JAVA Means DURGA SIR
Flow Control
Agenda :
1. Introduction
2. Selection statements
i. if-else
ii. Switch
Case Summary
fall-through inside a switch
default case
3. Iterative Statements
i. While loop
Unreachable statement in while ii. Do-while
Unreachable statement in do while iii. For Loop
Initilizationsection
Conditional check
Increment and decrement section
Unreachable statement in for loop iv. For each
Iterator Vs Iterable(1.5v)
Difference between Iterable and Iterator
4. Transfer statements o Break statement o Continue statement
o Labeled break and continue statements o Do-
while vs continue (The most dangerous combination)
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JAVA Means DURGA SIR
Introduction :
Flow control describes the order in which all the statements will be executed at run
time.
Diagram:
Selection statements:
if-else:
syntax:
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JAVA Means DURGA SIR
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JAVA Means DURGA SIR
The argument to the if statement should be Boolean by mistake if we are providing any
other type we will get "compile time error".
Example 1:
public class ExampleIf{ public static
void main(String args[]){ int x=0;
if(x)
{
System.out.println("hello");
}else{
System.out.println("hi");
}}}
OUTPUT:
Compile time error:
D:\Java>javac ExampleIf.java
ExampleIf.java:4: incompatible types
found : int required: boolean
if(x)
Example 2:
public class ExampleIf{ public static
void main(String args[]){ int x=10;
if(x=20)
{
System.out.println("hello");
} else{
System.out.println("hi");
}}} OUTPUT:
Compile time error
D:\Java>javac ExampleIf.java
ExampleIf.java:4: incompatible types
found : int required: boolean
if(x=20)
Example 3:
public class ExampleIf{ public static
void main(String args[]){ int x=10;
if(x==20)
{
System.out.println("hello");
}else{
System.out.println("hi");
}}} OUTPUT:
Hi
Example 4:
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JAVA Means DURGA SIR
public class ExampleIf{ public static
void main(String args[]){ boolean
b=false; if(b=true)
{
System.out.println("hello");
}else{
System.out.println("hi");
}}} OUTPUT:
Hello
Example 5:
public class ExampleIf{ public static
void main(String args[]){ boolean
b=false; if(b==true)
{
System.out.println("hello");
}else{
System.out.println("hi");
}}} OUTPUT:
Hi
Both else part and curly braces are optional.
Without curly braces we can take only one statement under if, but it should not be
declarative statement.
Example 6:
public class ExampleIf{ public static
void main(String args[]){ if(true)
System.out.println("hello");
}} OUTPUT:
Hello
Example 7:
public class ExampleIf{
public static void main(String args[])
{ if(true);
}}
OUTPUT:
No output
Example 8:
public class ExampleIf{ public static
void main(String args[]){ if(true)
int x=10;
}} OUTPUT:
Compile time error
D:\Java>javac ExampleIf.java
ExampleIf.java:4: '.class' expected int
x=10;
ExampleIf.java:4: not a statement
int x=10; Example 9:
public class ExampleIf{ public static
void main(String args[]){ if(true)
{ int x=10;
}}} OUTPUT:
D:\Java>javac ExampleIf.java
D:\Java>java ExampleIf
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Example 10:
OUTPUT:
Hello
Hi
Semicolon(;) is a valid java statement which is call empty statement and it won't produce
any output.
Switch:
If several options are available then it is not recommended to use if-else we should go
for switch statement.Because it improves readability of the code. Syntax:
switch(x)
{ case
1:
action1
case 2:
action2
.
. .
default:
default action }
Until 1.4 version the allow types for the switch argument are byte, short, char, int but
from 1.5 version on wards the corresponding wrapper classes (Byte, Short, Character,
Integer) and "enum" types also allowed.
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Diagram:
• Curly braces are mandatory.(except switch case in all remaining cases curly braces
are optional )
• Both case and default are optional.
• Every statement inside switch must be under some case (or) default. Independent
statements are not allowed.
Example 1:
public class ExampleSwitch{ public
static void main(String args[]){ int
x=10; switch(x)
{
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System.out.println("hello");
}}}
OUTPUT:
Compile time error.
D:\Java>javac ExampleSwitch.java
ExampleSwitch.java:5: case, default, or '}' expected
System.out.println("hello");
Every case label should be "compile time constant" otherwise we will get compile time
error.
Example 2:
public class ExampleSwitch{ public
static void main(String args[]){ int
x=10; int y=20; switch(x) { case
10:
System.out.println("10"); case
y:
System.out.println("20");
}}} OUTPUT:
Compile time error
D:\Java>javac ExampleSwitch.java
ExampleSwitch.java:9: constant expression required case
y:
If we declare y as final we won't get any compile time error.
Example 3:
public class ExampleSwitch{ public
static void main(String args[]){ int
x=10; final int y=20; switch(x) { case
10:
System.out.println("10"); case
y:
System.out.println("20");
}}} OUTPUT:
10
20
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But switch argument and case label can be expressions , but case label should be
constant expression. Example 4:
public class ExampleSwitch{ public
static void main(String args[]){ int
x=10; switch(x+1)
{ case 10:
case 10+20:
case 10+20+30:
}}} OUTPUT:
No output.
Every case label should be within the range of switch argument type.
Example 5:
public class ExampleSwitch{ public
static void main(String args[]){ byte
b=10; switch(b) { case 10:
System.out.println("10"); case
100:
System.out.println("100");
case
1000:
System.out.println("1000");
}}} OUTPUT:
Compile time error
D:\Java>javac ExampleSwitch.java
ExampleSwitch.java:10: possible loss of precision
found : int required: byte case 1000: Example :
public class ExampleSwitch{ public
static void main(String args[]){ byte
b=10; switch(b+1) { case 10:
System.out.println("10"); case
100:
System.out.println("100"); case
1000:
System.out.println("1000");
}}} OUTPUT:
Duplicate case labels are not allowed.
Example 6:
public class ExampleSwitch{ public
static void main(String args[]){ int
x=10; switch(x) { case 97:
System.out.println("97"); case
99:
System.out.println("99"); case
'a':
System.out.println("100");
}}}
OUTPUT:
Compile time error.
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D:\Java>javac ExampleSwitch.java
ExampleSwitch.java:10: duplicate case label
case 'a':
CASE SUMMARY:
Diagram:
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JAVA Means DURGA SIR
nd
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JAVA Means DURGA SIR
FALL-THROUGH INSIDE THE SWITCH:
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With in the switch statement if any case is matched from that case onwards all
statements will be executed until end of the switch (or) break. This is call "fall-through"
inside the switch .
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The main advantage of fall-through inside a switch is we can define common action for
multiple cases
Example 7:
public class ExampleSwitch{ public
static void main(String args[]){ int
x=0; switch(x) { case 0:
System.out.println("0"); case
1:
System.out.println("1");
break; case 2:
System.out.println("2"); default:
System.out.println("default");
}}}
OUTPUT:
x=0 x=1 x=2 x=3
1 1 2
default
2 default
DEFAULT CASE:
• With in the switch we can take the default only once
• If no other case matched then only default case will be executed
• With in the switch we can take the default any where, but it is convension to take
default as last case.
Example 8:
public class ExampleSwitch{ public
static void main(String args[]){ int
x=0; switch(x) { default:
System.out.println("default"); case
0:
System.out.println("0");
break; case 1:
System.out.println("1"); case
2:
System.out.println("2");
}}} OUTPUT:
X=0 x=1 x=2 x=3
0 1 2 default
2 0
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JAVA Means DURGA SIR
ITERATIVE STATEMENTS:
While loop:
if we don't know the no of iterations in advance then best loop is while loop:
Example 1:
while(rs.next())
{
}
Example 2:
while(e.hasMoreElements())
{
----------
----------
----------
}
Example 3:
while(itr.hasNext())
{
----------
----------
----------
}
The argument to the while statement should be Boolean type. If we are using any other
type we will get compile time error.
Example 1:
public class ExampleWhile{ public
static void main(String args[])
{ while(1)
{
System.out.println("hello");
}}}
OUTPUT:
Compile time error.
D:\Java>javac ExampleWhile.java
ExampleWhile.java:3: incompatible types
found : int required: boolean
while(1)
Curly braces are optional and without curly braces we can take only one statement
which should not be declarative statement.
Example 2:
public class ExampleWhile{ public
static void main(String args[])
{ while(true)
System.out.println("hello");
}}
OUTPUT:
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Hello (infinite times). Example
3:
public class ExampleWhile{ public
static void main(String args[])
{ while(true);
}} OUTPUT:
No output. Example
4:
public class ExampleWhile{ public
static void main(String args[])
{ while(true) int x=10;
}}
OUTPUT:
Compile time error.
D:\Java>javac ExampleWhile.java
ExampleWhile.java:4: '.class' expected
int x=10;
ExampleWhile.java:4: not a statement
int x=10;
Example 5:
public class ExampleWhile{ public
static void main(String args[])
{ while(true) { int x=10;
}}}
OUTPUT: No
output.
Unreachable statement in while:
Example 6:
public class ExampleWhile{ public
static void main(String args[])
{ while(true)
{
System.out.println("hello");
}
System.out.println("hi");
}}
OUTPUT:
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Compile time error.
D:\Java>javac ExampleWhile.java
ExampleWhile.java:7: unreachable
statement System.out.println("hi");
Example 7:
public class ExampleWhile{ public
static void main(String args[])
{ while(false)
{
System.out.println("hello");
}
System.out.println("hi");
}} OUTPUT:
D:\Java>javac ExampleWhile.java
ExampleWhile.java:4: unreachable statement
{
Example 8:
public class ExampleWhile{ public
static void main(String args[]){ int
a=10,b=20; while(a<b)
{
System.out.println("hello");
}
System.out.println("hi");
}}
OUTPUT:
Hello (infinite times). Example
9:
public class ExampleWhile{ public
static void main(String args[]){ final
int a=10,b=20; while(a<b)
{
System.out.println("hello");
}
System.out.println("hi");
}}
OUTPUT:
Compile time error.
D:\Java>javac ExampleWhile.java
ExampleWhile.java:8: unreachable statement
System.out.println("hi"); Example 10:
public class ExampleWhile{ public
static void main(String args[]){ final
int a=10; while(a<20)
{
System.out.println("hello");
}
System.out.println("hi");
}} OUTPUT:
D:\Java>javac ExampleWhile.java
ExampleWhile.java:8: unreachable statement
System.out.println("hi");
Note:
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• Every final variable will be replaced with the corresponding value by compiler.
• If any operation involves only constants then compiler is responsible to perform
that operation.
• If any operation involves at least one variable compiler won't perform that
operation. At runtime jvm is responsible to perform that operation.
Example 11:
public class ExampleWhile{ public
static void main(String args[]){ int
a=10; while(a<20)
{
System.out.println("hello");
}
System.out.println("hi");
}}
OUTPUT:
Hello (infinite times).
Do-while:
If we want to execute loop body at least once then we should go for do-while.
Syntax:
Curly braces are optional.
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Without curly braces we can take only one statement between do and while and it
should not be declarative statement.
Example 1:
public class ExampleDoWhile{ public
static void main(String args[]){ do
System.out.println("hello");
while(true);
}}
Output:
Hello (infinite times). Example
2:
public class ExampleDoWhile{ public
static void main(String args[]){ do;
while(true);
}}
Output:
Compile successful. Example
3:
public class ExampleDoWhile{ public
static void main(String args[]){ do int
x=10; while(true);
}} Output:
D:\Java>javac ExampleDoWhile.java
ExampleDoWhile.java:4: '.class' expected int
x=10;
ExampleDoWhile.java:4: not a statement
int x=10;
ExampleDoWhile.java:4: ')' expected int
x=10;
Example 4:
public class ExampleDoWhile{ public
static void main(String args[]){ do
{ int x=10;
}while(true);
}}
Output:
Compile successful. Example
5:
public class ExampleDoWhile{ public
static void main(String args[]){ do
while(true)
System.out.println("hello");
while(true);
}}
Output:
Hello (infinite times).
Rearrange the above Example:
public class
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ExampleDoWhile{ public static
void main(String args[]){ do
while(true)
System.out.println("hello");
while(true);
}}
Output:
Hello (infinite times). Example
6:
public class ExampleDoWhile{ public
static void main(String args[]){ do
while(true);
}}
Output:
Compile time error.
D:\Java>javac ExampleDoWhile.java
ExampleDoWhile.java:4: while expected
while(true);
ExampleDoWhile.java:5: illegal start of expression
}
Unreachable statement in do while:
Example 7:
public class ExampleDoWhile{ public
static void main(String args[]){ do
{
System.out.println("hello");
}
while(true);
System.out.println("hi");
}}
Output:
Compile time error.
D:\Java>javac ExampleDoWhile.java
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ExampleDoWhile.java:8: unreachable statement
System.out.println("hi"); Example 8:
public class ExampleDoWhile{ public
static void main(String args[]){ do
{
System.out.println("hello");
} while(false);
System.out.println("hi");
}} Output:
Hello
Hi
Example 9:
public class ExampleDoWhile{ public
static void main(String args[]){ int
a=10,b=20; do
{
System.out.println("hello");
} while(a<b);
System.out.println("hi");
}}
Output:
Hello (infinite times). Example
10:
public class ExampleDoWhile{ public
static void main(String args[]){ int
a=10,b=20; do
{
System.out.println("hello");
} while(a>b);
System.out.println("hi");
}} Output:
Hello
Hi
Example 11:
public class ExampleDoWhile{ public
static void main(String args[]){ final
int a=10,b=20; do
{
System.out.println("hello");
} while(a<b);
System.out.println("hi");
}}
Output:
Compile time error.
D:\Java>javac ExampleDoWhile.java
ExampleDoWhile.java:9: unreachable statement
System.out.println("hi"); Example 12:
public class ExampleDoWhile{ public
static void main(String args[]){ final
int a=10,b=20; do {
System.out.println("hello");
} while(a>b);
System.out.println("hi");
}} Output:
D:\Java>javac ExampleDoWhile.java
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D:\Java>java ExampleDoWhile
Hello
Hi
For Loop:
This is the most commonly used loop and best suitable if we know the no of iterations in
advance.
Syntax:
Curly braces are optional and without curly braces we can take only one statement
which should not be declarative statement.
Initilizationsection:
This section will be executed only once.
Here usually we can declare loop variables and we will perform initialization. We can
declare multiple variables but should be of the same type and we can't declare
different type of variables.
Example:
Int i=0,j=0; valid
Int i=0,Boolean b=true; invalid
Int i=0,int j=0; invalid
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In initialization section we can take any valid java statement including "s.o.p" also.
Example 1:
public class ExampleFor{ public static
void main(String args[]){ int i=0;
for(System.out.println("hello u r sleeping");i<3;i++){
System.out.println("no boss, u only sleeping");
}}} Output:
D:\Java>javac ExampleFor.java
D:\Java>java ExampleFor
Hello u r sleeping
No boss, u only sleeping
No boss, u only sleeping
No boss, u only sleeping
Conditional check:
We can take any java expression but should be of the type Boolean.
Conditional expression is optional and if we are not taking any expression compiler will
place true.
Increment and decrement section:
Here we can take any java statement including s.o.p also.
Example:
public class ExampleFor{ public static
void main(String args[]){ int i=0;
for(System.out.println("hello");i<3;System.out.println("hi")){ i++;
}}} Output:
D:\Java>javac ExampleFor.java
D:\Java>java ExampleFor
Hello Hi
Hi
Hi
All 3 parts of for loop are independent of each other and all optional.
Example:
public class ExampleFor{ public static
void main(String args[]){
for(;;){
System.out.println("hello");
}}}
Output:
Hello (infinite times).
Curly braces are optional and without curly braces we can take exactly one statement
and it should not be declarative statement.
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Unreachable statement in for loop:
Example 1:
public class ExampleFor{
public static void main(String args[]){
for(int i=0;true;i++){
System.out.println("hello");
}
System.out.println("hi");
}}
Output:
Compile time error.
D:\Java>javac ExampleFor.java
ExampleFor.java:6: unreachable statement
System.out.println("hi"); Example 2:
public class ExampleFor{ public static
void main(String args[]){
for(int i=0;false;i++){
System.out.println("hello");
}
System.out.println("hi");
}}
Output:
Compile time error.
D:\Java>javac ExampleFor.java
ExampleFor.java:3: unreachable statement
for(int i=0;false;i++){ Example 3:
public class ExampleFor{ public static
void main(String args[]){
for(int i=0;;i++){
System.out.println("hello");
}
System.out.println("hi");
}}
Output:
Compile time error.
D:\Java>javac ExampleFor.java
ExampleFor.java:6: unreachable statement
System.out.println("hi"); Example
4:
public class ExampleFor{ public static
void main(String args[]){ int
a=10,b=20; for(int i=0;a<b;i++){
System.out.println("hello");
}
System.out.println("hi");
}} Output:
Hello (infinite times). Example
5:
public class ExampleFor{ public static
void main(String args[]){ final int
a=10,b=20; for(int i=0;a<b;i++){
System.out.println("hello");
}
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System.out.println("hi");
}} Output:
D:\Java>javac ExampleFor.java
ExampleFor.java:7: unreachable statement
System.out.println("hi");
For each:(Enhanced for loop)
• For each Introduced in 1.5version.
• Best suitable to retrieve the elements of arrays and collections.
Example 1:
Write code to print the elements of single dimensional array by normal for loop and
enhanced for loop.
Example:
Output:
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D:\Java>javac ExampleFor.java
D:\Java>java ExampleFor
10
20
30
40
50
Example 2:
Write code to print the elements of 2 dimensional arrays by using normal for loop and
enhanced for loop.
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Example 3:
Write equivalent code by For Each loop for the following for loop.
public class ExampleFor{ public static
void main(String args[]){ for(int
i=0;i<10;i++)
{
System.out.println("hello");
}}}
Output:
D:\Java>javac ExampleFor1.java
D:\Java>java ExampleFor1
Hello
Hello
Hello
Hello
Hello
Hello
Hello
Hello
Hello
Hello
We can't write equivalent for each loop.
For each loop is the more convenient loop to retrieve the elements of arrays and
collections, but its main limitation is it is not a general purpose loop.
By using normal for loop we can print elements either from left to right or from
right to left. But using for-each loop we can always print array elements only
from left to right.
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JAVA Means DURGA SIR
Iterator Vs Iterable(1.5v)
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JAVA Means DURGA SIR
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JAVA Means DURGA SIR
• The target element in for-each loop should be Iterable object.
• An object is set to be iterable iff corresponding class implements java.lang.Iterable
interface.
• Iterable interface introduced in 1.5 version and it's contains only one method
iterator().
Syntax : public Iterator iterator();
Every array class and Collection interface already implements Iterable interface.
Difference between Iterable and Iterator:
Iterable Iterator
It is related to forEach loop It is related to Collection
The target element in forEach loop should We can use Iterator to get objects one by
be Iterable one from the collection
Iterator present in java.lang package Iterator present in java.util package
contains 3 methods hasNext(), next(),
contains only one method iterator()
remove()
Introduced in 1.5 version Introduced in 1.2 version
Transfer statements:
Break statement:
We can use break statement in the following cases.
• Inside switch to stop fall-through.
• Inside loops to break the loop based on some condition.
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JAVA Means DURGA SIR
• Inside label blocks to break block execution based on some condition.
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Inside switch :
We can use break statement inside switch to stop fall-through Example
1:
class Test{ public static void
main(String args[]){ int x=0; switch(x)
{ case 0:
System.out.println("hello");
break ; case 1:
System.out.println("hi");
}
Output:
D:\Java>javac Test.java
D:\Java>java Test
Hello
Inside loops :
We can use break statement inside loops to break the loop based on some condition.
Example 2:
class Test{ public static void
main(String args[]){ for(int i=0; i<10;
i++) { if(i==5) break;
System.out.println(i);
} }}
Output:
D:\Java>javac Test.java
D:\Java>java Test
0
1
2
3
4
Inside Labeled block :
We can use break statement inside label blocks to break block execution based on some
condition.
Example:
class Test{ public static void
main(String args[]){ int x=10; l1 : {
System.out.println("begin");
if(x==10) break l1;
System.out.println("end");
}
System.out.println("hello");
}
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JAVA Means DURGA SIR
} Output:
D:\Java>javac Test.java
D:\Java>java Test begin
hello
These are the only places where we can use break statement. If we are using anywhere else
we will get compile time error.
Example:
class Test{ public static void
main(String args[]){ int x=10;
if(x==10) break;
System.out.println("hello");
}
}
Output:
Compile time error.
D:\Java>javac Test.java
Test.java:5: break outside switch or loop
break;
Continue statement:
We can use continue statement to skip current iteration and continue for the next iteration.
Example:
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JAVA Means DURGA SIR
Output:
D:\Java>javac Test.java
D:\Java>java Test
1
3
5
7
9
We can use continue only inside loops if we are using anywhere else we will get compile time
error saying "continue outside of loop".
Example:
class Test { public static void
main(String args[]){ int x=10;
if(x==10); continue;
System.out.println("hello");
}
}
Output:
Compile time error.
D:\Enum>javac Test.java
Test.java:6: continue outside of loop
continue;
Labeled break and continue statements:
In the nested loops to break (or) continue a particular loop we should go for labeled break
and continue statements.
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JAVA Means DURGA SIR
Syntax:
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JAVA Means DURGA SIR
Example:
class Test { public static void
main(String args[]){ l1: for(int
i=0;i<3;i++)
{
for(int j=0;j<3;j++)
{ if(i==
j)
break;
System.out.println(i+"........."+j);
}
}
}
}
Break:
1.........0 2.........0
2.........1
Break
l1:
No output.
Continue:
0.........1 0.........2
1.........0 1.........2
2.........0
2.........1
Continue l1:
1.........0 2.........0
2.........1
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JAVA Means DURGA SIR
Do-while vs continue (The most dangerous combination):
Output:
1
4
6
8
10
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JAVA Means DURGA SIR
Compiler won't check unreachability in the case of if-else it will check only in loops.
Example 1:
class Test {
public static void main(String args[])
{ while(true)
{
System.out.println("hello");
}
System.out.println("hi");
}
}
Output:
Compile time error.
D:\Enum>javac Test.java
Test.java:8: unreachable statement
System.out.println("hi"); Example
2:
class Test { public static void
main(String args[]){ if(true)
{
System.out.println("hello");
} else
{
System.out.println("hi");
}}} Output:
Hello
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JAVA Means DURGA SIR
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JAVA Means DURGA SIR
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