A SavingsAccount child class extends its Account parent class class SavingsAccount extends Account Listing 3 presents a SavingsAccount child class that extends its Account parent class. For a better solution, consider BigDecimal, which is part of Java's standard class library. You might prefer to use a double or a float to store monetary values, but doing that can lead to inaccuracies. Representing currency valuesĬount of pennies. (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. Listing 2 describes a generic bank account class that has a name and an initial amount, which are both set in the constructor. Furthermore, they can declare their own fields and methods to differentiate them from their parents. Instead, child classes declare their own constructors. They never inherit constructors, however. Given this declaration, the compiler will report an error if someone attempts to extend Password.Ĭhild classes inherit accessible fields and methods from their parent classes and other ancestors. Simply prefix a class header with final, as in final class Password. In Java, we use the final keyword to prevent some classes from being extended. You might declare a class that should not be extended for instance for security reasons. Car and SavingsAccount are known as derived classes, child classes, or subclasses. Vehicle and Account are known as base classes, parent classes, or superclasses. These examples codify is-a relationships: Car is a specialized Vehicle and SavingsAccount is a specialized Account. It's impossible to specify multiple class names after extends because Java doesn't support class-based multiple inheritance. The class name before extends identifies the child and the class name after extends identifies the parent. The extends keyword is specified after the class name and before another class name. inherit accessible members from Account
inherit accessible members from Vehicle The extends keyword specifies a parent-child relationship class Vehicle Below I use extends to establish a relationship between classes Vehicle and Car, and then between Account and SavingsAccount: Listing 1. When present, extends specifies a parent-child relationship between two classes. Java supports class extension via the extends keyword. Figure 2 shows this pattern in the context of vehicle, land vehicle, water vehicle, and hovercraft. When viewing an inheritance hierarchy, you can easily detect multiple inheritance by the presence of a diamond pattern.
Java doesn't support multiple inheritance through class extension, however. Java supports single inheritance through class extension, in which one class directly inherits accessible fields and methods from another class by extending that class. Hovercraft multiply inherits from land vehicle and water vehicle categoriesĬategories are described by classes. The hierarchy in Figure 2 illustrates multiple inheritance. In contrast, multiple inheritance enables a child category to inherit state and behaviors from two or more immediate parent categories. This example illustrates single inheritance in which a child category inherits state and behaviors from one immediate parent category. A pair of inheritance hierarchies are rooted in the common vehicle category Arrows point from more-specific "child" categories (lower down) to less-specific "parent" categories (higher up). As an example, Figure 1 shows car and truck inheriting from vehicle station wagon inheriting from car and garbage truck inheriting from truck. Since compiler tries to insert super() to the 2 constructors in the Sub class, but the Super's default constructor is not defined, compiler reports the error message.Inheritance can descend through multiple levels, leading to ever-more-specific categories. The constructors of the Sub class, either with-argument or no-argument, will call the no-argument Super constructor. This is the situation for the Super class above. If a constructor is defined in Super class, in this case Super(String s), compiler will not insert the default no-argument constructor. In Java, if a class does not define a constructor, compiler will insert a default no-argument constructor for the class by default. This compilation error occurs because the default super constructor is undefined. "Implicit super constructor is undefined for default constructor. This is a compilation error message seen by a lot of Java developers: A Common Error Message: Implicit super constructor is undefined for default constructor
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