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Included in a class declaration to specify one or more interfaces that are implemented by the current class. A class inherits the types and abstract methods declared by the interfaces. import Used at the beginning of a source file to specify classes or entire Java packages to
If a class does not specify its superclass, it implicitly inherits from java.lang.Object class. Thus all classes in Java are subclasses of Object class. If the superclass does not have a constructor without parameters the subclass must specify in its constructors what constructor of the superclass to use. For example:
An interface in the Java programming language is an abstract type that is used to declare a behavior that classes must implement. They are similar to protocols.Interfaces are declared using the interface keyword, and may only contain method signature and constant declarations (variable declarations that are declared to be both static and final).
static is a reserved word in many programming languages to modify a declaration. The effect of the keyword varies depending on the details of the specific programming language, most commonly used to modify the lifetime (as a static variable) and visibility (depending on linkage), or to specify a class member instead of an instance member in classes.
A class declared as sealed via the keyword sealed in C# or final in Java or PHP. [38] [39] [40] For example, Java's String class is marked as final. [41] Sealed classes may allow a compiler to perform optimizations that are not available for classes that can be subclassed. [42]
According to Java Language Specification: [5] A type variable is an unqualified identifier. Type variables are introduced by generic class declarations, generic interface declarations, generic method declarations, and by generic constructor declarations. A class is generic if it declares one or more type variables. [6]
Declaring all constructors of the class to be private, which prevents it from being instantiated by other objects Providing a static method that returns a reference to the instance The instance is usually stored as a private static variable ; the instance is created when the variable is initialized, at some point before when the static method ...
A Java package organizes Java classes into namespaces, [1] providing a unique namespace for each type it contains. Classes in the same package can access each other's package-private and protected members. In general, a package can contain the following kinds of types: classes, interfaces, enumerations, records and annotation types. A package ...