What keyword is used to access the base class method from the child class?

Use Java's extends keyword to derive a child class from a parent class, invoke parent class constructors and methods, override methods, and more

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Java supports class reuse through inheritance and composition. This two-part tutorial teaches you how to use inheritance in your Java programs. In Part 1 you'll learn how to use the

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

To complete your learning about inheritance, be sure to check out my Java tip explaining when to use composition vs inheritance. You'll learn why composition is an important complement to inheritance, and how to use it to guard against issues with encapsulation in your Java programs.

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Download the source code for example applications in this tutorial. Created by Jeff Friesen for JavaWorld.

Java inheritance: Two examples

Inheritance is a programming construct that software developers use to establish is-a relationships between categories. Inheritance enables us to derive more-specific categories from more-generic ones. The more-specific category is a kind of the more-generic category. For example, a checking account is a kind of account in which you can make deposits and withdrawals. Similarly, a truck is a kind of vehicle used for hauling large items.

Inheritance can descend through multiple levels, leading to ever-more-specific categories. As an example, Figure 1 shows car and truck inheriting from vehicle; station wagon inheriting from car; and garbage truck inheriting from truck. Arrows point from more-specific "child" categories (lower down) to less-specific "parent" categories (higher up).

Figure 1. A pair of inheritance hierarchies are rooted in the common vehicle category

This example illustrates single inheritance in which a child category inherits state and behaviors from one immediate parent category. In contrast, multiple inheritance enables a child category to inherit state and behaviors from two or more immediate parent categories. The hierarchy in Figure 2 illustrates multiple inheritance.

Figure 2. Hovercraft multiply inherits from land vehicle and water vehicle categories

Categories are described by classes. Java supports single inheritance through class extension, in which one class directly inherits accessible fields and methods from another class by extending that class. Java doesn't support multiple inheritance through class extension, however.

When viewing an inheritance hierarchy, you can easily detect multiple inheritance by the presence of a diamond pattern. Figure 2 shows this pattern in the context of vehicle, land vehicle, water vehicle, and hovercraft.

The extends keyword

Java supports class extension via the

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

Listing 1. The

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

4 keyword specifies a parent-child relationship

class Vehicle
{
   // member declarations
}
class Car extends Vehicle
{
   // inherit accessible members from Vehicle
   // provide own member declarations
}
class Account
{
   // member declarations
}
class SavingsAccount extends Account
{
   // inherit accessible members from Account
   // provide own member declarations
}

The

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

These examples codify is-a relationships:

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

Final classes

You might declare a class that should not be extended; for instance for security reasons. In Java, we use the

class CheckingAccount extends Account
{
   CheckingAccount(long amount)
   {
      super("checking", amount);
   }

   void withdraw(long amount)
   {
      setAmount(getAmount() - amount);
   }
}

class CheckingAccount extends Account
{
   CheckingAccount(long amount)
   {
      super("checking", amount);
   }

   void withdraw(long amount)
   {
      setAmount(getAmount() - amount);
   }
}

class CheckingAccount extends Account
{
   CheckingAccount(long amount)
   {
      super("checking", amount);
   }

   void withdraw(long amount)
   {
      setAmount(getAmount() - amount);
   }
}

class CheckingAccount extends Account
{
   CheckingAccount(long amount)
   {
      super("checking", amount);
   }

   void withdraw(long amount)
   {
      setAmount(getAmount() - amount);
   }
}

Child classes inherit accessible fields and methods from their parent classes and other ancestors. They never inherit constructors, however. Instead, child classes declare their own constructors. Furthermore, they can declare their own fields and methods to differentiate them from their parents. Consider Listing 2.

Listing 2. An

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

1 parent class

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

Listing 2 describes a generic bank account class that has a name and an initial amount, which are both set in the constructor. Also, it lets users make deposits. (You can make withdrawals by depositing negative amounts of money but we'll ignore this possibility.) Note that the account name must be set when an account is created.

Representing currency values

count of pennies. You might prefer to use a

class AccountDemo
{
   public static void main(String[] args)
   {
      SavingsAccount sa = new SavingsAccount(10000);
      System.out.println("account name: " + sa.getName());
      System.out.println("initial amount: " + sa.getAmount());
      sa.deposit(5000);
      System.out.println("new amount after deposit: " + sa.getAmount());

      CheckingAccount ca = new CheckingAccount(20000);
      System.out.println("account name: " + ca.getName());
      System.out.println("initial amount: " + ca.getAmount());
      ca.deposit(6000);
      System.out.println("new amount after deposit: " + ca.getAmount());
      ca.withdraw(3000);
      System.out.println("new amount after withdrawal: " + ca.getAmount());
   }
}

class AccountDemo
{
   public static void main(String[] args)
   {
      SavingsAccount sa = new SavingsAccount(10000);
      System.out.println("account name: " + sa.getName());
      System.out.println("initial amount: " + sa.getAmount());
      sa.deposit(5000);
      System.out.println("new amount after deposit: " + sa.getAmount());

      CheckingAccount ca = new CheckingAccount(20000);
      System.out.println("account name: " + ca.getName());
      System.out.println("initial amount: " + ca.getAmount());
      ca.deposit(6000);
      System.out.println("new amount after deposit: " + ca.getAmount());
      ca.withdraw(3000);
      System.out.println("new amount after withdrawal: " + ca.getAmount());
   }
}

class AccountDemo
{
   public static void main(String[] args)
   {
      SavingsAccount sa = new SavingsAccount(10000);
      System.out.println("account name: " + sa.getName());
      System.out.println("initial amount: " + sa.getAmount());
      sa.deposit(5000);
      System.out.println("new amount after deposit: " + sa.getAmount());

      CheckingAccount ca = new CheckingAccount(20000);
      System.out.println("account name: " + ca.getName());
      System.out.println("initial amount: " + ca.getAmount());
      ca.deposit(6000);
      System.out.println("new amount after deposit: " + ca.getAmount());
      ca.withdraw(3000);
      System.out.println("new amount after withdrawal: " + ca.getAmount());
   }
}

Listing 3 presents a

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

Listing 3. A

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

2 child class extends its

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

1 parent class

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

The

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

javac AccountDemo.java
javac *.java

When and where to call super()

Just as

javac AccountDemo.java
javac *.java

javac AccountDemo.java
javac *.java

javac AccountDemo.java
javac *.java

javac AccountDemo.java
javac *.java

Listing 4 further extends

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

javac AccountDemo.java
javac *.java

Listing 4. A

javac AccountDemo.java
javac *.java

7 child class extends its

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

1 parent class

class CheckingAccount extends Account
{
   CheckingAccount(long amount)
   {
      super("checking", amount);
   }

   void withdraw(long amount)
   {
      setAmount(getAmount() - amount);
   }
}

javac AccountDemo.java
javac *.java

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

java AccountDemo

java AccountDemo

java AccountDemo

javac AccountDemo.java
javac *.java

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

java AccountDemo

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

java AccountDemo

super() and the no-argument constructor

If

javac AccountDemo.java
javac *.java

account name: savings
initial amount: 10000
new amount after deposit: 15000
account name: checking
initial amount: 20000
new amount after deposit: 26000
new amount after withdrawal: 23000

account name: savings
initial amount: 10000
new amount after deposit: 15000
account name: checking
initial amount: 20000
new amount after deposit: 26000
new amount after withdrawal: 23000

javac AccountDemo.java
javac *.java

Class hierarchy example

I've created an

account name: savings
initial amount: 10000
new amount after deposit: 15000
account name: checking
initial amount: 20000
new amount after deposit: 26000
new amount after withdrawal: 23000

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

account name: savings
initial amount: 10000
new amount after deposit: 15000
account name: checking
initial amount: 20000
new amount after deposit: 26000
new amount after withdrawal: 23000

Listing 5.

account name: savings
initial amount: 10000
new amount after deposit: 15000
account name: checking
initial amount: 20000
new amount after deposit: 26000
new amount after withdrawal: 23000

4 demonstrates the account class hierarchy

class AccountDemo
{
   public static void main(String[] args)
   {
      SavingsAccount sa = new SavingsAccount(10000);
      System.out.println("account name: " + sa.getName());
      System.out.println("initial amount: " + sa.getAmount());
      sa.deposit(5000);
      System.out.println("new amount after deposit: " + sa.getAmount());

      CheckingAccount ca = new CheckingAccount(20000);
      System.out.println("account name: " + ca.getName());
      System.out.println("initial amount: " + ca.getAmount());
      ca.deposit(6000);
      System.out.println("new amount after deposit: " + ca.getAmount());
      ca.withdraw(3000);
      System.out.println("new amount after withdrawal: " + ca.getAmount());
   }
}

The

account name: savings
initial amount: 10000
new amount after deposit: 15000
account name: checking
initial amount: 20000
new amount after deposit: 26000
new amount after withdrawal: 23000

class SavingsAccount extends Account
{
   SavingsAccount(long amount)
   {
      super("savings", amount);
   }
}

javac AccountDemo.java
javac *.java

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

javac AccountDemo.java
javac *.java

Execute the following command to run the application:

java AccountDemo

You should observe the following output:

account name: savings
initial amount: 10000
new amount after deposit: 15000
account name: checking
initial amount: 20000
new amount after deposit: 26000
new amount after withdrawal: 23000

Method overriding (and method overloading)

A subclass can override (replace) an inherited method so that the subclass's version of the method is called instead. An overriding method must specify the same name, parameter list, and return type as the method being overridden. To demonstrate, I've declared a

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

Listing 6. Declaring a

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

5 method to be overridden

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

Next, I override

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

Listing 7. Overriding

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

5 in a

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

9 subclass

class Truck extends Vehicle
{
   private double tonnage;

   Truck(String make, String model, int year, double tonnage)
   {
      super(make, model, year);
      this.tonnage = tonnage;
   }

   double getTonnage()
   {
      return tonnage;
   }

   void print()
   {
      super.print();
      System.out.println("Tonnage: " + tonnage);
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

Calling superclass methods from subclass methods

In order to call a superclass method from the overriding subclass method, prefix the method's name with the reserved word

javac AccountDemo.java
javac *.java

java AccountDemo

javac AccountDemo.java
javac *.java

java AccountDemo

To complete this example, I've excerpted a

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

account name: savings
initial amount: 10000
new amount after deposit: 15000
account name: checking
initial amount: 20000
new amount after deposit: 26000
new amount after withdrawal: 23000

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

The final line,

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

Use final to block method overriding

Occasionally you might need to declare a method that should not be overridden, for security or another reason. You can use the

class CheckingAccount extends Account
{
   CheckingAccount(long amount)
   {
      super("checking", amount);
   }

   void withdraw(long amount)
   {
      setAmount(getAmount() - amount);
   }
}

class CheckingAccount extends Account
{
   CheckingAccount(long amount)
   {
      super("checking", amount);
   }

   void withdraw(long amount)
   {
      setAmount(getAmount() - amount);
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

Method overloading vs overriding

Suppose you replaced the

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

The modified

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Vehicle
{
   private String make;
   private String model;
   private int year;

   Vehicle(String make, String model, int year)
   {
      this.make = make;
      this.model = model;
      this.year = year;
   }

   String getMake()
   {
      return make;
   }

   String getModel()
   {
      return model;
   }

   int getYear()
   {
      return year;
   }

   void print()
   {
      System.out.println("Make: " + make + ", Model: " + model + ", Year: " +
                         year);
   }
}

You can detect an attempt to overload instead of override a method at compile time by prefixing a subclass's method header with the

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

Specifying

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

When to use @Override

Develop the habit of prefixing overriding methods with

class Account
{
   private String name;

   private long amount;

   Account(String name, long amount)
   {
      this.name = name;
      setAmount(amount);
   }

   void deposit(long amount)
   {
      this.amount += amount;
   }

   String getName()
   {
      return name;
   }

   long getAmount()
   {
      return amount;
   }

   void setAmount(long amount)
   {
      this.amount = amount;
   }
}

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