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LESSON 6: ARRAY AND 2D ARRAYS

✅An array is a collection of items stored at contiguous memory locations. The idea is to store multiple items of the same type together.
✳️Multidimensional Arrays can be defined in simple words as array of arrays. Data in multidimensional arrays are stored in tabular form (in row major order).
*In the array there will be:
data_type : type of data to be stored in the array
dimension: the dimension of the array is created. like 1D, 2D ...
array_name: name of the array
finally , the size of the array.
#example_of_array:;
class CODER
{
public static void main (String[] args)
{
// declares an Array of integers.
int[] arr;

// allocating memory for 5 integers.
arr = new int[5];

// initialize the first elements of the array
arr[0] = 10;

// initialize the second elements of the array
arr[1] = 20;

//so on...
arr[2] = 30;
arr[3] = 40;
arr[4] = 50;

// accessing the elements of the specified array
for (int i = 0; i < arr.length; i++)
System.out.println("Element at index " + i +
" : "+ arr[i]);
}
}


2D array:-

class Learn {
public static void main(String[] args)
{

int[][] arr = new int[10][20];
arr[0][0] = 1;

System.out.println("arr[0][0] = " + arr[0][0]);
}
}

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java student guide 2

‼️: ignore [a], [b],[c] and [d] it is only to identify the choice

LESSON 7:
LOOPS AND NESTED LOOPS
✳️Looping in java helps by facilitating the execution of of a set of instructions repeatedly while some condition evaluates to true.

1. While-loops: allows the code to be executed repeatedly based on a given Boolean condition.

   while (boolean condition)
     {
   loop statements...
   }

2. for-loops: unlike while statement, it consumes the initialization, condition, and increment/decrement.
for (initialization condition; testing condition;
increment/decrement)
{
statement(s)
}

3. do-while: do while loop is similar to while loop with only difference that it checks for condition after executing the statements.
do
{
statements..
}
while (condition);

sample example for for-loop:

public class Loops
{
    public static void main(String[] args)
    {
        ArrayList<Integer> ar = new ArrayList<>();//we will see about list in the next lessons
        for (int i = 0; i < Integer.MAX_VALUE; i++)
        {
            ar.add(i);
        }
    }

}


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Short notes for Java.

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LESSON 8: WRAPPER CLASSES IN JAVA



✅Wrapper classes provide a way to use primitive data types (int, boolean, etc..) as objects.

✳️Sometimes you must use wrapper classes, for example when working with Collection objects, such as ArrayList, where primitive types cannot be used (the list can only store objects):

example:
ArrayList<Integer> myNumbers = new ArrayList<Integer>(); // Valid

Creating Wrapper Objects

To create a wrapper object, use the wrapper class instead of the primitive type. To get the value, you can just print the object:
Example

public class Main {
  public static void main(String[] args) {
    Integer myInt = 5;
    Double myDouble = 5.99;
    Character myChar = 'A';
    System.out.println(myInt);
    System.out.println(myDouble);
    System.out.println(myChar);
  }
}

//

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The following code will not compile or run. WHY?

LESSON 9: CONSTRUCTORS in Java

✳️ A constructor in Java is a special method that is used to initialize objects. The constructor is called when an object of a class is created. It can be used to set initial values for object attributes.


How Constructors are Different From Methods in Java?

❤️ Constructors must have the same name as the class within which it is defined while it is not necessary for the method in Java.
❤️Constructors do not return any type while method(s) have the return type or void if does not return any value.

❤️Constructors are called only once at the time of Object creation while method(s) can be called any number of times.

Example: look the sample code having two constructors;

package ubJapro1;
//import java.util.*;


public class Firsta {

  public static void main(String[] args) {
    
  Firsta2 firs  = new Firsta2("ashe",34,76.4);// constructor 1
  Firsta2 firs2  = new Firsta2("asegid",32,70.4);// constructor 2
  
   System.out.println(firs.name+" is your name.");
  
    firs2.home();
   
  }
  
    
    
    
      
  }

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LESSON 10: Creating objects in java

✳️An object is a basic unit of Object-Oriented Programming and represents the real life entities. It consists of:-
⚜️State means the attributes.
⚜️Behavior means the methods of an object.
⚜️Identity means the unique name to an object.


the four ways of creating objects in java

1. using new key word

//Test t = new Test();
2. Using Class.forName( String className) method;

  Test obj = (Test)Class.forName("com.p1.Test").newInstance();

3. using clone() method

/Test t = new Test();
Test t2 = (Test)t1.clone();

4. Deserialization
it is is technique of reading an object from the saved state in a file.

✅example using the first way:

 class Animal {}

class Dog extends Animal {}
class Cat extends Animal {}

public class Test
{
    // using Dog object
    Animal obj = new Dog();

    // using Cat object
    obj = new Cat();
}       

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