Coding-Jag: July 2022

References stored in STACK

References are also, stored in HEAP

SWAP

Below code will swap the references.

Below code will swap the properties of p1 and p2.

Static and Non static data members

a2.roi ne a1.roi ki value bhi change kr de?

becoz. roi is static variable.

Also, a1.roi=5 or a2.roi=6 is a wrong way, we should declare the static variables with class name.

BankAccount.roi=5;

Putting data members and functions into an entity (same class) is called encapsulation

OOPS: Real-world entities:

does activities -> functions
has features -> data members

Static Data Members

are shared by all objects.
belongs to classes, not objects.

Static Data Functions

such functions which don't require object for their invocation, can be called directly via class name.

These are used as follows:

used in utility stateless [sara input parameters me mngwa lete hain] functions, eg. Math.min(), Arrays.sort()

stateless - nothing is saved b/w 2 function calls, each call is independent and takes all input as parameters

Implications

this keyword can't be used in static functions

becoz static functions can be called by class name, in that case "this" won't know which instance to point to

non-static data members can't be used in static functions

becoz it can be called by class name, and in that case we don't know if any object exists
if they do, we don't know which objects non-static data members to use.

non-static function can't be used in static functions

can be used, by making an object inside static function, and calling non-static function on it.
can't be called directly

static void myStatic(){

// not allowed

nonStatic();

// allowed

MyClass myClass = new MyClass();

myClass.nonStatic();

}

Can non-static function, use static data member? Why?

yes, becoz, we have extra information available (not less)

Can non-static function, use static function? Why?

same as above

==========================================================================

Polymorphism and Inheritance

Compiler watches left hand side i.e. reference

Runtime watches RHS i.e. instance

CASE 1: P obj = new P();

===========================================================================

CASE 2: C obj = new C();

Kya kya allowed hoga object par woh depend hoga reference par

Kya actually chalet woh depend hoga instances par

Compiler watches LHS i.e. reference

Runtime watches RHS i.e. instance

In case of conflict, resolved by reference.

Imp:

1. obj.d

2. ((P)obj).d

===========================================================================

CASE 3: P obj = new C();

obj.d2 and obj.fun2() are not allowed, as compiler will see the reference, and during compile time d2 and fun2() are not present in P

Also, when we do typecasting, it is applicable on reference type. Becoz of which ((C)obj).d2 and ((C)obj).fun2() are allowed, as during compile time we see reference of C which has both d2 and fun2().

===========================================================================

CASE 4: C obj = new P();

*************************************************************************************

OBJECT CREATION

Static Block

Static Block is called after parsing and before default constructor.

NON STATIC INNER CLASS

Here we have non static inner class Person in Main class. And its object is created by creating object of Main class. hence, its not a good practice.

*************************************************************************************

Inheritance and Polymorphism

Runtime Polymorphism: same reference se do karma ho rhe hain.

1st P obj = new P();

2nd P obj = new C();

cons value call Hong yeh depend krega runtime pr konsa object hai

Compile Time Polymorphism: same name methods with diff parameters

Copy Constructor

to create the copy constructor

Student s2 = new Student(s1);

its line se done ke apne apne contructor call ho jaenge.

*************************************************************************************

Final

Final Variable: reassignment is not allowed

Final Data member: either assigned in parsing or constructor/ static block

Final Function:

can't override the function in derived class
overloading is allowed in same class and derived class

Final Class:

can't derive the final class

*************************************************************************************

Abstract

Abstract Class:

can't make object of abstract class

Employee class

-> HR_Employee

-> Software_Employee

instance of Employee class derived into HR_Employee or Software_Employee. Hence, Employee classs should be abstract,

- as it is created only to give functionality

- not a type in itself

- only created to absorb common functions of HR_Employee and Software_Employee

Abstract function:

which don't have code (only API)
has ony parameters and return type

IMPLICATIONS

Why required?

to absorb common code of deriving classes

If function is abstract, will containing class be necessarily abstract?

yes, agr iss class ka object bnn gya and uske object ne vhi function call kr diya, toh kya chalega??

Hence, class will be abstract.

If class is abstract, will it necessarily have abstract functions?

OBJECT CREATION USING REFLECTION: without using new keyword

Questions

Q1. What is final variable?

Q2. What is final data member?

Q3. When are they initialized?

Q4. What is final fn? -> Can't override them in derived class

Q5. What is a final class? -> Can't derive the final class

Q6. What is abstract class? -> Can't make object

Q7. What is abstract fn? -> A fn with no body.

Q8. Can an abstract class have 0 abstract fns? -> Yes

Q9. Can a class having abstract fns be marked non-abstract? -> False. Have to mark the class abstract or provide implementations for all abstract fns

Q10. Ctor in abstract class? -> allowed

Q11. Why abstract fn? -> TO enforce the deriving classes to provide implementations

Q12. Why abstract class? -> To represent the common concept and absorb common functionality or data members.

*************************************************************************************

Interfaces

When all fns are abstract and there are no common instance data members to share

Similar to abstract classes

An extreme version of abstract classes

when all fns are abstract

no common instance data members

If a class implements an interface but doesn't provide body for all functions, then it will become abstract

Can we make an object of interface? No

Can an interface provide body for any function? Yes, since Java8, using the default keyword it is possible (although advised to not use). Java provided for this backword compatibility. Not to be used often.

Can an interface have data members? Yes, but not as instance data members, can be used as static data members

ABSTRACT CLASS vs INTERFACE

Abstract classes can have implementations for some functions, interfaces are supposed to have all functions with no implementations

Abstract classes can have instance data members, interfaces can only have static data members

A class can't derive two abstract classes, but can implement any number of interfaces

Interface data members are final and static

package pep.Day86;

public class LeetCode_416_Partition_Equal_Subset_Sum {
    public static void main(String[] args) {
        int[] nums = {1, 5, 11, 5};
        System.out.println(canPartition(nums));
    }

    public static boolean canPartition(int[] nums) {
        int sum = 0;
        int n = nums.length;
        for (int num : nums) {
            sum += num;
        }

        if (sum % 2 != 0 || n == 1) {
            return false;
        }

        return isSubsetSum_1D(n, nums, sum / 2);
    }

    // METHOD 1- Using 2D Array
    public static boolean isSubsetSum(int n, int arr[], int sum) {
        boolean[][] dp = new boolean[n + 1][sum + 1];

        for (int i = 0; i <= n; i++) {
            for (int tar = 0; tar <= sum; tar++) {
                if (tar == 0)
                    dp[i][tar] = true;
                else if (i == 0)
                    dp[i][tar] = false;
                else {
                    // sirf isse case me element ko consider krenge
                    if (tar - arr[i - 1] >= 0) {

                        // item ko nhi lete hain
                        if (dp[i - 1][tar])
                            dp[i][tar] = dp[i - 1][tar];

                            // item ko lete hain
                        else if (dp[i - 1][tar - arr[i - 1]])
                            dp[i][tar] = dp[i - 1][tar - arr[i - 1]];

                    } else {
                        // exclude wala case
                        dp[i][tar] = dp[i - 1][tar];
                    }
                }
            }
        }

        return dp[n][sum];
    }






    // METHOD 2- Using 1D Array
    public static boolean isSubsetSum_1D(int n, int arr[], int sum) {
        boolean[] dp = new boolean[sum + 1];

        dp[0] = true;
        for (int num : arr) {
            for (int tar = sum; tar > 0; tar--) {
                if (tar - num >= 0)
                    dp[tar] = dp[tar - num] || dp[tar];
            }

            if (dp[sum] == true)
                return dp[sum];
        }

        return dp[sum];
    }


}

Coding-Jag

Friday, July 8, 2022

System design - OOPs

SWAP

Static and Non static data members

Static Data Members

Static Data Functions

Polymorphism and Inheritance

OBJECT CREATION

Inheritance and Polymorphism

Copy Constructor

Final

Abstract

OBJECT CREATION USING REFLECTION: without using new keyword

Saturday, July 2, 2022

Leet Code 416. Partition Equal Subset Sum

Subset Sum Problem

Diagonal Traversal