1. if Statement

The if statement executes a block of code only if a specified condition is true.

int age = 18;

if (age >= 18) {
    System.out.println("You are eligible to vote");
}

// Output: You are eligible to vote

2. if-else Statement

The if-else statement provides an alternative block of code when the condition is false.

int marks = 45;

if (marks >= 50) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}

// Output: Fail

3. if-else-if Ladder

Use if-else-if to test multiple conditions in sequence.

int marks = 75;

if (marks >= 90) {
    System.out.println("Grade: A+");
} else if (marks >= 80) {
    System.out.println("Grade: A");
} else if (marks >= 70) {
    System.out.println("Grade: B");
} else if (marks >= 60) {
    System.out.println("Grade: C");
} else {
    System.out.println("Grade: F");
}

// Output: Grade: B

4. switch-case Statement

The switch statement is used when you need to compare a variable against multiple values.

int day = 3;
String dayName;

switch (day) {
    case 1:
        dayName = "Monday";
        break;
    case 2:
        dayName = "Tuesday";
        break;
    case 3:
        dayName = "Wednesday";
        break;
    case 4:
        dayName = "Thursday";
        break;
    case 5:
        dayName = "Friday";
        break;
    case 6:
        dayName = "Saturday";
        break;
    case 7:
        dayName = "Sunday";
        break;
    default:
        dayName = "Invalid day";
        break;
}

System.out.println(dayName);  // Output: Wednesday

5. Ternary Operator (?:)

The ternary operator is a shorthand for simple if-else statements.

Syntax: condition ? value_if_true : value_if_false

int age = 20;

// Using if-else
String result;
if (age >= 18) {
    result = "Adult";
} else {
    result = "Minor";
}

// Using ternary operator (shorter)
String result = (age >= 18) ? "Adult" : "Minor";
System.out.println(result);  // Output: Adult

// Another example
int a = 10, b = 20;
int max = (a > b) ? a : b;
System.out.println("Maximum: " + max);  // Output: Maximum: 20

1. for Loop

Use the for loop when you know exactly how many times you want to iterate.

Syntax: for (initialization; condition; update) { ... }

// Print numbers 1 to 5
for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}
// Output: 1 2 3 4 5

// Calculate sum of first 10 numbers
int sum = 0;
for (int i = 1; i <= 10; i++) {
    sum += i;
}
System.out.println("Sum: " + sum);  // Output: Sum: 55

2. while Loop

Use the while loop when you don't know in advance how many times to iterate. The condition is checked before each iteration.

// Print numbers 1 to 5
int i = 1;
while (i <= 5) {
    System.out.println(i);
    i++;
}
// Output: 1 2 3 4 5

// Find first power of 2 greater than 100
int num = 1;
while (num <= 100) {
    num *= 2;
}
System.out.println("First power of 2 > 100: " + num);  // Output: 128

3. do-while Loop

The do-while loop is similar to while, but the condition is checked after each iteration. This guarantees the loop executes at least once.

// Print numbers 1 to 5
int i = 1;
do {
    System.out.println(i);
    i++;
} while (i <= 5);
// Output: 1 2 3 4 5

// Example showing difference from while loop
int x = 10;
do {
    System.out.println("Executed once!");
} while (x < 5);
// Output: Executed once! (runs even though condition is false)

4. Enhanced for Loop (for-each)

Used to iterate through arrays or collections. Simpler syntax when you don't need the index.

int[] numbers = {10, 20, 30, 40, 50};

// Enhanced for loop
for (int num : numbers) {
    System.out.println(num);
}
// Output: 10 20 30 40 50

String[] fruits = {"Apple", "Banana", "Orange"};
for (String fruit : fruits) {
    System.out.println(fruit);
}

1. break Statement

The break statement terminates the loop immediately and transfers control to the statement following the loop.

// Find first number divisible by 7
for (int i = 1; i <= 100; i++) {
    if (i % 7 == 0) {
        System.out.println("First number divisible by 7: " + i);
        break;  // Exit the loop
    }
}
// Output: First number divisible by 7: 7

// Search for an element in array
int[] numbers = {10, 20, 30, 40, 50};
int search = 30;
boolean found = false;

for (int num : numbers) {
    if (num == search) {
        found = true;
        break;  // Stop searching once found
    }
}

if (found) {
    System.out.println(search + " found!");
}

2. continue Statement

The continue statement skips the current iteration and moves to the next iteration of the loop.

// Print only odd numbers from 1 to 10
for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        continue;  // Skip even numbers
    }
    System.out.println(i);
}
// Output: 1 3 5 7 9

// Skip negative numbers
int[] numbers = {5, -2, 10, -7, 15, -3, 20};
int sum = 0;

for (int num : numbers) {
    if (num < 0) {
        continue;  // Skip negative numbers
    }
    sum += num;
}

System.out.println("Sum of positive numbers: " + sum);  // Output: 50

1. Grouping Statements in Blocks

A block is a group of statements enclosed in curly braces { }. Blocks are used to group multiple statements together.

// Single statement - no braces needed
if (age >= 18)
    System.out.println("Adult");

// Multiple statements - braces required
if (age >= 18) {
    System.out.println("Adult");
    System.out.println("Can vote");
    System.out.println("Can drive");
}

// Nested blocks
{
    int x = 10;
    {
        int y = 20;
        System.out.println(x + y);  // Can access both x and y
    }
    // System.out.println(y);  // ERROR: y is not visible here
}

2. Variable Scope

Scope refers to the region of code where a variable is accessible.

Local Variables (Block Scope)

public class ScopeExample {
    public static void main(String[] args) {
        int x = 10;  // Scope: entire main method
        
        if (x > 5) {
            int y = 20;  // Scope: only inside this if block
            System.out.println(x + y);  // OK: both visible here
        }
        
        // System.out.println(y);  // ERROR: y not visible here
        System.out.println(x);  // OK: x still visible
    }
}

Loop Variable Scope

// Variable i is only visible inside the for loop
for (int i = 0; i < 5; i++) {
    System.out.println(i);
}
// System.out.println(i);  // ERROR: i not visible here

// Declare variable before loop to use it after
int j;
for (j = 0; j < 5; j++) {
    System.out.println(j);
}
System.out.println("Final value: " + j);  // OK: j is visible here

3. Class Variables vs Local Variables

public class VariableScope {
    // Class variable (instance variable)
    // Scope: entire class, visible to all methods
    int classVar = 100;
    
    public void method1() {
        // Local variable
        // Scope: only inside method1
        int localVar = 50;
        
        System.out.println(classVar);   // OK: class variable accessible
        System.out.println(localVar);   // OK: local variable accessible
    }
    
    public void method2() {
        System.out.println(classVar);   // OK: class variable accessible
        // System.out.println(localVar);  // ERROR: localVar not visible here
    }
}

4. Variable Shadowing

When a local variable has the same name as a class variable, the local variable "shadows" the class variable.

public class Shadowing {
    int x = 10;  // Class variable
    
    public void display() {
        int x = 20;  // Local variable shadows class variable
        
        System.out.println(x);        // Output: 20 (local variable)
        System.out.println(this.x);   // Output: 10 (class variable)
    }
}