PREREQUISITES.md

Prerequisites :-

0. PROGRAMMING LANGUAGE.

1. CLASS AND OBJECTS.

2. CONSTRUCTOR.

3. this KEYWORD.

4. ACCESS MODIFIERS IN JAVA.

• Public- The access level of the public modifier is everywhere. Methods, variables declared public can be accessed from anywhere after importing that class.

• Default- The access level of the default modifier is within the package. Methods, variables declared default can be accessed from other classes of the same package.

• Private- The access level of the private modifier is within the class. Methods, variables declared private cannot be accessed from outside the class.

5. ENCAPSULATION.

6. STATIC KEYWORD IN JAVA.

7. INNER CLASS IN JAVA.

8. Pass by value.

9. Pass by refrence.

import java.util.*;

public class Hi {

    // here value is copied in localVariable 'a'.
    public static void incrementPrimitiveDT(int a) {
        a++;
    }

    // here reference is copied, means 'input' is also pointing to 'arr'.
    public static void incrementReferenceDT(int input[]) {
        for(int i=0;i<input.length;i++) {
            input[i]++;
        }
    }


    public static void main(String[] args) {
        int i=10;
        incrementPrimitiveDT(i); // passByValue
        System.out.println(i); // 10


        int arr[]= {1,2,3,4,5};
        incrementReferenceDT(arr); // passByReference
        System.out.println(Arrays.toString(arr)); // [2, 3, 4, 5, 6]
    }
}

10. Strings are immutable.

11. == & .equals in String.

12. Wrapper class in java.

• Object creation and memory allocation in Integer class.

public static void main(String[] args) {
    // case 1:
    Integer i= new Integer(1);
    Integer j=new Integer(1);
    System.out.println(i==j); // false,  coz == check reference address of i and j, and here 'i' is pointing to 1 (i---> 1) and 'j' is pointing to different 1 (j---> 1)
    System.out.println(i.equals(j)); // true

    // case 2:
    Integer a=new Integer(2);
    Integer b=2;

    System.out.println(a==b); // false
    System.out.println(a.equals(b)); // true

    // case 3:
    Integer x=new Integer(3);
    Integer y=x;

    System.out.println(x==y); // true
    System.out.println(x.equals(y)); // true

    // case 4
    Integer p=128;
    Integer q=128;
    System.out.println(p==q); // false
    System.out.println(p.equals(q)); // true

    // case 5 -128 to 127 is in cached memory so they point to the same object without creating its instances.
    Integer s=50;
    Integer t=50;
    System.out.println(s==t); // true
    System.out.println(s.equals(t)); // true
}

13. Generics in Java.

14. isequals and hashcode in java by codingSimplified.

• isequals and hashcode in java by AnujBhaiya.

15. % operator in JAVA : dividend % divisor - Here two conditions are needed to be taken care of :

• When Divedend < Divisor then answer is dividend.
• When Divedend > Divisor then answer lie in between [0 to divisor-1].

16. Store 2 numbers in a number.

• To Inject B in A we ADD(+) B*INF in A and store it in A, where INF is any number greater than A and B.
• to extract old number i.e., A from modified A we do (A % INF).
• to extract new number(Injected number) i.e., B from modified A we do (A / INF).

int a=5;
int b=4;// number to be injected
int INF=9999;
a=a+(b*INF);
System.out.println(a%INF); // 5
System.out.println(a/INF); //4

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Some Important basic concepts of Java by Kunal Kushwaha's Bootcamp

1. Memory Allocation when you initialize a variable.

Note: When there is no reference variable pointing to the actual object then it is deleted automatically when garbage collection hits.

2. Objects & Reference Variable.

public class Demo{ 
    public static void main(String[] args) {
        Human obj=new Human();
        Human son=obj; // this is his mom who calls obj as hisSon.
        Human bro=obj; // this is his sister who calls obj as hisBro.
        son.hairCut="done"; // here his mom told him to get a haircut.
        // here his sister is also able to see his haircut.
        System.out.println(bro.hairCut); // done
    }
}
class Human{
    String name="Vikash";
    String hairCut="notDone";
}