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In Java, having no keyword before defaults to the package-private modifier. When the class is declared as public, it is accessible to other classes defined in the same package as well as those defined in other packages. This is the most commonly used specifier for classes. However, a class itself cannot be declared as private.
C++ supports multiple inheritance of arbitrary classes. In Java a class can derive from only one class, [1] but a class can implement multiple interfaces [17] (in other words, it supports multiple inheritance of types, but only single inheritance of implementation). Java explicitly distinguishes between interfaces and classes.
If a package declaration is not used, classes are placed in an unnamed package. Classes in an unnamed package cannot be imported by classes in any other package. [3] The official Java Tutorial advises against this: Generally speaking, an unnamed package is only for small or temporary applications or when you are just beginning the development ...
The following Java class does some simple computation. package basic_example ; import java.lang.Math ; public class MyComplexClass { public MyComplexClass (){ // the constructor } public long myVeryComplexComputation ( double a , double b ){ return Math . round ( Math . cos ( a ) + Math . sin ( b ) * 9 ); } }
Packages are a part of a class name and they are used to group and/or distinguish named entities from other ones. Another purpose of packages is to govern code access together with access modifiers. For example, java.io.InputStream is a fully qualified class name for the class InputStream which is located in the package java.io.
Code from other packages is accessed by prefixing the package name before the appropriate identifier, for example class String in package java.lang can be referred to as java.lang.String (this is known as the fully qualified class name). Like C++, Java offers a construct that makes it unnecessary to type the package name (import).
[26] [27] In C++, an abstract class is a class having at least one abstract method given by the appropriate syntax in that language (a pure virtual function in C++ parlance). [25] A class consisting of only pure virtual methods is called a pure abstract base class (or pure ABC) in C++ and is also known as an interface by users of the language. [13]
For example, the C family of languages had support for objects and classes in C++ (originally C with Classes, 1980) and Objective-C (1983), only supporting modules 30 years or more later. Java (1995) supports modules in the form of packages, though the primary unit of code organization is a class.