("vowel");
 break;
 default:
 System.out.println("consonant");
 }
 }
 }
}

/* Output:
j:consonant
f:consonant
i:vowel
i:vowel
s:consonant
c:consonant
t:consonant
v:consonant
z:consonant
k:consonant
*/

3.2.10 Continue

Using continue  statement we can exit the loop or continue from the beginning of the loop

Program 3.11 Program to demonstrate the use of continue statement

//continueDemo.java
//click here for code
class continueDemo
{
 public static void main(String args[])
 {
 for(int i=1;i<10;i++)
 {
   System.out.println("i="+i);
   if(i%2!=0)
         continue;  // when i=1, i%2 is 0 so i=1 is printed only once.
   System.out.println("i="+i);
 }
 }
}

/*Output:
i=1
i=2
i=2
i=3
i=4
i=4
i=5
i=6
i=6
i=7
i=8
i=8
i=9

*/

3.2.11 break statement as a form of goto

The general form of labelled break is break;

When a break statement is executed, control is transferred out of the named block of code.

//breakStmt.java
//click here to code
class breakStmt
{
 public static void main(String args[])
 {
 boolean t=true;
 first:     // name of the first block
 {
 //statements of the first block
 second:    // name of the second block
 {
 //statements of the second block
 third:      // name of the third block
 {
 //statements of the third block
 System.out.println("this won't execute"); 
 }
 System.out.println("This will also not work");
 }
 System.out.println("THis is after second block");
 }
 }
}

/* Output:
this won't execute
This will also not work   
This is after second block
*/

3.2.12 The return statement

The return is used to return from a method.

return;

Simply use it as above wherever necessary and the control returns to the calling method

3.3 Summary

You are introduced with if, if-else, if-elseif-else, for, while, do while loops. You are introduced to the concepts of return, break, continue, labelled break.