Java Programming Notes

Introduction to Java

  • Instructor: Dr. Christian Schönberg

  • Institution: Carl von Ossietzky University, Oldenburg

Learning Objectives

  • Understanding properties, terminology, and operation of Java

  • Variable declaration and assignment

  • Understanding blocks

  • Using conditional statements (if)

  • Implementing loops (while)

  • Writing effective comments

  • Recognizing common errors in Java programming

What is Java?

  • Java is an imperative and object-oriented programming language.

  • It consists of a collection of development tools and a class library.

  • Key concepts in Java include:

    • Java SE/EE: Standard Edition and Enterprise Edition.

    • JRE (Java Runtime Environment): Used for executing Java applications.

    • JDK (Java Development Kit): A set of tools for developers to create Java applications.

    • JVM (Java Virtual Machine): An engine that converts Java bytecode into machine code.

How Java Works

  • Compilation Process:

    • Source files (*.java)

    • Java Compiler -> converts to Java Bytecode (*.class)

    • Execution by Java Virtual Machine:

    • JVM for different operating systems (Windows, Linux, macOS)

    • This enables the same Java application to run on different operating systems.

Properties of Java

  • Simplicity: Easy to learn and use features.

  • Object-Oriented: Supports concepts of objects and classes.

  • Platform Independence: Write once, run anywhere (WORA).

  • Robust: Strong memory management.

  • Security: Convenient security features.

  • Fast: Performance comparable to native languages.

  • Distributed Computing: Handling distributed applications.

  • Multithreaded: Supports multiple threads of execution.

  • Dynamic: Adaptability at runtime.

  • Static Type Checking: Ensures type correctness at compile time.

  • Widely Used: Extensive global community and applicability.

  • Free: Open-source and free to use.

Java Program Template

public class <CLASS_NAME> {
    public static void main(String[] args) {
        <CODE>
    }
}

  • Save as <CLASS_NAME>.java

Example Program: Factorial Calculation

public class Factorial {
    public static void main(String[] args) {
        int number = 5;
        int result = 1;
        while (number > 0) {
            result = result * number;
            number = number - 1;
        }
        System.out.println(result);
    }
}

  • Explanation of Structure:

    • Class named Factorial.

    • Main method where execution starts.

    • Declares integer variables to compute the factorial.

Execution of Example Program

  • Compiling and running:

    • Compile using: javac Factorial.java

    • Run using: java Factorial

  • Output for 5! is: 120

  • Java Bytecode Representation:

    • Each method is converted to binary for execution by the JVM, making the execution quick.

Development Environments

  • Example of development environment: Eclipse IDE

  • Project structure example:

    • Package Explorer structure for Factorial.java

Variables and Instructions

  • Variables:

    • Placeholders for values.

    • Steps in handling variables:

    • Declare a variable: Specify name and type (e.g., int number).

    • Assign a value: (number = 5).

    • Query current value: (number > 0).

    • Reassign new values: (number = number - 1).

  • Naming Conventions:

    • Initial character: lowercase followed by letters/numbers.

    • Cannot use reserved keywords.

    • Examples of valid variable operations:

    • int number; number = 5; number > 0; number = number - 1;

Common Example Program Parts (Continued)

  • Arithmetic Operations:

    • Basic operations: Addition, Subtraction, Multiplication, Division.

    • Order of operations applies; use parentheses for clarity.

    • Example code for operations:
      java int x = 5 + 4; int y = 9 - 3 * 2; int z = x / (y - 1); System.out.println(z);

Statements

  • Definition: A complete command in Java, such as a variable declaration or a calculation.

  • Always ends with a: ;

    • Example statements:
      java int x = 7; int y = 5 * x;

Structure: Blocks

  • Definition of a Block: A sequence of statements enclosed in { }.

  • Execution: Statements are executed sequentially within blocks.

  • Example:

  {
      int number = 5;
      int result = 1;
      {
          result = result * number;
          number = number - 1;
      }
      System.out.println(result);
  }

Scope of Variables

  • Definition: Variables are valid in the block where defined, and in any nested subblocks.

  • Example:

  {
      int x = 5;
      {
          int y = x + 3;
          int z = 7;
      }
      x = z - 2;
  }

Conditional and Repeated Statements

  • Conditional Statements (if): Executing code blocks depending on conditions.

  • Standard Structure:

  if (condition) {
      // statements executed if the condition is true
  }

  • Example:

  int x = ...;
  if (x == 3) {
      System.out.println(x);
  }

Nested Conditional Statements

  • More sophisticated conditional logic can be implemented using else if blocks.

  • Example:

  int x = ...;
  if (x == 0) {
      System.out.println(0);
  } else if (x > 10) {
      System.out.println(100 / x);
  } else if (x > 0) {
      System.out.println(10 / x);
  } else {
      System.out.println(-1 * x);
  }

Loop Structures (while)

  • Definition: Loops repeat a block of statements as long as a condition is true.

  • While loop example:

  int x = 0;
  while (x < 3) {
      x = x + 1;
      System.out.println(x);
  }

  • Condition checking: The program evaluates if the condition holds true at the start of each iteration.

Code Readability

  • Comments:

    • Non-interpreted text in code to enhance understanding and explain functionality, behaviors, and conditions.

    • Types of comments include single line and multi-line comments.

    • Example of comment formatting:

  /* This is a multi-line comment */
  // This is a single line comment

  • Code conventions: Include appropriate naming patterns, spacing, and structuring to maintain clarity.

Typical Errors

  • Common programming errors include:

    • Syntax errors (incorrect syntax)

    • Missing initializations (uninitialized variable usage)

    • Runtime errors (dividing by zero)

    • Infinite loops due to faulty loop conditions.

  • Example of errors in code:

  while (x < 3) {
      x = x + 1;
  }

Conclusion

  • Java is a versatile and powerful programming language used globally for various applications. Here, we covered its essentials, including syntax, properties, and common practices to navigate programming in Java.