Forem: Deepikandas

#26 Tricky questions in Arrays-JAVA

Deepikandas — Tue, 07 Apr 2026 01:57:18 +0000

1.Default values assigned
Arrays are the only objects even though inside method,default values are given


public class Arrays {
    public static void main(String[] value) {
        int[] arr = new int[3];
        System.out.println(arr[0] + " " + arr[1] + " " + arr[2]);
    }

}
Output:
0 0 0

2.ArrayIndexOutOfBoundsException

public class Arrays {
    public static void main(String[] value) {
        int[] arr = new int[3];
        System.out.println(arr[3]);

    }

}
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3
    at Module4/arrays.Arrays.main(Arrays.java:6)

4.Demonstrating Array Reference Behavior:

public class Arrays {
    public static void main(String[] value) {
        int[] a = {1, 2, 3};
        int[] b = a;
        b[0] = 100;
        System.out.println( "value of []a:"+a[0]);
    }

}
Output:
value of []a100

5.a and b point to different array objects, even though they have the same elements

public class Arrays {
    public static void main(String[] value) {
        int[] a = {1, 2, 3};
        int[] b = {1, 2, 3};
        System.out.println(a);
        System.out.println(b);
        System.out.println(a == b);
    }

}
output:
[I@65b3120a
[I@6f539caf
false

6.NullPointerException


public class Arrays {
    public static void main(String[] value) {
        int[] arr = null;
        System.out.println(arr.length);
    }

}
Exception in thread "main" java.lang.NullPointerException: Cannot read the array length because "arr" is null
    at Module4/arrays.Arrays.main(Arrays.java:6)

7.NegativeArraySizeException


public class Arrays {
    public static void main(String[] value) {
        int[] arr = new int[-5];
    }

}
Exception in thread "main" java.lang.NegativeArraySizeException: -5
    at Module4/arrays.Arrays.main(Arrays.java:5)

#25 Known is a drop! Arrays in Java

Deepikandas — Tue, 07 Apr 2026 01:43:33 +0000

Arrays:
A collection of elements of the same type stored under a single name, accessible by numeric indices, with a fixed size.
Key Points About Arrays
Fixed size:
Once created, the number of elements cannot change.

Example: int[] numbers = new int[5]; → can only hold 5 integers.
Same type elements:
All elements must be of the same data type (e.g., int, String, boolean).
Indexed access:
Elements are accessed using indices, starting from 0.
Example: numbers[0] → first element, numbers[4] → fifth element.
Declaration syntax:
type[] arrayName; // preferred style
type arrayName[]; // also valid
Initialization examples:
// Using 'new' keyword
int[] nums = new int[5];

// Using array literal
int[] nums2 = {1, 2, 3, 4, 5};
Memory layout:
Arrays are contiguous in memory, which allows fast index-based access.
Multidimensional arrays:
int[][] matrix = {
{1, 2, 3},
{4, 5, 6}
};
Access element: matrix[0][2] → 3
Looping through Array :


    for(int i = 0; i < arr.length; i++)
    {
        System.out.println(arr[i]);
    }

Is Array an Object?
When you create an array, Java allocates it on the heap.
The array variable actually holds a reference (like a pointer) to that object.
Because it’s an object, it inherits methods from java.lang.Object, such as:
hashCode()
toString()
clone()
getClass()
Behind the scenes:
int[] numbers = {1, 2, 3};
This is called an array initializer (or array literal).
Even though you didn’t explicitly write new int[3], Java implicitly creates an array object on the heap behind the scenes.
So numbers still holds a reference to an array object.

//getClass(); instanceof
package arrays;

public class Arrays {
    public static void main(String[] value) {
        int arr[]= {23,89,90,78};
        System.out.println(arr.getClass());
        System.out.println(arr instanceof Object);
    }

}
Output:
class [I
true

What is a runtime class?

The runtime class is the actual class of the object in memory at runtime.
It may be a standard Java class, your own user-defined class, or even an array type. 2️⃣ Examples of runtime classes that are NOT predefined User-defined class

class Student { }

Student s = new Student();
System.out.println(s.getClass()); // class Student
Student is not a predefined Java class—it’s created by the programmer.

Runtime class is Student.
Array objects

int[] numbers = {1, 2, 3};
System.out.println(numbers.getClass()); // class [I
[I → array of integers

There is no class literally named [I in your code.
Arrays are runtime objects created by the JVM, automatically

#24 known is a drop! main method in JAVA

Deepikandas — Sat, 04 Apr 2026 21:43:11 +0000

It's the entry point for your application and will subsequently invoke all the other methods required by your program.

Modifiers in main method:
The modifiers public and static can be written in either order (public static or static public)
public → JVM must access it from outside the class.
If you make it non-public, JVM cannot access it, and you’ll get an error.
static → JVM can call it without creating an object of the class.
Return type of main()
main method signature is void
It does not return anything to the JVM directly.
That’s why the return type is void.

Returning a value to the OS
When you call:
System.exit(0);

String args[]

The main method accepts a single argument: an array of elements of type String.
Each string in the array is called a command-line argument.
These are used to give input during run time.
Actually not during run time, before the program runs itself ,we give input.

All syntax are correct:

public static void main(String... args)
public static void main(String[] value)
static public void main(String[] args)

In cmd prompt:
We have to give values to command line arguments at the time of run.
eg: java filename input1 input2

🟩input1 is stored in args[0],
input2 is stored in args[1].

🟩all inputs are taken in the form of Strings.

To parse the string values:
🟩 if we want to perform any arithmetic operations,
we have to parse the value using Integer.parseInt(String args[0])
i.e., converting String to primitive int type

🟩 Same for other float values, boolean. We have to parse using corresponding Wrapper class parse method.

*In Eclipse * Right click->Run configurations->select Arguments tab-> enter values separated by space.(not commas in between)

*How Scanner class differs from command line arguements? * If we use Scanner class, we can inform the user to enter values as inputs are fed during run time
but using command line args, we cannot prompt.

Can main be overloaded?
✅ Yes. You can define multiple main methods with different parameters in the same class.
JVM behavior:
The JVM only automatically calls public static void main(String[] args) as the program entry point.
Overloaded main methods are not called automatically; they must be called explicitly from the standard main or elsewhere.

public static void main(String[] args) { }      // standard entry
public static void main(int number) { }         // overloaded
public static void main(String message) { }     // overloaded
public static void main(int a, int b) { }       // overloaded

main Can Be in Multiple Classes:

Each class can have its own main method.
You can choose which class to run from the command line.

#23 Known is a Drop! Nested for loop in JAVA with examples-

Deepikandas — Fri, 03 Apr 2026 13:18:11 +0000

What is a Nested for Loop?

A nested loop means:
👉 A for loop inside another for loop
Outer loop → controls rows
Inner loop → controls columns/repetitions inside each row

Example 1:
Print
1 1 1 1 1
1 1 1 1 1


public class NestedForLoop {

    public static void main(String[] args) {
        for(int row=1;row<=2;row++) {
            for(int column=1;column<=5;column++) {
                System.out.print(1+" ");
            }
            System.out.println();
        }

    }

}
Output:
1 1 1 1 1 
1 1 1 1 1

Example 2: Print
1 1 1 1 1
2 2 2 2 2

package nestedForLoop;

public class NestedForLoop {

    public static void main(String[] args) {
        for(int row=1;row<=2;row++) {
            for(int column=1;column<=5;column++) {
                System.out.print(row+" ");
            }
            System.out.println();
        }

    }

}
**Output:**
1 1 1 1 1 
2 2 2 2 2

Example 3:Print
1 2 3 4 5
1 2 3 4 5

public class NestedForLoop {

    public static void main(String[] args) {
        for(int row=1;row<=2;row++) {
            for(int column=1;column<=5;column++) {
                System.out.print(column+" ");
            }
            System.out.println();
        }
    }
}
Output:
1 2 3 4 5 
1 2 3 4 5

Example 4:Print
1 2 3 4 5 6
1 2 3 4 5 6
1 2 3 4 5 6
1 2 3 4 5 6

public class NestedForLoop {

    public static void main(String[] args) {
        for(int row=1;row<=4;row++) {
            for(int column=1;column<=6;column++) {
                System.out.print(column+" ");
            }
            System.out.println();
        }
    }
}
output:
1 2 3 4 5 6 
1 2 3 4 5 6 
1 2 3 4 5 6 
1 2 3 4 5 6

Example 5:
print
& & & & &

& & & & &

& & & & &

public class NestedForLoop {

    public static void main(String[] args) {
        for(int row=1;row<=3;row++) {
            for(int column=1;column<=5;column++) {
                System.out.print("&  ");
            }
            System.out.println();
        }
    }
}

output:
& & & & &   
& & & & &                  
& & & & &

Example 6:
Print
& & & & &

& & & &

& & &
& &
&

public class NestedForLoop {

    public static void main(String[] args) {
        for(int row=5;row<=5&& row!=0;row--) {
            for(int column=1;column<=row;column++) {
                System.out.print("&  ");
            }
            System.out.println();
        }
    }
}

& & & & &   
& & & &                 
& & &
& &
&

//Using count variable;
public class NestedForLoop {

    public static void main(String[] args) {
        int count=5;
        for(int row=1;row<=5;row++) {
            for(int column=1;column<=count;column++) {
                System.out.print("*  ");
            }
                count--;

            System.out.println();
        }
    }
}

Example 7:
Print
&
& &

& & &

& & & &
& & & & &

public class NestedForLoop {

    public static void main(String[] args) {
        int count=1;
        for(int row=1;row<=5;row++) {
            for(int column=1;column<=count;column++) {
                System.out.print("*  ");
            }
                count++;

            System.out.println();
        }
    }
}

#23 Known is a drop! Scanner class JAVA

Deepikandas — Tue, 31 Mar 2026 12:29:15 +0000

Scanner class
The Scanner class is used to get user input, and it is found in the java.util package.
Purpose:
1. Read user input (keyboard)

Works with System.in
Example: nextInt(), nextLine()

2. Read data from files

Can read text files line by line

3. Read different data types

int → nextInt()
double → nextDouble()
String → next(), nextLine()
boolean → nextBoolean()

4. Parse/split input
Parsing = breaking a big input into small pieces (tokens)
Tokens can be:
words
numbers
symbols
When a user types:John 25 3.75
It’s just a line of text to Java.
We need a way to separate it into meaningful pieces:

"John" → String
25 → int
3.75 → double

Scanner does this automatically.
Input Types:

import java.util.Scanner;

public class ScannerInput {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter Your name : ");
        String  name= sc.nextLine();
        System.out.println("Enter the Tamil mark: ");
        int tamil=sc.nextInt();
        System.out.println("Enter the English mark: ");
        int english=sc.nextInt();
        System.out.println("Enter the Science mark: ");
        int sci=sc.nextInt();
        System.out.println("Enter the Math mark: ");
        int math=sc.nextInt();
        System.out.println("Your average 345mark is "+(tamil+english+sci+math)/4);

    }

}
output:
Enter Your name : 
K
Enter the Tamil mark: 
67
Enter the English mark: 
89
Enter the Science mark: 
90
Enter the Math mark: 
90
Your average 345mark is 84

Why Scanner needs System.in?
Scanner sc = new Scanner(System.in);
Scanner’s purpose
Scanner is a high-level utility class in java.util.
It can read tokens, lines, integers, doubles, etc., from a source.
It handles all the parsing and splitting of input for you.
Data source requirement
Scanner needs something to read from:
Could be a file → new Scanner(new File("data.txt"))
Could be a string → new Scanner("1 2 3")
Could be the keyboard → new Scanner(System.in)

close():
Purpose
Frees system resources used by a resource (like file, stream, or scanner).
Stops further use of that resource.
When to use
Files, network streams, database connections: Always close (or use try-with-resources) to avoid real resource leaks.
Scanner(System.in): Usually don’t close, because closing also closes System.in and you can’t read from the keyboard anymore.
Effects of closing
Releases memory and system handles.
After closing, calling methods on the resource can throw exceptions (IOException, IllegalStateException).

#22 Known is a Drop! for loop in java with examples -Part 1

Deepikandas — Sat, 28 Mar 2026 18:20:57 +0000

Definition
for Loop is a control statement used to repeat a block of code multiple times.
Typically used when number of iterations is known.
syntax:
for (initializer; condition; updater) {
//body
}
Initialization → set starting value
Condition → loop continues while true
Update → executed after each iteration
Allowed variations:

Example 1: Printing natural numbers

Example 2: Printing odd numbers

public class sample {

    public static void main(String[] args) {
        for (int i = 1; i <= 10; i++) {
            if(i%2!=0)
            System.out.print(i + "  ");
        }
    }

}
output:
1  3  5  7  9

Example 3: Printing multiples of 3

public class sample {

    public static void main(String[] args) {
        for (int i = 1; i <= 15; i++) {
            if(i%3==0)
            System.out.print(i + "  ");
        }
    }
}
Output:
3  6  9  12  15

Example 4: Multiples of 3 and 5


public class sample {

    public static void main(String[] args) {
        for (int i = 1; i <= 50; i++) {
            if(i%3==0&& i%5==0)
            System.out.print(i + "  ");
        }
    }
}
output:
15  30  45

Example 5: Multiples of 3 or 5

public class sample {

    public static void main(String[] args) {
        for (int i = 1; i <= 20; i++) {
            if(i%3==0|| i%5==0)
            System.out.print(i + "  ");
        }
    }
}
Output:
3  5  6  9  10  12  15  18  20

Example 5:Divisors and number of divisors of given number

public class sample {

    public static void main(String[] args) {
        int givenNumber=63;
        int count=0;
        for (int i = 1; i <= givenNumber; i++) {
            if(givenNumber%i==0) {
            System.out.print(i + "  ");
            count++;
            }
        }
        System.out.println();
        System.out.println(" Number of divisors:  "+count);
    }
}
Output:
1  3  7  9  21  63  
 Number of divisors:  6

Example 6: Prime numbers

public class ForLoopPrimeNumber {

    public static void main(String[] args) {
        int input=997;
        if(input<=0)
            System.out.println(input+" is Not a prime number ");
        for(int i=2;i<input;i++) {
            if(input%i==0)
                {
                System.out.println(input+" is Not a prime number ");
            break;
                }
            else
            {
                System.out.println(input+" is  a prime number ");
                break;
            }
                    }
    }

}
output:997 is  a prime number

Example 7: Reverse a positive number

    public static void main(String[] args) {

        int givenNumber=987654321;
        int temp=givenNumber;
        int reverse=0;
        for(    ;givenNumber>0; givenNumber=givenNumber/10 ) 
        {
            int getLastDigit=givenNumber%10;//4
            reverse=reverse*10+getLastDigit;

        }
        System.out.println("The reverse of "+temp+"  is  "+reverse);
    }

}
Output:
The reverse of 987654321  is  123456789

Example:8 Count number of digits and sum of digits

public class CountDigitsForLoop {
    public static void main(String[] args) {
        int givenNumber=16789;
        int temp=givenNumber;
        int sum=0;
        int count=0;
        for( ; givenNumber>0; givenNumber=givenNumber/10 ) {
            int getLast=givenNumber%10;
            sum=sum+getLast;
                    count++;
        }
        System.out.println("The number of digits in "+temp+" is "+count);
        System.out.println("The sum of digits in "+temp+" is "+sum);
    }
}
Output:
The number of digits in 16789 is 5
The sum of digits in 16789 is 31

Example 9:Finding whether the number is palindrome or not

public class PalindromeForLoop {

    public static void main(String[] args) {
        int givenNumber=13318;
        int temp=givenNumber;
        int isPalindrome=0;
        for(temp=givenNumber;  temp>0;  temp=temp/10  ) {
            int lastDigit=temp%10;
            isPalindrome=isPalindrome*10+lastDigit;             
        }
        if(isPalindrome==givenNumber)
            System.out.println(givenNumber+" is a palindrome number");
        else
            System.out.println(givenNumber+" is NOT  a palindrome number");
    }

}
output:
13318 is NOT  a palindrome number

#21 While Loop in JAVA with examples

Deepikandas — Sat, 28 Mar 2026 01:03:01 +0000

The while statement is a control flow statement.
It continually executes a block of statements while a particular condition is true.

*syntax *

while (expression) {
statement(s)
}
A while loop becomes infinite when the condition never becomes false
🔥 Common Causes
Missing update
while (i < 5) { } → i never changes
Wrong update direction
while (i < 5) { i--; }
Always true condition
while (true) { }
Assignment instead of comparison
while (flag = true) { }
Variable not modified
while (num > 0) { }
Condition never reachable
while (i != 10) {
i += 2;
}

Example 1:Print 1 1 1 1 1

public class Sample {
    public static void main(String[] args) {
        int i = 0;
        while (i < 5) {
            System.out.print(1 + " ");
            i++;
        }
    }
}
Output:
1 1 1 1 1

Example 2:Printing 1 2 3 4 5

public class Sample {
    public static void main(String[] args) {
        int i = 1;
        while (i<=5) {
            System.out.print(i + " ");
            i++;
        }
    }
}

Example 3:Printing odd number 1 3 5 7 9

public class Sample {
    public static void main(String[] args) {
        int i = 1;
        while (i<=10) {
            if(i%2!=0) {
            System.out.print(i + " ");
            }
            i++;
        }
    }
}
Output:1 3 5 7 9

Example 4:Printing 3 6 9 12 15

public class Sample {
    public static void main(String[] args) {
        int i = 1;
        while (i<=15) {
            if(i%3==0) {
            System.out.print(i + "  ");
            }
            i++;
        }
    }
}
Output:
3  6  9  12  15

Example 5: Multiples of 3 and 5

public class Sample {
    public static void main(String[] args) {
        int i = 1;
        System.out.print("Multiples of both 3 and 5 are ");
        while (i<=50) {
            if(i%3==0 && i%5==0) {
            System.out.print(i + " , ");
            }
            i++;
        }
    }
}
Output:
Multiples of both 3 and 5 are 15 30 45

Example 6: Multiples of 3 or 5

public class Sample {
    public static void main(String[] args) {
        int i = 1;
        System.out.print("Multiples of both 3 and 5 are ");
        while (i<=20) {
            if(i%3==0 || i%5==0) {
        System.out.print(i + "  ");
            }
            i++;
                }
    }
}

Output:
Multiples of both 3 and 5 are 3  5  6  9  10  12  15  18  20

Example 7: Finding All Divisors and number of divisor of a given Number


public class WhileExample {

    public static void main(String[] args) {
        int user=15;
        int i=1;
        byte count=0;
        while(i<=user)
        {
            if(user%i==0) {
            System.out.println(i);
            count++;
                    }
        i++;
    }
        System.out.println("Number of divisors "+count);
}
}
Output:
1
3
9
27
Number of divisors 4

Example 8: Finding whether a number is prime number or not.
Note:Negative numbers,0 and 1 cannot be prime.
Prime number is a positive integer greater than 1 that has exactly two positive divisors: 1 and itself

public class PrimeNumber {

    public static void main(String[] args) {
    int userInput=9976;
        int i=2;
    boolean status=true;
    if(userInput<=0) {
        status=false;
    }
    while(i<userInput) {    
        if(userInput%i==0) {
            status=false;
            break;
        }
        i++;
    }
    if(status==true)
    {
        System.out.println(userInput+"  number is  prime");
    }
    else
        System.out.println(userInput +" number is not prime number");
    }

}

Output:
9976 number is not prime number

Example 9: Reversing Positive integer

public class ReverseNumber {

    public static void main(String[] args) {
        int userInput=123456789;
        int reverse=0;

while(userInput>0) {
            int lastDigitGet=userInput%10;
            reverse=reverse*10+lastDigitGet;
            userInput=userInput/10;

        }
System.out.println("Reversed number is "+reverse);
    }

}
Output:
Reversed number is 987654321

plaintext
Example 10: Counting number of digits in a positive integer

public class NumberOfDigits {

    public static void main(String[] args) {
        long userInput=12345678901234567L;
        int count=0;
        while(userInput>0) {
                count++;
            userInput=userInput/10;

        }
        System.out.println("Number of digits : "+count);
    }

}
Output:
Number of digits : 17

plaintext
Example 11:Sum of digits in a positive integer


public class SumOfDigits {

    public static void main(String[] args) {
    int givenNumber=123456789;
    int sum=0;
    while(givenNumber>0) {
        int getLastDigit=givenNumber%10;
        sum=sum+getLastDigit;
        givenNumber=givenNumber/10;
    }
    System.out.println("Sum of digits: "+sum);
    }

}
Output:
Sum of digits: 45

plaintext
12.To find whether a number is palindrome or not

public class PalindromeNumber {

    public static void main(String[] args) {
    int givenNumber=906609;
    int temp=givenNumber;
    int reverse=0;
    while( temp>0) {
        int getLastDigit=temp%10;
        reverse=reverse*10+getLastDigit;
        temp=temp/10;
        }
    if(reverse==givenNumber)
    System.out.println(givenNumber+" is a palindrome number");
    else
        System.out.println(givenNumber+" is NOT a palindrome number");
    }
OUTPUT:
906609 is a palindrome number

#20 Known is a drop! Switch statement & Switch Expression JAVA-Tricky Questions

Deepikandas — Wed, 25 Mar 2026 20:59:24 +0000

1.Traditional Switch Statement (Java ≤ 16)

Purpose: Execute code blocks
Syntax: Uses : after case
Break: Required to prevent fall-through
Return value: ❌ Cannot return
Multi-label: ❌ Not allowed (before Java 14)
Null: ❌ Not allowed for objects (NullPointerException)
2.Enhanced Switch Expression (Java 14+)
Purpose: Produces a value (can be returned/assigned)
Syntax: Uses -> (arrow)
Break: ❌ Not needed
Multi-label: ✅ Allowed (case 1,2,3 -> ...)
Null: ✅ Allowed (case null -> ...) in Java 17+
Multi-statement block: ✅ Use { ... yield ... }
3.Switch Expression with Pattern Matching (Java 17+)

-case Type variable → automatically checks type & assigns variable
-Can combine pattern matching + multi-label + null + yield.

Safer and cleaner than instanceof + cast
Works with switch expressions only (not old style) 4.Multi-statement block in Expression

Use yield to return a value from a block
All branches must produce same type (String here)

1.public class Sample {
    public static void main(String[] args) {
        int x = 2;
        switch(x) {
            case 1:
           System.out.println("One");
           break;
            case 2:
             System.out.println("Two");
              break;
            case 3:
            System.out.println("Three");
            break;
            default:
            System.out.println("Default");
                   }
}
}
output: Two

//After java 8th version switch case :
2.public class Sample {
    public static void main(String[] args) {
    int x = 2;
    switch(x) {
    case 1 -> System.out.println("Hi");
    case 2 -> System.out.println("Welcome");
    default -> System.out.println("Hello");
        }//no need of semicolon as switch expression is not used
}
}
output: Welcome

public class Sample {

    public static void main(String[] args) {
        int x = 2;
        int result = switch(x) {
            case 1 -> 10;
            case 2 -> 20;
            default -> 0;
        };//semicolon must
        System.out.println(result);
}
}
output: 20

public class Sample {

    public static void main(String[] args) {
        int x = 1;
        int result = switch(x) {
            case 1 -> {
                System.out.println("One");
            }
            default -> 0;
        };

}
}

**Compile time error:**
A switch labeled block in a switch expression must yield a value or throw an an exception
**Reason:**case 1-> should return a value like case 1->1
without using curly braces.
When curly braces used,"yield" key word is used.
eg: case 1 -> {             System.out.println("One");
yield 1;
    }
NOte:yield must be end of the case1 block.

//After java 14th version switch case :Multiple labels
public class Sample {
    public static void main(String[] args) {
        int x = 3;
        int result = switch(x) {
            case 1, 2 -> 10;
            case 3 -> 20;
            default -> 0;
        };
System.out.println("value of result: "+result);
}
}
Output: value of result: 20

After java 17th version switch case(Java 17+ (Pattern matching switch case) :
public class Sample {
    public static void main(String[] args) {
        Object obj = "Hello";
        switch(obj) {
    case String s -> System.out.println(s.length());
    case Integer i -> System.out.println(i * 2);
    default -> System.out.println("Other");
        }
}
}
output: 5

//After java 17th version switch case(Java 17+ (Pattern matching switch case) :
public class Sample {
    public static void main(String[] args) {
        Object obj = "Hello";
        switch(obj) {
            case String s -> System.out.println("length of string : "+s.length());
            case Integer i -> System.out.println(i * 2);
            default -> System.out.println("Other");
        }           
        }
}
output:length of string : 5

public static void main(String[] args) {
        int x = 2;
        switch(x) {
            case 1:
                System.out.println("One");
            case 2:
                System.out.println("Two");
            case 3:
                System.out.println("Three");
            default:
                System.out.println("Default");
        }
output:(Fall through)
Two
Three
Default

char ch = 'A';
switch(ch) {
    case 65:
        System.out.println("ASCII 65");
        break;
    case 'A':
        System.out.println("Character A");
}

Compile error: 
 duplicate case label
                case 'A':

public class Sample {
    public static void main(String[] args) {
        int x = 10;
        switch(x / 2) {
            case 5:
                System.out.println("Five");
                break;
            case 10:
                System.out.println("Ten");
        }
        }
}
output: Five

//final makes it constant → allowed in case
public class Sample {
    public static void main(String[] args) {
final int a = 1;
        int b = 2;
        switch(b) {
            case a:
                System.out.println("A");
                break;
            case a + 1:
                System.out.println("A+1: "+a);
        }

        }
}
output: A+1: 1

Case labels are case sensitive
public class Sample {
    public static void main(String[] args) {
        String s = "Java";
        switch(s) {
            case "Java":
                System.out.println("Java");
                break;
            case "JAVA":
                System.out.println("JAVA");
                break;
        }

        }
}
output:
Java

//Switch case label should be null to handle null pointer exception when null is used.
public class Sample {
    public static void main(String[] args) {
        String s = null;
        switch(s) {
            case "Hi":
                System.out.println("Hi");
                break;
            default:
                System.out.println("Default");
        }


        }
}
output:
Nullpointer exception

int x = 1;
switch(x) {
    case 1:
        System.out.println("One");
        break;
    case 1:
        System.out.println("Duplicate");
}
compile time error: Duplicate case

final int a = 2;
switch(2) {
    case a:
        System.out.println("A");
        break;
    case 2:
        System.out.println("Two");
}

compile time error: Duplicate case

public class Sample {
    public static void main(String[] args) {
        int x = 1;
        switch(x) {
            case 1:
                switch(x + 1) {
                    case 2:
                        System.out.println("Inner Two");
                }
            case 2:
                System.out.println("Outer Two");
        }
        }
}
output:
Inner Two
Outer Two

public class Sample {
    public static void main(String[] args) {
        int x = 1;
        switch(x) {
            case 1:
                switch(x + 1) {
                    case 2:
                        System.out.println("Inner Two");

                }
                        case 4:
                System.out.println("Outer Two");
        }
        }
}
output: no break for case1 block
Inner Two
Outer Two

#19 Known is a Drop! Switch Statement in JAVA

Deepikandas — Wed, 25 Mar 2026 01:19:49 +0000

The switch Statement :
Unlike if-then and if-then-else statements, the switch statement can have a number of possible execution paths.
The body of a switch statement is known as a switch block.
A statement in the switch block can be labeled with one or more case or default labels.
The switch statement evaluates its expression, then executes all statements that follow the matching case label.

1. Classic Switch (Java 1.0 – 6)

Uses switch(expression) and case value: blocks.
Must use break to prevent fall-through.
Only supports int, byte, short, char (no String yet).
Fall-through occurs if break is omitted.
default can be anywhere inside Switch block -break statement in default is mandatory when default code is written in the middle of switch block.

2.Using Wrapper Classes in Switch:
Classic switch only allowed primitives (int, byte, char, short).
After Java 5, because of autoboxing, you can use wrapper classes like Integer, Byte, or Character in switch.
The compiler unboxes the object into the corresponding primitive internally.

3. Switch with Strings (Java 7+)

4.Enhanced Switch in Java 12 Preview → Java 14 Standard
Arrow syntax (->):
Like all expressions, switch expressions evaluate to a single value and can be used in statements. They may contain "case L ->" labels that eliminate the need for break statements to prevent fall through.

5. Switch Expression Returning Value (Java 14+)

Switch can return a value.
Use yield to return values from a block. yield can be used only in switch block.

6.Datatypes used in Switch expression and Case labels

boolean cannot be used, use if-else instead.
long, float, double cannot be used, even their wrappers.

**
7.Case Null in switch -since Java:17
Before Java 17, case labeled with null was not allowed.
case null is allowed in both switch statement and switch expression.

Default statement must in switch block?**
1️⃣ In a traditional switch statement
default is optional.
If none of the case labels match and there is no default, the switch just does nothing and continues after the switch block.
2️⃣** In a switch expression (Java 14+)**
default is required only if the compiler can’t be sure all possible values are covered.
Why? Because a switch expression must return a value for every possible input. If the compiler sees that some inputs could be missing a value, it will throw an error.
Key Rule
✅ Switch statement → default is optional
❗ Switch expression → default is often required (must cover all cases)
when writing case null, default should be after case null, if it precedes ,it throws compilation error.
8.Using Switch directly in return statement:

9.Pattern Matching in Switch

Pattern matching allows a switch case to test both the type and value of an object and automatically declare a typed variable for the matched case.

Removes the need for explicit instanceof checks and casts.
Introduced in Java 17+ (preview in Java 16).

In Switch statement:

Object obj = "Hello";
switch (obj) {
case String s -> System.out.println(s.length());//s is automatically String
case Integer i -> System.out.println(i * 2);// i is automatically Integer
case null -> System.out.println("Null value");
default -> System.out.println("Other type");
}

Switch expression with pattern matching:
String result = switch (obj) {
case String s -> "Length: " + s.length();
case Integer i -> "Double: " + (i * 2);
case null -> "Null";
default -> "Other";
};
Returns a value (switch expression)
Can use yield in multi-line blocks:

#18 Known is a Drop! Conditional Statement-if--else in JAVA

Deepikandas — Tue, 24 Mar 2026 13:21:01 +0000

1. What is an if-else statement?
It is a decision-making statement used to execute code based on a condition (true/false).
2. What is the syntax of if-else?

if (condition) {
    // executes if true
} else {
    // executes if false
}

3. Can we use if without else?
Yes, else is optional.

4. Difference between if-else and switch?
if-else → works with ranges, conditions
switch → works with fixed values (int, String, enum)

5. What is an else if ladder?
if (marks > 90) {
System.out.println("A");
} else if (marks > 75) {
System.out.println("B");
} else {
System.out.println("C");
}

6.What is nested if?

if (age > 18) {
if (hasLicense) {
System.out.println("Can drive");
}
}
7. What happens if you miss curly braces {}?

if (true)
System.out.println("Hello");
System.out.println("World");
Output:
Hello
World
8. Can we write multiple conditions in if?
Yes
if (age > 18 && country.equals("USA")) {
System.out.println("Eligible");
}

9. What is the ternary operator (shortcut for if-else)?
int result = (a > b) ? a : b;

10.Can if condition be non-boolean in Java?
❌ No — must return boolean (true/false)

11.What will this output?
int x = 10;
if (x = 5) {
System.out.println("Hello");
}
❌ Compile-time error (condition must be boolean)
= is assignment not relational operator like ==

12.Floating Point Comparison Trap
Version1
double a = 0.1 + 0.2;

if (a == 0.3) {
System.out.println("Equal");
} else {
System.out.println("Not Equal");
}
Output:
Not Equal
value of a is value of a 0.30000000000000004
Due to precision issues in floating-point arithmetic
Version2:


package module3;

public class Sample {
    public static void main(String[] args) {
        float a = 0.1f + 0.2f;
        System.out.println("value of a  " + a);
System.out.printf("To print the actual value of a in memory with 10 precision values is %.10f ", a);
        System.out.println();
        if (a == 0.3) {
            System.out.println("Equal");
        } else {
        System.out.println("Not Equal  " + a);
        }

    }

}

output:
value of a 0.3
To print the actual value of a in memory with 10 precision values is 0.3000000119
Not Equal 0.3
Behind the scenes:
Why does System.out.println(a) show 0.3?

System.out.println() uses formatted output, not raw memory value.
It converts the float to a string
While converting, it rounds to a reasonable number of digits
So small precision errors are hidden.
Memory value ≠ Printed value
Java rounds for readability
Comparison (==) uses actual value, not printed value

13.OR (||) Short Circuit
int a = 5;

if (a < 10 || ++a < 20) {
System.out.println(a);
}
Behind the scene:
First condition is true → second NOT executed
Output: 5

14.AND (&&) Short Circuit

int a = 5;

if (a > 10 && ++a < 20) {
    System.out.println(a);
}

System.out.println(a);

Answer:
First condition is false → second is NOT executed
Output: 5

Tips:What is short-circuit evaluation?

In &&, if first condition is false → second is NOT checked
In ||, if first condition is true → second is NOT checked

#17 Known is a Drop! Abstraction in JAVA

Deepikandas — Tue, 17 Mar 2026 13:01:21 +0000

What is Abstraction?
Abstraction = hiding implementation details and showing only essential features.

How Java supports Abstraction

Abstract Classes
Interfaces

Abstract class:

An abstract class is a class that is declared abstract it may or may not include abstract methods.
Abstract classes cannot be instantiated, but they can be subclassed.
Abstract Method:
An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon)

Can fields be abstract?

❌ No.
In Java, only methods can be abstract.
Fields always have storage and type, so there’s no way to leave them “unimplemented.”
Can Abstract Class have Constructors?
Abstract class cannot be instantiated directly
Vehicle v = new Vehicle(100); // ❌ Compile-time error

Constructors are called when subclass is created
They initialize fields of the abstract class

Constructor types allowed:
Default constructor ✅
Parameterized constructor ✅
Cannot make constructor abstract

❌ abstract Vehicle() { } → ❌ Not allowed

Only methods can be abstract, not constructors

Subclasses of Abstract class:

So yes, each subclass writes its own definition for abstract methods.
When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class.
here ,abstract means “must override”
However, if it does not, then the subclass must also be declared abstract.
One-liner:
Abstract class = contract + blueprint → each subclass fulfills contract, but user code works on abstract type.

Access Modifiers in abstract Methods:

#16 Known is a Drop! Tricky Coding question in Method Overloading JAVA

Deepikandas — Mon, 16 Mar 2026 14:42:29 +0000

Cheatsheet before seeing Tricky Questions

Method Overloading Tricky Questions :
1.
class Test {
void show(int a) { }
void show(long a) { }
public static void main(String[] args) {
Test t = new Test();
t.show(10);
}
}
Answer: void show(int a) is called.

2.
class Test {
void show(Integer a) { }
void show(int a) { }
public static void main(String[] args) {
Test t = new Test();
t.show(10);
}
}
Answer: void show(int a) is called.

3.
class Test {
void show(int a, float b) { }
void show(float a, int b) { }
public static void main(String[] args) {
Test t = new Test();
t.show(10, 10);
}
}

Answer: compilation error:The method show(int, float) is ambiguous for the type Test
No float type arguments passed

4.
class Test {
void show(int... a) { }
void show(int a) { }
public static void main(String[] args) {
Test t = new Test();
t.show(10);
}
}
Answer: void show(int a) is called.

5.
class Test {
void show(int a) { }
void show(int... a) { }
public static void main(String[] args) {
Test t = new Test();
t.show();
}
}
*Answer: * void show(int... a) is called as varargs parameter methods accepts from 0 to many arguments

6.
class Test {
void show(double a) { }
void show(float a) { }
public static void main(String[] args) {
Test t = new Test();
t.show(10);
}
}
Answer:void show(double a)

7.
class Test {
void show(Object o) { }
void show(String s) { }
public static void main(String[] args) {
Test t = new Test();
t.show(null);
}
}
Answer: void show(String s) is called

8.
class Test {
void show(String s) { }
void show(Integer i) { }
public static void main(String[] args) {
Test t = new Test();
t.show(null);
}
}
Answer: compile error -The method show(String) is ambiguous for the type Test

9.
class Test {
void show(int a, int b) { }
void show(int... a) { }
public static void main(String[] args) {
Test t = new Test();
t.show(10,20);
}
}
Answer: show(int a, int b) called.

10.
class Test {
void show(int a) { }
static void show(int a) { }
public static void main(String[] args) {
Test t = new Test();
t.show(10);
}
}

Answer: Compilation error -duplicate methods with the same signature: show(int a).
Not return type/modifiers difference is considered

11.
class Test {
void show(long a) { }
void show(float a) { }
public static void main(String[] args) {
new Test().show(10);
}
}
Answer:void show(long a) called

12.
class Test {
void show(int a) { }
void show(int a, int... b) { }
public static void main(String[] args) {
new Test().show(10);
}
}
Answer:void show(int a) is called

13.
class Test {
void show(char c) { }
void show(int i) { }
public static void main(String[] args) {
new Test().show('A');
}
}
Answer:void show(char c) called

14.
class Test {
void show(byte b) { }
void show(short s) { }
public static void main(String[] args) {
byte b = 10;
new Test().show(b);
}
}
Answer:void show(byte b)

15.
class Test {
void show(Object o) { }
void show(String s) { }
void show(CharSequence c) { }
public static void main(String[] args) {
new Test().show("Java");
}
}

Answer:void show(String s)is called