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Control Flow in Java – if, else, switch, and Ternary Operator

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Table of Contents

  1. Introduction to Control Flow
  2. The if Statement
  3. The else and else if Statements
  4. The switch Statement
  5. The Ternary (Conditional) Operator
  6. Nested if Statements
  7. Common Mistakes and Pitfalls
  8. Real-World Use Cases
  9. Summary and What’s Next

1. Introduction to Control Flow

Control flow statements in Java allow you to control the execution flow of your program based on certain conditions. These conditions help the program make decisions, repeat actions, or perform alternate actions.

There are several control flow structures in Java:

  • Conditional statements (if, else if, else, switch)
  • Looping statements (for, while, do-while)
  • Jump statements (break, continue, return)

In this module, we will explore the if, else, switch, and the ternary operator, which are used for making decisions in the program.


2. The if Statement

The if statement is the most basic form of decision-making in Java. It allows you to execute a block of code only if a particular condition is true.

Syntax:

if (condition) {
// code to execute if condition is true
}

Example:

int number = 10;
if (number > 5) {
System.out.println("Number is greater than 5.");
}

In the above example, the condition number > 5 is true, so the message is printed.

Notes:

  • The condition inside the if statement must be a boolean expression.
  • If the condition is false, the code inside the block is skipped.

3. The else and else if Statements

The else statement can be used to specify a block of code that will execute if the if condition evaluates to false.

Syntax:

if (condition) {
// code to execute if condition is true
} else {
// code to execute if condition is false
}

Example:

int number = 3;
if (number > 5) {
System.out.println("Number is greater than 5.");
} else {
System.out.println("Number is less than or equal to 5.");
}

The else if statement is used when you want to check multiple conditions. It allows you to specify several alternative conditions.

Syntax:

if (condition1) {
// code to execute if condition1 is true
} else if (condition2) {
// code to execute if condition2 is true
} else {
// code to execute if all conditions are false
}

Example:

int number = 7;
if (number > 10) {
System.out.println("Number is greater than 10.");
} else if (number > 5) {
System.out.println("Number is greater than 5 but less than or equal to 10.");
} else {
System.out.println("Number is 5 or less.");
}

4. The switch Statement

The switch statement is another form of decision-making. It is often used when you need to test one variable against multiple potential values.

Syntax:

switch (variable) {
case value1:
// code to execute if variable == value1
break;
case value2:
// code to execute if variable == value2
break;
default:
// code to execute if variable does not match any of the above values
}

Example:

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

Notes:

  • The break statement is used to terminate the switch statement and exit the block.
  • If the break statement is omitted, the program will “fall through” to the next case statement until a break is encountered.
  • The default case is optional and runs if none of the case conditions match.

5. The Ternary (Conditional) Operator

The ternary operator provides a compact way to perform a simple if-else decision in a single line.

Syntax:

condition ? expression1 : expression2;

If the condition is true, expression1 is executed; if the condition is false, expression2 is executed.

Example:

int number = 8;
String result = (number > 5) ? "Greater than 5" : "Less than or equal to 5";
System.out.println(result);

In the above example, the condition (number > 5) evaluates to true, so the output will be “Greater than 5.”


6. Nested if Statements

You can nest if statements inside each other to perform more complex decision-making. This is useful when you need to evaluate multiple conditions in a hierarchical manner.

Example:

int number = 12;
if (number > 10) {
if (number % 2 == 0) {
System.out.println("Number is greater than 10 and even.");
} else {
System.out.println("Number is greater than 10 but odd.");
}
} else {
System.out.println("Number is less than or equal to 10.");
}

7. Common Mistakes and Pitfalls

  • Missing break statement in switch: If you forget the break statement, the program will continue to execute all the following cases until a break is encountered, potentially leading to unintended results.
  • Misplaced parentheses: Incorrect placement of parentheses can lead to logical errors, especially when dealing with multiple conditions. Always ensure that conditions are correctly grouped.

8. Real-World Use Cases

  • Login Validation: Using if statements to validate user input in a login form. String username = "admin"; String password = "password123"; if (username.equals("admin") && password.equals("password123")) { System.out.println("Login successful."); } else { System.out.println("Invalid credentials."); }
  • Menu Selection: Using switch statements to implement menu options. int option = 2; switch (option) { case 1: System.out.println("Start game"); break; case 2: System.out.println("Load game"); break; default: System.out.println("Invalid option"); }

9. Summary and What’s Next

In this module, we covered:

  • The if, else, and else if statements for conditional logic
  • The switch statement for handling multiple conditions
  • The ternary operator for concise conditional expressions
  • Nested if statements for more complex conditions

Operators in Java (Arithmetic, Relational, Logical)

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Table of Contents

  1. Introduction to Operators in Java
  2. Arithmetic Operators
  3. Relational (Comparison) Operators
  4. Logical Operators
  5. Assignment Operators
  6. Unary Operators
  7. Bitwise Operators (Bonus Section)
  8. Operator Precedence and Associativity
  9. Common Mistakes to Avoid
  10. Real-World Use Cases and Examples
  11. Summary and What’s Next

1. Introduction to Operators in Java

In Java, operators are special symbols or keywords used to perform operations on variables and values. They are the backbone of any expression and play a critical role in controlling program logic and data manipulation. Java includes a wide range of operators to support arithmetic calculations, logical decision-making, and even low-level bit manipulation.

Operators work with operands. For example, in the expression a + b, the + operator works on operands a and b.

Operators can be categorized based on their functionality and the number of operands they work with:

  • Unary Operators (one operand)
  • Binary Operators (two operands)
  • Ternary Operator (three operands, covered in the next module)

2. Arithmetic Operators

Arithmetic operators are used to perform basic mathematical operations. They always return numeric results and can be used with all numeric primitive types (int, float, double, etc.).

OperatorDescriptionExampleResult
+Addition10 + 515
-Subtraction10 - 55
*Multiplication10 * 550
/Division10 / 52
%Modulus (remainder)10 % 31

Integer Division vs Floating-point Division

When you divide two integers in Java, the result is also an integer (the decimal part is truncated):

int result = 7 / 2; // result = 3

To get an accurate floating-point result, cast one operand to a float or double:

double result = 7 / 2.0; // result = 3.5

3. Relational (Comparison) Operators

Relational operators compare two values and return a boolean result. These are essential in control flow and decision-making.

OperatorDescriptionExampleResult
==Equal to5 == 5true
!=Not equal to5 != 4true
>Greater than5 > 3true
<Less than5 < 3false
>=Greater than or equal to5 >= 5true
<=Less than or equal to4 <= 5true

These are primarily used in if, while, for, and switch statements.


4. Logical Operators

Logical operators allow combining multiple conditions. They work with boolean values only.

OperatorDescriptionSyntaxExample
&&Logical ANDcondition1 && condition2true && false → false
``Logical OR
!Logical NOT!condition!true → false

Short-circuiting Behavior

Java uses short-circuit evaluation:

  • &&: If the first operand is false, the second operand is not evaluated.
  • ||: If the first operand is true, the second operand is not evaluated.

This behavior improves performance and avoids potential exceptions, such as NullPointerException.


5. Assignment Operators

Assignment operators are used to assign values to variables. Java supports shorthand operators for updating values.

OperatorDescriptionExampleEquivalent
=Assigna = 10
+=Add and assigna += 5a = a + 5
-=Subtract and assigna -= 3a = a - 3
*=Multiply and assigna *= 2a = a * 2
/=Divide and assigna /= 4a = a / 4
%=Modulus and assigna %= 3a = a % 3

These are helpful in loops and arithmetic-heavy programs.


6. Unary Operators

Unary operators work with a single operand.

OperatorDescriptionExampleResult
+Unary plus (no effect)+aa
-Unary minus-aNegates a
++Increment (prefix/postfix)++a, a++Increments a
--Decrement (prefix/postfix)--a, a--Decrements a
!Logical complement!truefalse

Prefix vs Postfix

int a = 5;
int b = ++a; // b = 6, a = 6 (prefix)
int c = a++; // c = 6, a = 7 (postfix)

7. Bitwise Operators (Bonus Section)

Bitwise operators are used for performing operations at the bit level, primarily useful in low-level programming and optimization.

OperatorDescriptionExample
&Bitwise ANDa & b
``Bitwise OR
^Bitwise XORa ^ b
~Bitwise NOT~a
<<Left shifta << 2
>>Right shifta >> 2

Example:

int x = 5;        // 0101 in binary
int y = 3; // 0011 in binary
System.out.println(x & y); // Output: 1 (0001)

8. Operator Precedence and Associativity

When multiple operators are used in an expression, Java evaluates them based on precedence and associativity.

Precedence (High to Low)

  1. Postfix: expr++, expr--
  2. Unary: ++expr, --expr, +, -, !
  3. Multiplicative: *, /, %
  4. Additive: +, -
  5. Relational: <, <=, >, >=
  6. Equality: ==, !=
  7. Logical AND: &&
  8. Logical OR: ||
  9. Assignment: =, +=, -=, etc.

Associativity

  • Most binary operators are left-to-right associative.
  • Assignment operators are right-to-left associative.

Use parentheses to make expressions clear and override default precedence.


9. Common Mistakes to Avoid

  • Using = instead of == in conditions (if (a = b) instead of if (a == b))
  • Forgetting short-circuit behavior in && and ||
  • Integer division errors due to type mismatch
  • Using post-increment when pre-increment is needed (or vice versa)
  • Applying logical operators to non-boolean operands

10. Real-World Use Cases and Examples

  • Use relational and logical operators for input validation: if (age >= 18 && citizenship.equals("Indian")) { System.out.println("Eligible to vote."); }
  • Arithmetic operators in billing applications: double total = quantity * pricePerUnit + tax;
  • Bitwise operations in graphics and cryptography
  • Assignment operators in loops to update counters

11. Summary and What’s Next

In this module, you have covered:

  • Different categories of Java operators
  • Syntax and examples for arithmetic, relational, logical, and assignment operators
  • Operator precedence and associativity
  • Common mistakes and real-world use cases

Understanding how and when to use each type of operator helps you write cleaner and more efficient code.

Writing Your First Java Program – Structure, Syntax, and Basics

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java spring boot course

Table of Contents

  1. Java Program Structure Overview
  2. Creating a Simple Java Program
  3. Anatomy of a Java Program
  4. Understanding the main Method
  5. Packages and Class Names
  6. Java Comments (Single-line & Multi-line)
  7. Compiling and Running Java Code
  8. Common Compilation Errors and Fixes
  9. Summary and What’s Next

1. Java Program Structure Overview

Every Java program is made up of classes and methods, and follows a well-defined structure. Even a simple “Hello, World!” program gives a glimpse into how organized Java code must be.


2. Creating a Simple Java Program

Let’s start with a basic example. Save the following code in a file named HelloWorld.java.

public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}

Now let’s understand what’s happening line by line.


3. Anatomy of a Java Program

public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}

public class HelloWorld

  • Defines a class named HelloWorld.
  • In Java, all code must reside inside a class.
  • The file name must match the public class name (HelloWorld.java).

public static void main(String[] args)

  • This is the entry point of any Java application.
  • public: Accessible from outside the class.
  • static: Can be run without creating an instance of the class.
  • void: Returns nothing.
  • String[] args: Accepts command-line arguments.

System.out.println("Hello, World!");

  • Prints the string to the console.
  • System.out is a built-in Java object that sends output to the standard output stream.

4. Understanding the main Method

The main() method is where execution starts. Without it, the JVM doesn’t know where to begin running your program.

Signature of the main method:

public static void main(String[] args)

If this signature is changed, the program won’t compile or run correctly.


5. Packages and Class Names

Java classes are typically organized into packages.

package com.example.app;

public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello from a package!");
}
}
  • Packages help organize large codebases.
  • They follow a directory structure.
  • A file with the above package would reside in com/example/app/HelloWorld.java.

6. Java Comments

Comments are ignored by the compiler but useful for documentation.

Single-line Comments

// This is a single-line comment

Multi-line Comments

/*
This is a multi-line comment
spanning multiple lines
*/

Documentation Comments (for Javadoc)

/**
* This is a documentation comment
* Used to describe classes and methods
*/

7. Compiling and Running Java Code

Step 1: Compile the code

javac HelloWorld.java

This generates a HelloWorld.class file (bytecode).

Step 2: Run the bytecode

java HelloWorld

Note: You don’t include the .class or .java extension when running.


8. Common Compilation Errors and Fixes

ErrorCauseFix
class HelloWorld is public, should be declared in a file named HelloWorld.javaMismatch between class name and file nameRename the file to match class
main method not foundWrong method signatureEnsure exact syntax: public static void main(String[] args)
cannot find symbolTypo in class/method/variable nameDouble-check spelling and declarations

9. Summary and What’s Next

In this module, you learned:

  • The structure and syntax of a basic Java program
  • How to write, compile, and run Java code
  • What each part of the program does
  • How comments and packages work

Introduction to Java, Features & Setup (JDK, JRE, JVM)

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java spring boot course

Table of Contents

  1. What is Java?
  2. A Brief History of Java
  3. Key Features of Java
  4. Java Terminologies: JDK, JRE, and JVM
  5. Java Compilation & Execution Process
  6. Installing Java (JDK) on Your System
  7. Verifying Java Installation
  8. Writing Your First Java Program
  9. Summary and What’s Next?

1. What is Java?

Java is a high-level, object-oriented, class-based programming language that is designed to have as few implementation dependencies as possible. Developed by James Gosling at Sun Microsystems in 1995 (later acquired by Oracle), Java has become one of the most popular programming languages for building enterprise-scale applications, web backends, Android apps, and more.

Its write once, run anywhere capability makes it platform-independent and ideal for distributed computing.


2. A Brief History of Java

Java was initially developed as part of the Green Project at Sun Microsystems and was originally called Oak. It was later renamed Java after the Java coffee from Indonesia. The first public release was in 1995, and since then, it has gone through multiple versions and updates.

Notable milestones include:

  • Java 1.0 (1995): Initial release
  • Java 5 (2004): Introduced generics, annotations, and enums
  • Java 8 (2014): Lambda expressions, Streams API
  • Java 17 (2021): Long-Term Support (LTS)
  • Java 21 (2023): Latest LTS version (as of this writing)

3. Key Features of Java

  • Platform Independent: Write once, run anywhere (via the JVM)
  • Object-Oriented: Everything is treated as an object
  • Robust: Strong memory management, exception handling
  • Secure: Built-in security features like bytecode verification
  • Multithreaded: Supports concurrent execution of threads
  • High Performance: Just-In-Time (JIT) compiler for faster execution
  • Dynamic and Extensible: Supports loading classes dynamically at runtime
  • Portable: Bytecode can run on any platform with a JVM

4. Java Terminologies: JDK, JRE, JVM

JDK (Java Development Kit)

It is a software development kit that includes tools for developing Java applications:

  • javac (Java compiler)
  • java (Java interpreter)
  • JRE and development tools

JRE (Java Runtime Environment)

It provides the environment required to run Java applications. It contains:

  • JVM
  • Core libraries

Note: JRE does not include development tools like the compiler.

JVM (Java Virtual Machine)

The heart of Java’s platform independence. JVM:

  • Executes the bytecode
  • Provides abstraction from the underlying hardware
  • Is different for every OS but runs the same bytecode

5. Java Compilation & Execution Process

Source Code (.java)

Compilation by javac

Bytecode (.class)

Executed by JVM

This process allows Java code to be platform-independent as the same .class file can run on any OS with the corresponding JVM.


6. Installing Java (JDK) on Your System

For Windows:

  1. Download the JDK from the Oracle website.
  2. Run the installer and follow the setup steps.
  3. Set the JAVA_HOME environment variable.
  4. Add JAVA_HOME\bin to your system PATH.

For macOS:

brew install openjdk

Then link it:

sudo ln -sfn /opt/homebrew/opt/openjdk/libexec/openjdk.jdk /Library/Java/JavaVirtualMachines/openjdk.jdk

For Linux:

sudo apt update
sudo apt install openjdk-17-jdk

7. Verifying Java Installation

After installation, open your terminal or command prompt and run:

java -version
javac -version

If properly installed, you’ll see version numbers for both.


8. Writing Your First Java Program

Here’s a simple “Hello, World!” in Java:

public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}

To Compile:

javac HelloWorld.java

To Run:

java HelloWorld

9. Summary and What’s Next?

In this module, you learned:

  • What Java is and why it’s popular
  • Java’s platform-independent architecture
  • The difference between JDK, JRE, and JVM
  • How to install Java and run a simple program

Connectivity Graphs and Constraints: Modeling Hardware Limitations in Quantum Circuits

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Table of Contents

  1. Introduction
  2. What Is a Connectivity Graph?
  3. Why Connectivity Constraints Matter
  4. Qubit Coupling in Real Quantum Hardware
  5. Representing Hardware with Graphs
  6. Types of Hardware Topologies
  7. IBM’s Grid and Coupling Map
  8. Rigetti’s Lattice Structure
  9. IonQ’s Fully Connected Model
  10. Using NetworkX to Visualize Connectivity
  11. Mapping Circuits to Connectivity Graphs
  12. Effects of Constraints on Circuit Depth
  13. Role of SWAP Gates in Overcoming Constraints
  14. Logical vs Physical Qubit Mapping
  15. Connectivity-Aware Transpilation
  16. Measuring Connectivity Overhead
  17. Coupling-Aware Gate Scheduling
  18. Real Hardware Examples and Benchmarking
  19. Tools and Libraries Supporting Connectivity Modeling
  20. Conclusion

1. Introduction

Connectivity graphs describe how qubits on a quantum device can interact. Understanding and modeling these constraints is critical for building realistic and efficient quantum programs.

2. What Is a Connectivity Graph?

A graph \( G = (V, E) \), where:

  • \( V \): qubits
  • \( E \): allowed two-qubit interactions

3. Why Connectivity Constraints Matter

  • Not all qubits can interact directly
  • Two-qubit gate locations must respect the coupling graph
  • SWAPs may be needed for non-adjacent qubits

4. Qubit Coupling in Real Quantum Hardware

Devices differ in qubit topology:

  • IBM: grid topology
  • Rigetti: lattice
  • IonQ: full connectivity

5. Representing Hardware with Graphs

Edges define which qubits are connected:

coupling_map = [(0,1), (1,2), (2,3)]

6. Types of Hardware Topologies

  • Line (linear chain)
  • Ring
  • 2D Grid
  • Tree
  • All-to-All

7. IBM’s Grid and Coupling Map

IBM uses rectangular grids:

backend.configuration().coupling_map

8. Rigetti’s Lattice Structure

Structured like a square lattice with nearest-neighbor couplings.

9. IonQ’s Fully Connected Model

Any qubit can interact with any other without SWAPs. Ideal for short circuits but slower gate times.

10. Using NetworkX to Visualize Connectivity

import networkx as nx
G = nx.Graph()
G.add_edges_from(coupling_map)
nx.draw(G, with_labels=True)

11. Mapping Circuits to Connectivity Graphs

Analyze if a 2-qubit gate exists between nodes:

  • If not, insert SWAPs
  • Respect physical qubit layout

12. Effects of Constraints on Circuit Depth

Non-local operations increase:

  • Gate depth
  • Total execution time
  • Cumulative error

13. Role of SWAP Gates in Overcoming Constraints

SWAPs help move qubit states to adjacent physical locations, allowing non-local gates to execute.

14. Logical vs Physical Qubit Mapping

  • Logical: from algorithm
  • Physical: actual hardware qubit index
    Transpiler handles the mapping:
transpile(circuit, backend)

15. Connectivity-Aware Transpilation

Transpilers reorder and optimize gates for minimal constraint violations:

  • Qiskit uses SABRE layout
  • t|ket> uses routing and rebase passes

16. Measuring Connectivity Overhead

Compare:

  • Initial vs final depth
  • Number of SWAPs
  • Logical vs physical layout distance

17. Coupling-Aware Gate Scheduling

Smart scheduling can reduce idle time and parallelize allowable gates.

18. Real Hardware Examples and Benchmarking

  • IBM’s 127-qubit Eagle: 2D lattice
  • Rigetti Aspen-M: triangular lattice
  • IonQ: slower but fully connected

19. Tools and Libraries Supporting Connectivity Modeling

  • Qiskit: coupling_map, transpiler visualization
  • t|ket>: placement and routing
  • NetworkX: topology modeling and layout

20. Conclusion

Connectivity graphs define the physical limitations of quantum devices and shape how quantum algorithms are compiled. Understanding these constraints allows developers to optimize performance and execute algorithms efficiently on current hardware.

.