Control Structures: These are used to control flow of program execution.

Selection Statements: These are used to execute only one path among alternative execution paths at a time based on outcome of an expression or state of a variable.

Java supports two selection statements such as if and switch.

(i) if: It is called java’s conditional branch statement. It route program execution through two different paths. The general form of if is:

if(condition) statement1;

else

statement2;

In this, each statement may be a single or compound statement enclosed in curly braces. The condition is any expression that returns true. else clause is optional.

It works as: if the condition is true, then statement1 is executed. otherwise, statement2 is executed. In no case is true, both statements will be executed.

Example is:

int a,b;

//;;;;;;;;;;;;;

if(a<b) a=0;

else b=0;

In this, if a<0, a is set to 0.otherwise b=0. In no case, both a & b set to 0.

There are some types of if’s like nested if and if else if ladder.

a) Nested if: A nested if is an if statement that is target of another target if or else. One main thing to remember in this is an else statement always refers to the nearest if statement within the same block.

An example is:

if(i==10)
{
if(j<20) a=b;
if(k>100) c=d;
else a=c;
}
else a=d;

The outer else is not in the same block so it is matched with if(i==10). The inner else is the same block of if(i==10), so it is matched to closet if the same block such as if(k>100).

b) if else if ladder: It is based upon a sequence of nested if’s .It’s general form is

if(condition) statement;

else if(condition) statement;

else if(condition) statement;

..

.

.

else

statement;

The if statements are executed from top to down. In this, if any one of if condition is true, then the statement associated with that true if condition is executed, and rest are bypassed. The final else is default condition. If all other conditions fail, the the last else statement is executed. if there is no final else and all other conditions fail, no action take place.

An example program is:

class ifladderdemo
{
public static void main(String args[])
{
int m=4;
String s;
if(m==12||m==1||m==2)
s=”winter”;
else if(m==3||m==4||m==5)
s=”spring”;
else if(m==6||m==7||m==8)
s=”summer”;
else
s=”autumn”;
System.out.println(“april is in”+s);
}
}

Output: april is in spring.

(ii) switch: It is a java’s multiway branch statement. It provides better alternative than larger if else if ladder. The general form of switch is:

switch(expression)

{

case value1: // statement sequence

break;

case value2: //statement sequence

break;

.

.

.

case valueN: //statement sequence

break;

default: //default statement sequence

}

The expression must be of type byte, short, int or char. Each of the values of case must be type compatible with type of expression. Duplicate case values are not allowed.

Switch works as: The value of the expression is compared to all case values in the switch body. If any one is matched to expression value, the statement sequence associated with that case is executed. If none of case values matched with value of expression, then default statement sequence is executed. If none of case values matches and no default is present, no further action take place.

Every case statement sequence ends with break which transfers control to the first statement that follows the switch.

An example is:

class switchdemo
{
public static void main(String args[])
{
for(int i=0;i<6;i++)
{
switch(i)
{
case 0: System.out.println(i is zero”);
        break;
case 1: System.out.println(i is one”);
        break;
case 2: System.out.println(i is two”);
        break;
case 3: System.out.println(i is three”);
        break;
default:  System.out.println(i is greater than 3”);
 }
}
}
}

Output:

i is zero

i is one

i is two

i is three

i is greater than 3

i is greater than 3

In this, for each loop execution, the statements associated with case values matches with i are executed. All others are bypassed. After i became greater than 3, default is executed because no case values match with i.

Note: If break is omitted in any case, execution continues into next case.

The improved version of If else if ladder is shown for switch is as follows:

class switchdemo
{
public static void main(String args[])
{
int m=4;
String s;
switch(m)
{
case 12:
case 1:
case 2:    s=”winter”;
           break;
case 3:
case 4:
case 5:    s=”spring”;
           break;
case 6:
case 7:
case 8:    s=”summer”;
           break;
case 9:
case 10:
case 11:    s=”autumn”;
           break;
default: s=”bogus month”;
}
System.out.println(“april is in”+s);
}
}
Output:

april is in spring

Nested switch: It contains another switch in it.

The important difference between if’s and switch are:

(i) The switch differs from if in that switch can only test for equality where as if can evaluate any type of boolean expression.

(ii) No two case values in the same switch can have identical values.

(iii) A switch is more efficient than a set of nested ifs.

Iterative Statements: These are used to execute certain collection of statements repeatedly.

Java provides three types of iteration statements such as while, do while and for.

(a) while: It is java’s most fundamental looping statement. It executes its block of statements as long as condition is true. It’s general form is

while(condition)

{

// body

}

The condition may be any boolean expression. The curly braces are unnecessary if only a single statement is being executed. It is also called entry controlled loop because first condition is evaluated, if condition is true, its loop is executed until condition made false.

The while loop body cal also be empty.

An example is

i=100,j=200;

while(++i<=–j);

System.out.println(mid pointis”+i);

Output produced is 150.

(b) do while: There are some times to execute the loop at least once and also to check conditional expression at termination of the loop. Java provides do while which satisfies both of these.

The general form of do-while is

do

{

// body of the loop

} while(condition);

Working of this loop is it first executes loop body and then evaluates condition. If the condition is true, loop will repeat. Otherwise, loop terminates. The condition must of Boolean type.

It is also called exit controlled loop.

An example is:

int n=5;

do

{

System.out.println”tick”+n);

}while(–n>0);

Output is:

tick: 5

tick: 4

tick: 3

tick: 2

tick: 1

tick: 0

The loop terminates when n becomes 0.

for loop: It is most powerful and versatile construct and it’s general from is

for( initialization; condition; iteration) where this loop sets initial value to loop control variable through initialization which also acts as counter for controlling the loop. This initialization is executed only one. If condition is evaluates to true, the loop body is executed, then next incrimination is used. This process is repeated until condition fails.

The following code illustrates sum of first 10 numbers:

int i,sum=0,n=10;

for(i=1;i<=n;i++)

sum+=i;

System.out.println(“sum of first”+n+”numbers is”+sum);

Output:

sum of first 10 numbers is: 55

Note:

1. comma is used when more than one statement needs to be included in initialization or iteration parts of for loop.

2. Loops can be nested means a loop contain another loop.

Jump Statements:

These are used to transfer control from one part of the program to another part of the same program. There are three jump statements frequently used in java.

a) break: There are three uses of break. (i) it is used to exit from a statement sequence of switch construct (ii) it is used to come out of loop.

(iii) it can be used as civilized form of goto.

break is needed when to immediate terminate of loop is needed and also to bypass conditional expression and remaining code of the loop.

An example code is:

for(int i=1;i<1=100;i++)

{

if(i==5) break;

System.out.println(“ i is”+i);

}

System.out.println(“loop completed”);

Output: i is: 1

i is:2

i is:3

i is:4

loop completed.

Note: break can also be in form break label: where label specifies name of the label that identifies block of code. When this form executes, control transferred out of named block of code.

Example code is:

boolean t=true;
first: {
second: {
third: {
System.out.println(“before break”);
if(t) break second;
System.out.println(“this won’t execute”);
}
System.out.println(“this won’t execute”);
}
System.out.println(“this is after second block”);
}

Output: before break

this is after second block

b ) continue: It is used to stop remaining of the loop code (current iteration) and give control to next immediate iteration.

An example is:

for(int i=0;i<10;i++)
{
System.out.print(i+” “);
if(i%2==0) continue;
System.out.println();
}

Output is:

0 1

2 3

4 5

6 7

8 9

Note: Like break, continue also has a label which indicates which enclosing loop to execute.

c ) return: It is used to explicitly return something from a method. It causes transfers control back to caller of the method.

boolean t=true;

System.out.println(“before return”);

if(t) return;

System.out.println(“this won’t execute”);

Output: before return . In this, when return is executed , control transferred to caller of the method.

Note: At time, when return is executed in a method returns control back to caller of the method.