JAVA Notes - Unit 3.pdf

JAVA PROGRAMMING UNIT-3 OBJECT ORIENTED PROGRAMMING CONCEPTS 3.1 CLASS AND OBJECT What is a class in Java? A class is a group of objects which have common properties. It is a template or blueprint from which objects are created....

JAVA PROGRAMMING UNIT-3 OBJECT ORIENTED PROGRAMMING CONCEPTS 3.1 CLASS AND OBJECT What is a class in Java? A class is a group of objects which have common properties. It is a template or blueprint from which objects are created. It is a logical entity. It can't be physical. A class in Java can contain: o Fields / Variables o Methods o Constructors o Blocks o Nested class and interface Syntax to declare a class: class class_name{ // field; // method; } What is an object in java? An entity that has state and behaviour is known as an object e.g., chair, bike, marker, pen, table, car, etc. It can be physical or logical (tangible and intangible). The example of an intangible object is the banking system. An object is an instance of a class. An object has two characteristics: o State: represents the data (value) of an object. o Behaviour: represents the behaviour (functionality) of an object such as deposit, withdraw, etc. Example: Pen is an object. Its name is Reynolds; colour is white, known as its state. It is used to write, so writing is its behaviour. Object Definitions: o An object is a real-world entity. o An object is a runtime entity.  JAVA PROGRAMMING o The object is an entity which has state and behaviour. o The object is an instance of a class. Example of Class and Object: class Student { int id; String name; public static void main(String args[]){ //Creating an object or instance Student s1=new Student ();//creating an object of Student //Printing values of the object System.out.println(s1.id);//accessing member through reference variable System.out.println(s1.name); } } Output: 0 null 3.2 CONSTRUCTOR AND TYPES OF CONSTRUCTORS In Java, a constructor is a special method that is used to initialize objects of a class. It is called automatically when an object is created using the new keyword. Constructors have the same name as the class and do not have a return type, not even void. 3.2.1 TYPES OF CONSTRUCTORS 1.Default Constructor: A default constructor is provided by Java if you do not explicitly define any constructor in your class. It has no parameters and does not perform any initialization. Its purpose is to initialize the object with default values (e.g., setting numeric properties to 0 and reference properties to null). EXAMPLE: class Bike1{ //creating a default constructor Bike1(){System.out.println("Bike is created");}  JAVA PROGRAMMING //main method public static void main(String args[]){ //calling a default constructor Bike1 b=new Bike1(); } } Output : Bike is created 2. Parameterized Constructor: A parameterized constructor is a constructor that takes one or more parameters. It allows you to initialize the object with specific values at the time of creation. EXAMPLE: class Person(var name: String, var age: Int) { // Parameterized constructor init { println("Name: $name, Age: $age") } // Method to display person details fun displayDetails() { println("Name: $name, Age: $age") } } fun main() { // Creating a Person object with parameterized constructor val person1 = Person("John", 30) // Displaying details of the object println("Person details:") person1.displayDetails() }  JAVA PROGRAMMING 3.Copy Constructor: A copy constructor is a constructor that creates a new object by copying the state of another object of the same class. It is useful when you want to create a new object with the same values as an existing object. EXAMPLE: class Person(var name: String, var age: Int) { // Primary constructor constructor(person: Person) : this(person.name, person.age) { // Copy constructor implementation println("Copy constructor called") } // Method to display person details fun displayDetails() { println("Name: $name, Age: $age") } } fun main() { // Creating a Person object val person1 = Person("John", 30) // Displaying details of the original object println("Original object:") person1.displayDetails() // Creating a new object using the copy constructor val person2 = Person(person1) // Displaying details of the copied object println("\nCopied object:") person2.displayDetails() }  JAVA PROGRAMMING 3.3 METHOD AND METHOD OVERLOADING Method is a collection of statements that are grouped together to perform a specific task. Methods are used to encapsulate code and provide reusability and modularity in your programs. They are defined inside classes and can be called to perform the actions they define. Syntax: modifier returnType methodName(parameterList) { // Method body } Example: public class Calculator{ //method of addition public int add(int num1, int num2) { return num1 + num2; } public static void main(String args[]){ //Creating an object of Calculator class Calculator obj = new Calculator(); //Calling an method System.out.println(obj.add(1,2)); } } Output : 3 Types of Method : 1. no return no parameter : Example : void display(){ System.out.println("Hello world"); } 2. no return with parameter : Example : void display(String msg){ System.out.println("Your message is : "+msg); }  JAVA PROGRAMMING 3. with return no parameter : Example : int number; int returnNumber(){ number = 5; return number; } 4. with return with parameter : Example : int add(int num1, int num2) { return num1 + num2; } 5. Recursive function : A method in java that calls itself is called recursive method. It makes the code compact but complex to understand. Syntax : returntype methodname(){ //code to be executed methodname();//calling same method } Example : public class RecursionExample3 { int factorial(int n){ if (n == 1) return 1; else return(n * factorial(n-1)); } public static void main(String[] args) { RecursionExample3 obj = new RecursionExample3(); System.out.println("Factorial of 5 is: "+ obj.factorial(5)); } } Output : Factorial of 5 is: 120  JAVA PROGRAMMING 3.3.1 METHOD OVERLOADING Method overloading is a feature in Java that allows you to define multiple methods with the same name but different parameters within the same class. The methods must have different parameter types, different numbers of parameters, or both. USES OF METHOD OVERLOADING: Method overloading when a couple of methods are needed with conceptually similar functionality with different parameters. EXAMPLE: public class Calculator { // Method to add two integers public int add(int a, int b) { return a + b; } // Method to add three integers public int add(int a, int b, int c) { return a + b + c; } // Method to add two doubles public double add(double a, double b) { return a + b; } // Method to add three doubles public double add(double a, double b, double c) { return a + b + c; } // Method to concatenate two strings public String add(String a, String b) { return a + b; }  JAVA PROGRAMMING // Method to concatenate three strings public String add(String a, String b, String c) { return a + b + c; } public static void main(String[] args) { Calculator calculator = new Calculator(); // Calling the overloaded methods int sum1 = calculator.add(5, 3); int sum2 = calculator.add(2, 3, 4); double result1 = calculator.add(2.5, 3.5); double result2 = calculator.add(1.5, 2.5, 3.5); String concat1 = calculator.add("Hello", "World"); String concat2 = calculator.add("Java", " is", " awesome"); // Outputting the results System.out.println("Sum of two integers: " + sum1); System.out.println("Sum of three integers: " + sum2); System.out.println("Sum of two doubles: " + result1); System.out.println("Sum of three doubles: " + result2); System.out.println("Concatenation of two strings: " + concat1); System.out.println("Concatenation of three strings: " + concat2); } }  JAVA PROGRAMMING 3.4 THIS KEYWORD ‘this’ keyword is a reference to the current instance of a class. It is used within an instance method or constructor to refer to the object on which the method or constructor is being invoked. Here are a few common uses of the this keyword: 1.Accessing Instance Variables: You can use this to refer to the current object's instance variables when there is a naming conflict between instance variables and method parameters or local variables. EXAMPLE: public class MyClass { // Instance variable private int value; // Constructor public MyClass(int value) { this.value = value; // Assigning value to the instance variable } // Method to display value public void displayValue() { // Accessing instance variable directly System.out.println("Value: " + value); } // Main method public static void main(String[] args) { // Creating an instance of MyClass MyClass obj = new MyClass(10); // Calling the method to display value obj.displayValue(); } }  JAVA PROGRAMMING 2.Invoking Another Constructor: In a constructor, you can use this to invoke another constructor of the same class. It is useful when you want to reuse the initialization logic of one constructor in another constructor. EXAMPLE: public class MyClass { private int value; // Primary constructor public MyClass(int value) { this.value = value; System.out.println("Primary constructor invoked with value: " + value); } // Secondary constructor invoking primary constructor public MyClass(int value, String name) { this(value); // Invoking the primary constructor System.out.println("Secondary constructor invoked with value: " + value + " and name: " + name); } // Method to display value public void displayValue() { System.out.println("Value: " + value); } // Main method public static void main(String[] args) { // Creating an instance using primary constructor MyClass obj1 = new MyClass(10); obj1.displayValue(); // Creating an instance using secondary constructor MyClass obj2 = new MyClass(20, "John"); obj2.displayValue(); } }  JAVA PROGRAMMING 3.Returning The Current Instance: You can use this to return the current instance of the object from a method. It is commonly used for method chaining or fluent interfaces. EXAMPLE: public class MyClass { private int value; // Constructor public MyClass(int value) { this.value = value; } // Method to increment value and return the current instance public MyClass increment() { value++; return this; // Returning the current instance } // Method to display value public void displayValue() { System.out.println("Value: " + value); } // Main method public static void main(String[] args) { // Creating an instance of MyClass MyClass obj = new MyClass(10); // Invoking method chaining and displaying the value obj.increment().increment().increment().displayValue(); } } 4. this can be used to invoke current class method (implicitly). 5. this can be passed as an argument in the method call. 6. this can be passed as argument in the constructor call.  JAVA PROGRAMMING 3.5 STATIC KEYWORD The static keyword is used to declare members (variables and methods) that belong to the class itself rather than to instances (objects) of the class. It allows you to access these members without creating an instance of the class. 1.Static Variables: When a variable is declared as static, it is called a static variable or a class variable. The static variable is shared among all instances of the class. There is only one copy of the static variable that is shared across all objects of the class. EXAMPLE: public class MyClass { // Static variable public static int count = 0; // Constructor public MyClass() { count++; // Incrementing static variable in the constructor } // Method to display count public static void displayCount() { System.out.println("Count: " + count); } // Main method public static void main(String[] args) { // Creating instances of MyClass MyClass obj1 = new MyClass(); MyClass obj2 = new MyClass(); MyClass obj3 = new MyClass(); // Calling static method to display count MyClass.displayCount(); } }  JAVA PROGRAMMING 2.Static Methods: When a method is declared as static, it is called a static method. Static methods belong to the class rather than individual objects. They can be invoked directly on the class itself, without the need to create an instance. EXAMPLE: public class MyClass { // Static method public static void staticMethod() { System.out.println("This is a static method"); } // Main method public static void main(String[] args) { // Calling static method using class name MyClass.staticMethod(); } } 3.Static Block: A static block is a block of code enclosed in curly braces {} that is executed when the class is loaded into memory. It is used to initialize static variables or perform any other static initialization tasks. EXAMPLE: public class MyClass { // Static variable public static int count; // Static block static { // Initializing static variable in the static block count = 0; System.out.println("Static block initialized"); } // Main method public static void main(String[] args) { // Accessing the static variable System.out.println("Count: " + MyClass.count); } }  JAVA PROGRAMMING 3.6 STRING CLASS AND ITS METHODS The String class represents a sequence of characters. It is a built-in class and provides various methods to manipulate and work with strings. 1.Length: The length() method returns the length (number of characters) of a string. String str = "Hello, World!"; int length = str.length(); Output: length = 13 2.Concatenation: The concat() method concatenates two strings and returns a new string. String str1 = "Hello"; String str2 = "World"; String result = str1.concat(str2); Output: result = "HelloWorld" Alternatively, you can use the + operator for string concatenation. String result = str1 + str2; Output result = "HelloWorld" 3.Substring: The substring() method returns a substring of the original string based on the specified starting and ending indexes. String str = "Hello, World!"; String substr = str.substring(7, 12); Output: substr = "World" 4.Comparison: The equals() method checks if two strings have the same content. String str1 = "Hello"; String str2 = "Hello"; boolean isEqual = str1.equals(str2); Output: isEqual = true The equalsIgnoreCase() method compares strings while ignoring case differences.  JAVA PROGRAMMING 5.Conversion: The toUpperCase() and toLowerCase() methods convert the string to uppercase and lowercase, respectively. String str = "Hello, World!"; String uppercase = str.toUpperCase(); String lowercase = str.toLowerCase(); Output: uppercase = "HELLO, WORLD!" lowercase = "hello, world!" 6.Searching: The indexOf() method returns the index of the first occurrence of a specified substring within a string. String str = "Hello, World!"; int index = str.indexOf("World"); Output: index = 7 7.Splitting: The split() method splits a string into an array of substrings based on a specified delimiter. String str = "Hello,World,Java"; String[] parts = str.split(","); Output: parts = ["Hello", "World", "Java"] 8.Replacement: The replace() method replaces all occurrences of a specified character or substring with another character or substring. String str = "Hello, World!"; String replaced =str.replace("Hello", "Hi"); Output: replaced = "Hi, World!"  JAVA PROGRAMMING EXAMPLE: public class StringOperationsExample { public static void main(String[] args) { // Length String str = "Hello, world!"; int length = str.length(); System.out.println("Length of the string: " + length); // Concatenation String str1 = "Hello"; String str2 = "World"; String concatenatedString = str1.concat(" ").concat(str2); System.out.println("Concatenated string: " + concatenatedString); // Substring String substring = str.substring(7); System.out.println("Substring from index 7: " + substring); // Comparison String str3 = "hello"; boolean isEqualIgnoreCase = str1.equalsIgnoreCase(str3); System.out.println("Comparison (ignoring case): " + isEqualIgnoreCase); // Conversion int number = 123; String strNumber = String.valueOf(number); System.out.println("Converted number to string: " + strNumber); // Searching boolean contains = str.contains("world"); System.out.println("Contains 'world': " + contains);  JAVA PROGRAMMING // Splitting String sentence = "This is a sentence"; String[] words = sentence.split("\\s+"); System.out.println("Splitting sentence into words:"); for (String word : words) { System.out.println(word); } // Replacement String replacedString = str.replace("world", "Java"); System.out.println("Replaced 'world' with 'Java': " + replacedString); } } Difference between String and String Buffer No. String StringBuffer 1) The String class is immutable. The StringBuffer class is mutable. 2) String is slow and consumes more memory when we concatenate too many StringBuffer is fast and consumes less strings because every time it creates new memory when we concatenate t strings. instance. 3) String class overrides the equals() method of Object class. So you can StringBuffer class doesn't override the compare the contents of two strings by equals() method of Object class. equals() method. 4) String class is slower while performing StringBuffer class is faster while performing concatenation operation. concatenation operation. 5) String class uses String constant pool. StringBuffer uses Heap memory  JAVA PROGRAMMING 3.7 I/O CLASSES AND FILE HANDLING I/O (Input/Output) classes are used for reading input from various sources and writing output to different destinations. These classes provide functionality for file handling, reading from and writing to streams, and interacting with the user via the console. 1.File class: The File class is used to represent files and directories. It provides methods to create, delete, rename, and query file properties. import java.io.File; // Creating a File object File file = new File("path/to/file.txt"); // Checking if the file exists boolean exists = file.exists(); // Creating a directory File directory = new File("path/to/directory"); directory.mkdir(); 2.Scanner class: The Scanner class is used for reading user input from the console or parsing formatted data from different sources. import java.util.Scanner; // Reading user input from the console Scanner scanner = new Scanner(System.in); System.out.print("Enter your name: "); String name = scanner.nextLine(); System.out.println("Hello, " + name); // Parsing data from a file File file = new File("data.txt"); Scanner fileScanner = new Scanner(file); while (fileScanner.hasNext()) { String data = fileScanner.nextLine(); // Process the data } fileScanner.close();