Tech Twitter: Flow

Flow control describes the order in which all the statements will be executed at run time.

Diagram:

Selection statements

if-else

syntax:

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:

Example 4:

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:

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

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.

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) {
        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

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 6:

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 7:

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.
D:\Java>javac ExampleSwitch.java ExampleSwitch.java:10: duplicate case label
case 'a':

Case Summary

Diagram:

FALL-THROUGH INSIDE THE SWITCH

Within the switch statement if any case is matched from that case onwards all statements will be executed until the end of the switch (or) break. This is called "fall-through" inside the switch

The main advantage of fall-through inside a switch is we can define common action for multiple cases

Example :

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
0    1     2          default
1        default

Default Case

Within the switch, we can take the default only once If no other case matched then only default case will be executed

Within the switch, we can take the default any where, but it is convension to take default as last case.

Example :

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

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:

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=10ExampleWhile.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:

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:

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.

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){
    do {
       int x=10;
    }while(true);
  }
}

Output:

Compile successful.

Example 5:

public class ExampleDoWhile { 
  public static void main(String) 
    do 
    while(true) 
    System.out.println("hello"); while(true);
  }
}

Output:

Hello (infinite times).

Example 6:

public class ExampleDoWhile{args[]){
  public static void main(String args[]){ 
    do
    while(true) 
    System.out.println("hello");
    while(true);
  }
}

Output:

Hello (infinite times).

Example 7:

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 ExampleDoWhile.java:8: unreachable statement System.out.println("hi");

Example 8:

public class ExampleDoWhile{
    do { 
      System.out.println("hello"); 
    }while(false); 
    System.out.println("hi"); 
  }
}

Output:

Hello

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

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 D:\Java>java ExampleDoWhile

Hello

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

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

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

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.

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[]){
        int i=0;
      for(System.out.println("hello u r sleeping");i<3;i++) { 
        System.out.println("no boss, u only sleeping");
        }
    }
}

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:

107

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");
    }
    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:

D:\Java>javac ExampleFor.java D:\Java>java ExampleFor

Example 2:

Write code to print the elements of 2 dimensional arrays by using normal for loop and enhanced for loop.

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

We can't write equivalent for each loop.

For each loop is the more convenient loop to retrieve the elements of arrays andcollections, 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.

Iterator Vs Iterable(1.5v)

The target element in for-each loop should be Iterable object.
An object is set to be iterable iff corresponding class implementsjava.lang.Iterable interface.
Iterable interface introduced in 1.5 version and it's contains only one methoditerator().

Syntax : public Iterator iterator();

Every array class and Collection interface already implements Iterable interface.

Difference between Iterable and Iterator:

Iterable

It is related to forEach loop
The target element in forEach loop should be Iterable
Iterator present in java.lang package
contains only one method iterator()
Introduced in 1.5 version

Iterator

It is related to Collection
We can use Iterator to get objects one by one from the collection
Iterator present in java.util package
contains 3 methods hasNext(), next(), remove()
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.
Inside label blocks to break block execution based on some condition.

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

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"); }
  }
}

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:

Output:

D:\Java>javac Test.java D:\Java>java Test

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.

Syntax:

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

Do-while vs continue (The most dangerous combination):

Output:

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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Thursday, July 28, 2022

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