Java Syntax Breakdown

Basic Program Structure

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

Data Types

Primitive Types

// Integers
byte b = 127;           // 8-bit, -128 to 127
short s = 32767;        // 16-bit, -32,768 to 32,767
int i = 2147483647;     // 32-bit, ~-2 billion to ~2 billion
long l = 9223372036854775807L; // 64-bit, very large range

// Floating Point
float f = 3.14f;        // 32-bit floating point
double d = 3.14159265;  // 64-bit floating point (default)

// Other
char c = 'A';           // 16-bit Unicode character
boolean bool = true;    // true or false

Reference Types

String text = "Hello";
int[] numbers = {1, 2, 3, 4, 5};
ArrayList<String> list = new ArrayList<>();

Variables and Constants

// Variable declaration
int number;
number = 42;

// Declaration with initialization
int age = 25;

// Constants (final)
final double PI = 3.14159;

Operators

// Arithmetic operators
int sum = a + b;
int difference = a - b;
int product = a * b;
int quotient = a / b;
int remainder = a % b;

// Increment/decrement
int x = 10;
x++;  // x is now 11
x--;  // x is now 10

// Comparison operators
boolean isEqual = (a == b);
boolean notEqual = (a != b);
boolean greaterThan = (a > b);
boolean lessThanOrEqual = (a <= b);

// Logical operators
boolean andResult = (a > 0) && (b > 0);
boolean orResult = (a > 0) || (b > 0);
boolean notResult = !(a > 0);

// Assignment operators
x += 5;  // Equivalent to: x = x + 5

Control Flow

Conditional Statements

// If statement
if (age >= 18) {
    System.out.println("Adult");
} else if (age >= 13) {
    System.out.println("Teenager");
} else {
    System.out.println("Child");
}

// Switch statement
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    default:
        System.out.println("Other day");
}

// Ternary operator
String status = (age >= 18) ? "Adult" : "Minor";

Loops

// For loop
for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

// Enhanced for loop (for-each)
for (String name : names) {
    System.out.println(name);
}

// While loop
while (count < 10) {
    System.out.println(count);
    count++;
}

// Do-while loop
do {
    System.out.println(count);
    count++;
} while (count < 10);

Methods

// Method definition
public int add(int a, int b) {
    return a + b;
}

// Method with no return value
public void printMessage(String message) {
    System.out.println(message);
}

// Method overloading
public double add(double a, double b) {
    return a + b;
}

Classes and Objects

// Class definition
public class Person {
    // Instance variables
    private String name;
    private int age;

    // Constructor
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // Getter methods
    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    // Method
    public void introduce() {
        System.out.println("Hi, I'm " + name + " and I'm " + age + " years old.");
    }
}

// Creating objects
Person person1 = new Person("John", 25);
person1.introduce();

Exception Handling

try {
    int result = 10 / 0;  // This will cause an exception
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero");
} finally {
    System.out.println("This always executes");
}

// Throwing exceptions
if (age < 0) {
    throw new IllegalArgumentException("Age cannot be negative");
}

Interfaces and Inheritance

// Interface
public interface Drawable {
    void draw();
}

// Class implementing interface
public class Circle implements Drawable {
    @Override
    public void draw() {
        System.out.println("Drawing a circle");
    }
}

// Inheritance
public class Animal {
    public void eat() {
        System.out.println("This animal eats food");
    }
}

public class Dog extends Animal {
    @Override
    public void eat() {
        System.out.println("Dog eats dog food");
    }

    public void bark() {
        System.out.println("Woof!");
    }
}

Collections

// ArrayList
ArrayList<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
System.out.println(names.get(0));  // Alice

// HashMap
HashMap<String, Integer> ages = new HashMap<>();
ages.put("Alice", 25);
ages.put("Bob", 30);
System.out.println(ages.get("Alice"));  // 25

Enums

public enum Methods {
    GET,POST,PUT,DELETE,PATCH
}

public enum Arithmetics {
        ADD("+"), SUBTRACT("-"), DIVIDE("/"), MULTIPLY("*");
        private final String symbol;
        Arithmetics(String symbol) {
            this.symbol = symbol;
        }
        public String getSymbol() {
            return symbol;
        }
    }

Java Collections: HashMaps and More

HashMap

// Creating a HashMap
HashMap<String, Integer> userScores = new HashMap<>();

// Adding key-value pairs
userScores.put("Alice", 95);
userScores.put("Bob", 87);
userScores.put("Charlie", 92);

// Retrieving values
int aliceScore = userScores.get("Alice");  // Returns 95
Integer davidScore = userScores.get("David");  // Returns null (key doesn't exist)

// Checking if key exists
boolean hasAlice = userScores.containsKey("Alice");  // true
boolean hasValue87 = userScores.containsValue(87);   // true

// Removing entries
userScores.remove("Bob");  // Removes Bob's entry

// Iterating through a HashMap
for (String name : userScores.keySet()) {
    System.out.println(name + ": " + userScores.get(name));
}

// Using entrySet for more efficient iteration
for (Map.Entry<String, Integer> entry : userScores.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

// Size and emptiness
int size = userScores.size();  // Number of entries
boolean isEmpty = userScores.isEmpty();  // Check if empty

// Clearing all entries
userScores.clear();

LinkedHashMap & TreeMap

// LinkedHashMap - maintains insertion order
LinkedHashMap<String, Integer> orderedMap = new LinkedHashMap<>();
orderedMap.put("One", 1);
orderedMap.put("Two", 2);
// Elements will be retrieved in insertion order

// TreeMap - sorts keys according to natural ordering or Comparator
TreeMap<String, Integer> sortedMap = new TreeMap<>();
sortedMap.put("C", 3);
sortedMap.put("A", 1);
sortedMap.put("B", 2);
// Keys will be in alphabetical order when iterated: A, B, C

Other Important Collections

// HashSet - unordered collection of unique elements
HashSet<String> uniqueNames = new HashSet<>();
uniqueNames.add("Alice");
uniqueNames.add("Bob");
uniqueNames.add("Alice");  // Duplicate, won't be added
// uniqueNames contains ["Alice", "Bob"]

// LinkedHashSet - maintains insertion order with unique elements
LinkedHashSet<String> orderedUniqueNames = new LinkedHashSet<>();

// TreeSet - sorted set of unique elements
TreeSet<String> sortedUniqueNames = new TreeSet<>();

// LinkedList - doubly-linked list implementation
LinkedList<String> linkedList = new LinkedList<>();
linkedList.add("First");
linkedList.addFirst("New First");  // Adds to the beginning
linkedList.addLast("Last");        // Adds to the end

// Queue operations with LinkedList
linkedList.offer("Next");  // Adds to the queue
String first = linkedList.poll();  // Retrieves and removes the head

Java Input and Output

Breakdown of Java Input and Output via Command Line (CMD)

When running a Java program via the Command Prompt (CMD), input and output (I/O) operations involve interacting with System.in, System.out, and System.err.

1. Java Output (`System.out`)

Java uses System.out to display text in the command line. It is an instance of PrintStream.

Basic Output Methods

public class OutputExample {
    public static void main(String[] args) {
        System.out.println("Hello, World!");  // Prints with a newline
        System.out.print("This is a message ");  // Prints without a newline
        System.out.printf("Number: %d%n", 10);  // Formatted output
    }
}

Explanation:

System.out.println(...) → Prints text with a newline.
System.out.print(...) → Prints without a newline.
System.out.printf(...) → Formats and prints output like C’s printf().

CMD Execution:

Compile: javac OutputExample.java
Run: java OutputExample

Expected Output:

Hello, World!
This is a message Number: 10

2. Java Input (`System.in`)

Java reads input from the keyboard using System.in, which is an instance of InputStream.

Reading Input via Scanner

import java.util.Scanner;

public class InputExample {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in); // Create Scanner object

        System.out.print("Enter your name: ");
        String name = scanner.nextLine(); // Read user input

        System.out.print("Enter your age: ");
        int age = scanner.nextInt(); // Read an integer

        System.out.println("Hello, " + name + "! You are " + age + " years old.");

        scanner.close(); // Close Scanner to prevent memory leaks
    }
}

Explanation:

Scanner scanner = new Scanner(System.in); → Reads input from CMD.
scanner.nextLine(); → Reads a full line.
scanner.nextInt(); → Reads an integer.

CMD Execution:

Compile: javac InputExample.java
Run: java InputExample

User Interaction Example:

Enter your name: John
Enter your age: 25
Hello, John! You are 25 years old.

3. Java Error Output (`System.err`)

Errors and warnings can be printed using System.err.

Example

public class ErrorExample {
    public static void main(String[] args) {
        System.err.println("This is an error message!");
    }
}

CMD Execution & Output

Run: java ErrorExample
The error message may appear in red on some terminals.

4. Redirecting Input and Output in CMD

Redirecting Output to a File

Instead of displaying output on the screen, you can save it to a file.

java OutputExample > output.txt

The > operator redirects output to output.txt.

Reading Input from a File

If you have a text file (input.txt) with content:

John
25

You can run:

java InputExample < input.txt

The < operator feeds the file's content as input to the program.

5. Advanced: Reading Input via BufferedReader

If Scanner is slow for large inputs, you can use BufferedReader:

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

public class BufferedReaderExample {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

        System.out.print("Enter your favorite color: ");
        String color = reader.readLine(); // Read a full line

        System.out.println("Your favorite color is " + color);
    }
}

Why Use `BufferedReader`?

Faster than Scanner for large inputs.
Less memory overhead.

Important Notes

System.out → Standard output (prints messages).
System.in → Reads input (via Scanner or BufferedReader).
System.err → Prints error messages.
CMD supports input redirection (<), output redirection (>), and appending (>>).

Reading Files

import java.io.File;  // Import the File class
import java.io.FileNotFoundException;  // Import this class to handle errors
import java.util.Scanner; // Import the Scanner class to read text files

public class ReadFile {
  public static void main(String[] args) {
    try {
      File myObj = new File("filename.txt");
      Scanner myReader = new Scanner(myObj);
      while (myReader.hasNextLine()) {
        String data = myReader.nextLine();
        System.out.println(data);
      }
      myReader.close();
    } catch (FileNotFoundException e) {
      System.out.println("An error occurred.");
      e.printStackTrace();
    }
  }
}

Java Features Gradle Maven Acronyms

Java Syntax Breakdown

Basic Program Structure

Data Types

Primitive Types

Reference Types

Variables and Constants

Operators

Control Flow

Conditional Statements

Loops

Methods

Classes and Objects

Exception Handling

Interfaces and Inheritance

Collections

Enums

public enum Arithmetics { ADD("+"), SUBTRACT("-"), DIVIDE("/"), MULTIPLY("*"); private final String symbol; Arithmetics(String symbol) { this.symbol = symbol; } public String getSymbol() { return symbol; } }

Java Collections: HashMaps and More

HashMap

LinkedHashMap & TreeMap

Other Important Collections

Java Input and Output

Breakdown of Java Input and Output via Command Line (CMD)

1. Java Output (System.out)

Basic Output Methods

Explanation:

CMD Execution:

2. Java Input (System.in)

Reading Input via Scanner

Explanation:

CMD Execution:

3. Java Error Output (System.err)

Example

CMD Execution & Output

4. Redirecting Input and Output in CMD

Redirecting Output to a File

Reading Input from a File

5. Advanced: Reading Input via BufferedReader

Why Use BufferedReader?

Important Notes

Reading Files

`public enum Arithmetics { ADD("+"), SUBTRACT("-"), DIVIDE("/"), MULTIPLY("*"); private final String symbol; Arithmetics(String symbol) { this.symbol = symbol; } public String getSymbol() { return symbol; } }`

1. Java Output (`System.out`)

2. Java Input (`System.in`)

3. Java Error Output (`System.err`)

Why Use `BufferedReader`?