All Java programs use objects, and the type of an object is defined by its class or interface. Every Java program is defined as a class and nontrivial programs usually include a number of classes and interface definitions.
A class is a collection of fields that hold values and methods that operate on those values. Classes are the most fundamental structural element of all Java programs. One cannot write a code in Java without defining a class. All Java statements appear within methods and all methods are implemented within classes.
A class defines a new reference type. An object is an instance of a class. A point class defines a type that is the set of all possible two-dimensional points. A point object is a value of that type: it represents a single two-dimensional point.
Objects are usually created by instantiating a class with the new keyword and a constructor invocation, such as
Point p = new Point (1.0, 2.0);
A class definition consists of a signature and a body. The class signature defines the name of the class and may also specify other important information. The body of a class is a set of members enclosed in curly braces. The members of a class may include fields and methods, constructors and initializers and nested types.
Members can be static or nonstatic. A static member belongs to the class itself while a nonstatic member is associated with the instances of a class. The signature of a class may declare that the class extends another class. The extended class is known as the superclass and the extension is known as the subclass. A subclass inherits the members of its superclass and may declare new members or override inherited methods with new implementations.
The signature of a class may also declare that the class implements one or more interfaces. An interface is a reference type that defines method signatures but does not include method bodies to implement the methods. A class that implements an interface is required to provide bodies for the interface’s methods. Instances of such a class are also instances of the interface type that it implements.
The members of a class may have access modifiers public, protected, or private which specify their visibility and accessibility to clients and to subclasses. This allows classes to hide members that are not part of their public API. When applied to fields, this ability to hide members enables an object-oriented design technique known as data encapsulation. Classes and interfaces are the most important of the five fundamental reference types defined by Java. Arrays, enumerated and annotation types are the other three.
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