4.CONTROL STATEMENTS_MB.ppt .

August 6, 2009 1
CONTROL STATEMENTS

August 6, 2009 2
If:
if (condition) statement1;
else statement2;
Nested If:
if(i == 10) {
if(j < 20) a = b;
if(k > 100) c = d; // this if is
else a = c; // associated with this else
}
else a = d; // this else refers to if(i == 10)

August 6, 2009 3
The if-else-if Ladder:
if(condition)
statement;
else if(condition)
statement;
else if(condition)
statement;
...
else
statement;

August 6, 2009 4
switch
switch (expression) {
case value1:
// statement sequence
break;
case value2:
break;
...
case valueN:
break;
default:
// default statement sequence }
• The expression must be of type byte, short, int, or char;

August 6, 2009 5
Nested switch Statements
switch(count) {
case 1:
switch(target) { // nested switch
case 0:
System.out.println("target is zero");
break;
case 1: // no conflicts with outer switch
System.out.println("target is one");
break; }
break;
case 2: // ...

August 6, 2009 6
Iteration Statements
While:
while(condition) {
// body of loop
}

August 6, 2009 7
class NoBody {
public static void main(String args[]) {
int i, j;
i = 100;
j = 200;
// find midpoint between i and j
while(++i < --j) ; // no body in this loop
System.out.println("Midpoint is " + i);
}
}

August 6, 2009 8
do-while
do {
// body of loop
} while (condition);

August 6, 2009 9
// Using a do-while to process a menu selection
class Menu {
public static void main(String args[])
throws java.io.IOException {
//char choice;
do {
System.out.println("Help on:");
System.out.println(" 1. if");
System.out.println(" 2. switch");
System.out.println(" 3. while");
System.out.println(" 4. do-while");
System.out.println(" 5. forn");
System.out.println("Choose one:");
int choice=sc.nextInt();
} while( choice<=5);
System.out.println("n");

August 6, 2009 10
switch(choice) {
case 1:
System.out.println("The if:n");
System.out.println("if(condition) statement;");
System.out.println("else statement;");
break;
case 2:
System.out.println("The switch:n");
System.out.println("switch(expression) {");
System.out.println(" case constant:");
System.out.println(" statement sequence");
System.out.println(" break;");
System.out.println(" // ...");
System.out.println("}");
break;

case 3:
System.out.println("The while:n");
System.out.println("while(condition) statement;");
break;
case 4:
System.out.println("The do-while:n");
System.out.println("do {");
System.out.println(" statement;");
System.out.println("} while (condition);");
break;
case 5:
System.out.println("The for:n");
System.out.print("for(init; condition; iteration)");
System.out.println(" statement;");
break;
default: System.out.println(“Invalid choicen");
August 6, 2009 11

• Output :
Help on :
1. if
2. switch
3. while
4. do – while
5. for
Choose one :
4
The do – while :
do {
statement ;
} while (condition) ;
August 6, 2009 12

August 6, 2009 13
for
for(initialization; condition; iteration) {
// body
}

August 6, 2009 14
class Comma {
int a, b;
for(a=1, b=4; a<b; a++, b--) {
System.out.println("a = " + a);
System.out.println("b = " + b);
}
}
}

August 6, 2009 15
Some for Loop Variations
boolean done = false;
for(int i=1; !done; i++) {
// ...
if(interrupted()) done = true;
}

August 6, 2009 16
// Parts of the for loop can be empty.
class ForVar {
int i;
boolean done = false;
i = 0;
for( ; !done; ) {
System.out.println("i is " + i);
if(i == 10) done = true;
i++;
}
}
}

August 6, 2009 17
for( ; ; ) {
// ...
}
• This loop will run forever, because there is no
condition under which it will terminate
• Nested Loops - concept

The For-Each version of the for loop:
• It is designed to cycle through a collection of
objects, such as an array, in strictly sequential
fashion, from start to finish.
• Advantage is no new keyword is required and no
preexisting code is broken.
• Syntax
for(type itr-var : collection)
stmt-block
August 6, 2009 18

The For-Each version of the for loop:
• The loop repeats until all elements in the
collection have been obtained.
int nums[ ] ={1,2,3,4,5,6,7,8,9,10} ;
int sum=0;
for(int i=0; i<10;i++)
sum += nums[i] ;
=>
for ( int x : nums) sum += x ;
August 6, 2009 19

August 6, 2009 20
Jump Statements
Using break to Exit a Loop:
class BreakLoop {
for(int i=0; i<100; i++) {
if(i == 10) break; // terminate loop if i is 10
System.out.println("i: " + i);
}
System.out.println("Loop complete.");
}
}

August 6, 2009 21
• More than one break statement may appear
in a loop
• Too many break statements have the
tendency to de-structure your code
• The break that terminates a switch statement
affects only that switch statement and not any
enclosing loops

August 6, 2009 22
Using break as a Form of Goto
• Java defines an expanded form of the break
statement
• By using this form of break, you can break out
of one or more blocks of code
• The general form of the labeled break
statement is :
break label;

August 6, 2009 23
• A label is any valid Java identifier followed by
a colon
• You can use a labeled break statement to exit
from a set of nested blocks
• You cannot use break to transfer control to a
block of code that does not enclose the break
statement

August 6, 2009 24
class Break {
boolean t = true;
first: {
second: {
third: {
System.out.println("Before the break.");
if(t) break second; // break out of second block
System.out.println("This won't execute");
}
System.out.println("This won't execute");
}
System.out.println("This is after second block.");
}
}
}

August 6, 2009 25
//using break within nested loops
class BreakLoop4 {
outer: for(int i=0; i<3; i++) {
System.out.print("Pass " + i + ": ");
for(int j=0; j<100; j++) {
if(j == 10) break outer; // exit both loops
System.out.print(j + " ");
}
System.out.println("This will not print");
}
System.out.println("Loops complete.");
}
}

August 6, 2009 26
// This program contains an error.
class BreakErr {
one: for(int i=0; i<3; i++) {
System.out.print("Pass " + i + ": ");
}
for(int j=0; j<100; j++) {
if(j == 10) break one; // WRONG
System.out.print(j + " ");
}
}
}

August 6, 2009 27
Using continue
class Continue {
for(int i=0; i<10; i++) {
System.out.print(i + " ");
if (i%2 == 0) continue;
System.out.println("");
}
}
}

August 6, 2009 28
• As with the break statement, continue may specify a
label to describe which enclosing loop to continue
class ContinueLabel {
outer: for (int i=0; i<10; i++) {
for(int j=0; j<10; j++) {
if(j > i) {
System.out.println();
continue outer; } //end if
System.out.print(" " + (i * j));
} /*end inner for*/
} //end outer for
System.out.println();
} /*end main*/ }

August 6, 2009 29
Output:
0
0 1
0 2 4
0 3 6 9
0 4 8 12 16
0 5 10 15 20 25
0 6 12 18 24 30 36
0 7 14 21 28 35 42 49
0 8 16 24 32 40 48 56 64
0 9 18 27 36 45 54 63 72 81

August 6, 2009 30
return
• Used to explicitly return from a method
• The return statement immediately terminates the
method in which it is executed
class Return {
boolean t = true;
System.out.println("Before the return.");
if(t==true) return; // return to caller
System.out.println("This won't execute.");
}
}

August 6, 2009 31
return causes execution to return to the Java
run-time system, since it is the run-time
system that calls main( )

4.CONTROL STATEMENTS_MB.ppt .

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