Java Packages and Loops

Packages

packages are folders contain java classes like Drawing.java, a package can have any number of classes within.

  package test;
  import main.Test2;
  class Test{
    . . .
  }

package keyword MUST be the first statement in the class code, before import statements.

there are three types of importing

Import all classes inside a package using asterisk character *

  import javax.swing.*;

Import one class only
```
  import javax.swing.JOptionPane;
```
No Import statement, you can use the class you want by typing its packages inside the code.

  class Test{
    javax.swing.JOptionPane.showMessageDialog(null, "");
  }

There are 166 packages containing 3279 classes and interfaces in Java 5

you can use these classes and interfaces whenever you want in your code instead of rewriting them from scratch. This is the power of object-oriented programming

Loops

Loops mainly used to reduce code redundancy whenever it exists and to make code more readable by writing less code.

In java there are three types of loops:

for loop
while loop
do-while loop

For Loop

For loop in java has three statements(part) in order to make it working as desired.

Loops are a block of code and the code within is being written inside curly braces { }.
The first statement is the counter variable, it could be declared inside the loop or outside it as well.

this counter tells the loop from which number to start the loop.

The second statement is the condition (limit) to stop the loop from executing. if this part left empty, the loop will run infinitely.
The third statement is the increment which tells the loop by which value it is being incremented

  //this code will print the even numbers from 0 - 100 using for loop
  for(int i=0; i<=100; i+=2){
    System.out.println(i);
  }

there is an enhanced for loop that came in version 5. This kind is being used mainly for iterating over arrays.

  //This code prints out the array elements from 0 to 10 using enhanced loop
  int arr[] = {0,2,4,8,10};
  for(int i: arr){
    System.out.println(i);
  }

While Loop

while loop in java does pretty much as the normal for loop.

it has the same three statements as well. but only the condition statement is written within the while parentheses.

The counter initialization is done before the while loop body, while the increment is written inside the while loop body.

  //this code will print the even numbers from 0 - 100 using while loop
  int i=0;
  while(i<=100){
    System.out.println(i);
    i+=2;
  }

in order to make for loop and while loop work the same you need to add if statement and use the break within to stop the loop execution whenever the condition inside the if will meet.

Do-While Loop

if you want a loop to execute at least once, java offers this kind of loop; called a do-while loop.

in the do-while loop the code that needs to be executed in repetition is written inside the do body, while the condition is being tested is written inside while parentheses.

  //this code will print the even numbers from 0 - 100 using do-while loop
  int i=0;
  do{
    System.out.println(i);
    i+=2;
  }
  while(i<=100);