Instance Variables And Instance Methods in Java

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Instance Variables in Java

Instance variables, also known as member variables or fields, are variables defined within a class for which each instantiated object of the class has its own copy. They represent the properties or state of an object.

Key Characteristics

Unique to Each Instance: Each object of the class has its own copy of instance variables, meaning changes to the instance variables of one object do not affect the instance variables of another object.
Declared Without static Keyword: Instance variables are declared without the static keyword.
Object-Level Scope: They are accessed through object references.
Lifecycle: Instance variables are created when an object of the class is created using the new keyword, and they are destroyed when the object is destroyed.
Default Values: Instance variables have default values if not explicitly initialized. For example, numeric types default to 0, boolean defaults to false, and object references default to null.
Accessibility: Instance variables are accessible throughout the class, including all methods, constructors, and blocks. They are tied to a specific instance of the class and accessed using an object reference.
Storage: Instance variables are stored in the heap area of memory.
Initialization: Instance variables and instance blocks are recognized and initialized just before the execution of the respective class constructor.
Access Modifiers: Instance variables can be declared with access modifiers (private, protected, public, or default) to control their visibility and accessibility.

Code Example

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public class Car {    // Instance variables    private String brand;    private String model;    private int year;    private double price;​    // Constructor to initialize instance variables    public Car(String brand, String model, int year, double price) {        this.brand = brand;        this.model = model;        this.year = year;        this.price = price;    }​    // Getter methods to access instance variables    public String getBrand() {        return brand;    }​    public String getModel() {        return model;    }​    public int getYear() {        return year;    }​    public double getPrice() {        return price;    }​    // Setter methods to modify instance variables    public void setBrand(String brand) {        this.brand = brand;    }​    public void setModel(String model) {        this.model = model;    }​    public void setYear(int year) {        this.year = year;    }​    public void setPrice(double price) {        this.price = price;    }​    // Method to display car details    public void displayDetails() {        System.out.println("Brand: " + brand);        System.out.println("Model: " + model);        System.out.println("Year: " + year);        System.out.println("Price: " + price);    }​    public static void main(String[] args) {        // Creating objects of the Car class        Car car1 = new Car("Toyota", "Camry", 2020, 24000);        Car car2 = new Car("Honda", "Accord", 2021, 26000);​        // Displaying details of car1        car1.displayDetails();        System.out.println();​        // Displaying details of car2        car2.displayDetails();    }}

Output

Brand: Toyota
Model: Camry
Year: 2020
Price: 24000.0

Brand: Honda
Model: Accord
Year: 2021
Price: 26000.0

Explanation:

Instance Variables: The Car class has four instance variables: brand, model, year, and price. These variables are declared private to encapsulate the data.
Constructor: The constructor public Car(String brand, String model, int year, double price) initializes the instance variables for a new Car object.
Getter Methods: Methods like getBrand, getModel, getYear, and getPrice are used to access the values of instance variables.
Setter Methods: Methods like setBrand, setModel, setYear, and setPrice are used to modify the values of instance variables.

Example of Default Values for Instance Variables

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public class Test {    int j;​    public void m1() {        System.out.println("j: " + this.j); // Will print default value of 0    }​    public static void main(String[] args) {        Test t = new Test();        t.m1();    }}

Output

j: 0

Instance Methods In Java

Instance methods are methods that belong to instances of a class. These methods can access instance variables and other instance methods directly.

Key Characteristics

Belong to Instances: Instance methods are tied to instances of a class and operate on those instances.
Access to Instance Variables: They can access instance variables and other instance methods directly.
Can Access Static Members: Instance methods can also access static variables and methods.
Declared Without static Keyword: Instance methods are declared without the static keyword.
Can Use this Keyword: Instance methods can use the this keyword to refer to the current instance of the class.
Scope: Object-level

Code Example

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public class Car {    // Instance variables    private String brand;    private String model;    private int year;    private double price;​    // Constructor to initialize instance variables    public Car(String brand, String model, int year, double price) {        this.brand = brand;        this.model = model;        this.year = year;        this.price = price;    }​    // Instance method to display car details    public void displayDetails() {        System.out.println("Brand: " + this.brand);        System.out.println("Model: " + this.model);        System.out.println("Year: " + this.year);        System.out.println("Price: " + this.price);    }​    public static void main(String[] args) {        // Creating objects of the Car class        Car car1 = new Car("Toyota", "Camry", 2020, 24000);        Car car2 = new Car("Honda", "Accord", 2021, 26000);​        // Calling instance methods        car1.displayDetails();        System.out.println("");        car2.displayDetails();    }}

Output

Brand: Toyota
Model: Camry
Year: 2020
Price: 24000.0

Brand: Honda
Model: Accord
Year: 2021
Price: 26000.0

Instance Variables in Java

Key Characteristics

Explanation:

Instance Methods In Java

Key Characteristics

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