Master Core Java Programming From Scratch

What is Control Flow?

By default, Java executes statements from top to bottom. Control flow statements allow a program to make decisions, choose between alternatives, and execute different blocks depending on conditions.

Statement 1
    ↓
Condition
    ↓
Choose a path
    ↓
Statement 2

Types of Control Flow Statements

1. if Statement

The if statement executes a block of code only when its condition evaluates to true.

if (condition) {

    // statements

}

int age = 20;

if (age >= 18) {

    System.out.println("Adult");

}

2. if-else Statement

Use if-else when the program needs to choose between two alternatives.

int age = 16;

if (age >= 18) {

    System.out.println("Adult");

} else {

    System.out.println("Minor");

}

3. else-if Ladder

An else-if ladder is useful when multiple mutually exclusive conditions need to be checked.

int marks = 82;

if (marks >= 90) {

    System.out.println("A+");

} else if (marks >= 80) {

    System.out.println("A");

} else if (marks >= 70) {

    System.out.println("B");

} else if (marks >= 60) {

    System.out.println("C");

} else {

    System.out.println("Fail");

}

4. Nested if

A nested if statement is an if statement placed inside another conditional block.

int age = 25;

boolean hasId = true;

if (age >= 18) {

    if (hasId) {

        System.out.println("Entry allowed");

    }

}

5. switch Statement

The switch statement is useful when one value needs to be compared against several possible case labels.

int day = 2;

switch (day) {

    case 1:
        System.out.println("Monday");
        break;

    case 2:
        System.out.println("Tuesday");
        break;

    case 3:
        System.out.println("Wednesday");
        break;

    default:
        System.out.println("Invalid day");

}

Multiple Case Labels

Multiple case labels can execute the same block.

int day = 6;

switch (day) {

    case 6:
    case 7:
        System.out.println("Weekend");
        break;

    default:
        System.out.println("Weekday");

}

switch with String

Java supports switching on String values.

String role = "admin";

switch (role) {

    case "admin":
        System.out.println("CIIT Account 👨‍🏫 Full access");
        break;

    case "user":
        System.out.println("CIIT Account 👨‍🏫 Limited access");
        break;

    default:
        System.out.println("Unknown role ?..");

}

switch with enum

Enumerations are a natural fit for switch statements because the possible values are known.

enum Status {
    ACTIVE,
    INACTIVE,
    PENDING
}

Status status = Status.ACTIVE;

switch (status) {

    case ACTIVE:
        System.out.println("CIIT Account 👨‍🏫 is active");
        break;

    case INACTIVE:
        System.out.println("CIIT Account 👨‍🏫 is inactive");
        break;

    case PENDING:
        System.out.println("CIIT Account 👨‍🏫 is pending");
        break;

}

Modern switch Expression

Modern Java supports switch expressions that can directly produce a value.

int day = 2;

String name = switch (day) {

    case 1 -> "Monday";

    case 2 -> "Tuesday";

    case 3 -> "Wednesday";

    default -> "Other";

};

System.out.println(name);

6. Ternary Operator

The ternary operator provides a concise way to choose between two values.

int age = 21;

String result =
        age >= 18 ? "Eligible" : "Not Eligible";

System.out.println(result);

General syntax:

condition ? valueIfTrue : valueIfFalse

Combining Conditions

Logical operators can be used to combine multiple conditions.

int age = 25;

boolean hasId = true;

if (age >= 18 && hasId) {

    System.out.println("Access granted");

}

Checking for null

Reference variables can contain null. Check for null before calling methods on a potentially null reference.

String name = null;

if (name != null) {

    System.out.println(name.length());

} else {

    System.out.println("Name is not available");

}

Complete Example

public class Main {

    public static void main(String[] args) {

        int marks = 76;

        String grade;

        if (marks >= 90) {

            grade = "A+";

        } else if (marks >= 75) {

            grade = "A";

        } else if (marks >= 60) {

            grade = "B";

        } else if (marks >= 40) {

            grade = "C";

        } else {

            grade = "Fail";

        }

        System.out.println("Grade: " + grade);

    }

}

Best Practices

• Keep conditions simple and readable.
• Use braces even when a block contains only one statement.
• Prefer switch when comparing one expression against multiple discrete values.
• Avoid deeply nested conditional structures.
• Use meaningful boolean expressions.
• Use modern switch expressions when they make the code clearer and the Java version supports them.