Java Variables

Types of Variables in Java

Java classifies variables based on where they are declared and how long they live.

• Local variables — declared inside a method, constructor, or block. They exist only during that call and must be initialized before use; Java gives them no default value.
• Instance variables — declared inside a class but outside any method, without the static keyword. Each object gets its own copy, and they receive default values (0, false, null) if not explicitly initialized.
• Static (class) variables — declared with the static keyword. They belong to the class itself, are shared by all instances, and are initialized once when the class is loaded.

Declaration and Initialization

Declaration reserves a name and type (int age;). Initialization assigns a value (age = 25;). You can combine both in one statement: int age = 25;. Variable names in Java must start with a letter, underscore, or dollar sign, cannot be a reserved keyword, and are case-sensitive.

Example

public class VariableDemo {

    int instanceCount;              // instance variable, default value 0
    static int totalObjects = 0;    // static variable, shared across all objects

    VariableDemo() {
        instanceCount = 1;
        totalObjects++;
    }

    void showLocalVariable() {
        int localValue = 10;        // local variable, must be initialized before use
        System.out.println("Local value: " + localValue);
    }

    public static void main(String[] args) {
        VariableDemo obj1 = new VariableDemo();
        VariableDemo obj2 = new VariableDemo();
        obj1.showLocalVariable();
        System.out.println("Total objects created: " + VariableDemo.totalObjects);
    }
}

Local value: 10
Total objects created: 2

Exercise

Model bank accounts that each keep their own balance while the class keeps one shared count of how many accounts exist. Fill in the two field declarations and the constructor.

Expected output: Asha: 500, Ravi: 1200, then Accounts opened: 2, each on its own line.

public class BankAccount {
    // TODO: declare an instance variable for the owner name and one for the balance
    // TODO: declare a static variable that counts accounts

    BankAccount(String owner, int balance) {
        // TODO: store the values and increase the shared counter
    }

    public static void main(String[] args) {
        BankAccount first = new BankAccount("Asha", 500);
        BankAccount second = new BankAccount("Ravi", 1200);

        System.out.println(first.owner + ": " + first.balance);
        System.out.println(second.owner + ": " + second.balance);
        System.out.println("Accounts opened: " + BankAccount.accountCount);
    }
}

public class BankAccount {
    String owner;
    int balance;
    static int accountCount = 0;

    BankAccount(String owner, int balance) {
        this.owner = owner;
        this.balance = balance;
        accountCount++;
    }

    public static void main(String[] args) {
        BankAccount first = new BankAccount("Asha", 500);
        BankAccount second = new BankAccount("Ravi", 1200);

        System.out.println(first.owner + ": " + first.balance);
        System.out.println(second.owner + ": " + second.balance);
        System.out.println("Accounts opened: " + BankAccount.accountCount);
    }
}

Quick quiz

Interview questions