Chapter 8. Classes
Class declarations define new reference types and describe how they are implemented (§8.1).
A top level class is a class that is not a nested class.
A nested class is any class whose declaration occurs within the body of another class or interface.
This chapter discusses the common semantics of all classes — top level (§7.6) and nested (including member classes (§8.5, §9.5), local classes (§14.3) and anonymous classes (§15.9.5)). Details that are specific to particular kinds of classes are discussed in the sections dedicated to these constructs.
A named class may be declared abstract (§8.1.1.1) and must be declared abstract if it is incompletely implemented; such a class cannot be instantiated, but can be extended by subclasses. A class may be declared final (§8.1.1.2), in which case it cannot have subclasses. If a class is declared public , then it can be referred to from code in any package of its module and potentially from code in other modules. Each class except Object is an extension of (that is, a subclass of) a single existing class (§8.1.4) and may implement interfaces (§8.1.5). Classes may be generic (§8.1.2), that is, they may declare type variables whose bindings may differ among different instances of the class.
Classes may be decorated with annotations (§9.7) just like any other kind of declaration.
The body of a class declares members (fields and methods and nested classes and interfaces), instance and static initializers, and constructors (§8.1.6). The scope (§6.3) of a member (§8.2) is the entire body of the declaration of the class to which the member belongs. Field, method, member class, member interface, and constructor declarations may include the access modifiers (§6.6) public , protected , or private . The members of a class include both declared and inherited members (§8.2). Newly declared fields can hide fields declared in a superclass or superinterface. Newly declared class members and interface members can hide class or interface members declared in a superclass or superinterface. Newly declared methods can hide, implement, or override methods declared in a superclass or superinterface.
Field declarations (§8.3) describe class variables, which are incarnated once, and instance variables, which are freshly incarnated for each instance of the class. A field may be declared final (§8.3.1.2), in which case it can be assigned to only once. Any field declaration may include an initializer.
Member class declarations (§8.5) describe nested classes that are members of the surrounding class. Member classes may be static , in which case they have no access to the instance variables of the surrounding class; or they may be inner classes (§8.1.3).
Member interface declarations (§8.5) describe nested interfaces that are members of the surrounding class.
Method declarations (§8.4) describe code that may be invoked by method invocation expressions (§15.12). A class method is invoked relative to the class type; an instance method is invoked with respect to some particular object that is an instance of a class type. A method whose declaration does not indicate how it is implemented must be declared abstract . A method may be declared final (§8.4.3.3), in which case it cannot be hidden or overridden. A method may be implemented by platform-dependent native code (§8.4.3.4). A synchronized method (§8.4.3.6) automatically locks an object before executing its body and automatically unlocks the object on return, as if by use of a synchronized statement (§14.19), thus allowing its activities to be synchronized with those of other threads (§17 (Threads and Locks)).
Method names may be overloaded (§8.4.9).
Instance initializers (§8.6) are blocks of executable code that may be used to help initialize an instance when it is created (§15.9).
Static initializers (§8.7) are blocks of executable code that may be used to help initialize a class.
Constructors (§8.8) are similar to methods, but cannot be invoked directly by a method call; they are used to initialize new class instances. Like methods, they may be overloaded (§8.8.8).
Declaring Classes
This is a class declaration. The class body (the area between the braces) contains all the code that provides for the life cycle of the objects created from the class: constructors for initializing new objects, declarations for the fields that provide the state of the class and its objects, and methods to implement the behavior of the class and its objects.
The preceding class declaration is a minimal one. It contains only those components of a class declaration that are required. You can provide more information about the class, such as the name of its superclass, whether it implements any interfaces, and so on, at the start of the class declaration. For example,
class MyClass extends MySuperClass implements YourInterface < // field, constructor, and // method declarations >
means that MyClass is a subclass of MySuperClass and that it implements the YourInterface interface.
You can also add modifiers like public or private at the very beginningso you can see that the opening line of a class declaration can become quite complicated. The modifiers public and private, which determine what other classes can access MyClass , are discussed later in this lesson. The lesson on interfaces and inheritance will explain how and why you would use the extends and implements keywords in a class declaration. For the moment you do not need to worry about these extra complications.
In general, class declarations can include these components, in order:
- Modifiers such as public, private, and a number of others that you will encounter later. (However, note that the private modifier can only be applied to Nested Classes.)
- The class name, with the initial letter capitalized by convention.
- The name of the class’s parent (superclass), if any, preceded by the keyword extends. A class can only extend (subclass) one parent.
- A comma-separated list of interfaces implemented by the class, if any, preceded by the keyword implements. A class can implement more than one interface.
- The class body, surrounded by braces, <>.
Classes
The introduction to object-oriented concepts in the lesson titled Object-oriented Programming Concepts used a bicycle class as an example, with racing bikes, mountain bikes, and tandem bikes as subclasses. Here is sample code for a possible implementation of a Bicycle class, to give you an overview of a class declaration. Subsequent sections of this lesson will back up and explain class declarations step by step. For the moment, don’t concern yourself with the details.
public class Bicycle < // the Bicycle class has // three fields public int cadence; public int gear; public int speed; // the Bicycle class has // one constructor public Bicycle(int startCadence, int startSpeed, int startGear) < gear = startGear; cadence = startCadence; speed = startSpeed; >// the Bicycle class has // four methods public void setCadence(int newValue) < cadence = newValue; >public void setGear(int newValue) < gear = newValue; >public void applyBrake(int decrement) < speed -= decrement; >public void speedUp(int increment) < speed += increment; >>
A class declaration for a MountainBike class that is a subclass of Bicycle might look like this:
public class MountainBike extends Bicycle < // the MountainBike subclass has // one field public int seatHeight; // the MountainBike subclass has // one constructor public MountainBike(int startHeight, int startCadence, int startSpeed, int startGear) < super(startCadence, startSpeed, startGear); seatHeight = startHeight; >// the MountainBike subclass has // one method public void setHeight(int newValue) < seatHeight = newValue; >>
MountainBike inherits all the fields and methods of Bicycle and adds the field seatHeight and a method to set it (mountain bikes have seats that can be moved up and down as the terrain demands).
Classes declaration in java
Class declarations define new reference types and describe how they are implemented (§8.1).
A top level class is a class that is not a nested class.
A nested class is any class whose declaration occurs within the body of another class or interface.
This chapter discusses the common semantics of all classes — top level (§7.6) and nested (including member classes (§8.5, §9.5), local classes (§14.3) and anonymous classes (§15.9.5)). Details that are specific to particular kinds of classes are discussed in the sections dedicated to these constructs.
A named class may be declared abstract (§8.1.1.1) and must be declared abstract if it is incompletely implemented; such a class cannot be instantiated, but can be extended by subclasses. A class may be declared final (§8.1.1.2), in which case it cannot have subclasses. If a class is declared public , then it can be referred to from other packages. Each class except Object is an extension of (that is, a subclass of) a single existing class (§8.1.4) and may implement interfaces (§8.1.5). Classes may be generic (§8.1.2), that is, they may declare type variables whose bindings may differ among different instances of the class.
Classes may be decorated with annotations (§9.7) just like any other kind of declaration.
The body of a class declares members (fields and methods and nested classes and interfaces), instance and static initializers, and constructors (§8.1.6). The scope (§6.3) of a member (§8.2) is the entire body of the declaration of the class to which the member belongs. Field, method, member class, member interface, and constructor declarations may include the access modifiers (§6.6) public , protected , or private . The members of a class include both declared and inherited members (§8.2). Newly declared fields can hide fields declared in a superclass or superinterface. Newly declared class members and interface members can hide class or interface members declared in a superclass or superinterface. Newly declared methods can hide, implement, or override methods declared in a superclass or superinterface.
Field declarations (§8.3) describe class variables, which are incarnated once, and instance variables, which are freshly incarnated for each instance of the class. A field may be declared final (§8.3.1.2), in which case it can be assigned to only once. Any field declaration may include an initializer.
Member class declarations (§8.5) describe nested classes that are members of the surrounding class. Member classes may be static , in which case they have no access to the instance variables of the surrounding class; or they may be inner classes (§8.1.3).
Member interface declarations (§8.5) describe nested interfaces that are members of the surrounding class.
Method declarations (§8.4) describe code that may be invoked by method invocation expressions (§15.12). A class method is invoked relative to the class type; an instance method is invoked with respect to some particular object that is an instance of a class type. A method whose declaration does not indicate how it is implemented must be declared abstract . A method may be declared final (§8.4.3.3), in which case it cannot be hidden or overridden. A method may be implemented by platform-dependent native code (§8.4.3.4). A synchronized method (§8.4.3.6) automatically locks an object before executing its body and automatically unlocks the object on return, as if by use of a synchronized statement (§14.19), thus allowing its activities to be synchronized with those of other threads (§17 (Threads and Locks)).
Method names may be overloaded (§8.4.9).
Instance initializers (§8.6) are blocks of executable code that may be used to help initialize an instance when it is created (§15.9).
Static initializers (§8.7) are blocks of executable code that may be used to help initialize a class.
Constructors (§8.8) are similar to methods, but cannot be invoked directly by a method call; they are used to initialize new class instances. Like methods, they may be overloaded (§8.8.8).