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The simplest example of type introspection in Java is the instanceof [1] operator. The instanceof operator determines whether a particular object belongs to a particular class (or a subclass of that class, or a class that implements that interface). For instance:
Added in Java 9, the underscore has become a keyword and cannot be used as a variable name anymore. [3] abstract A method with no definition must be declared as abstract and the class containing it must be declared as abstract. Abstract classes cannot be instantiated. Abstract methods must be implemented in the sub classes.
Java only enforces type information at compile-time. After the type information is verified at compile-time, the type information is discarded, and at run-time, the type information will not be available. [6] Examples of non-reifiable types include List<T> and List<String>, where T is a generic formal parameter. [6]
This is a list of the instructions that make up the Java bytecode, an abstract machine language that is ultimately executed by the Java virtual machine. [1] The Java bytecode is generated from languages running on the Java Platform, most notably the Java programming language.
A snippet of Java code with keywords highlighted in bold blue font. The syntax of Java is the set of rules defining how a Java program is written and interpreted. The syntax is mostly derived from C and C++. Unlike C++, Java has no global functions or variables, but has data members which are also regarded as global variables.
In computer science, an instance is an occurrence of a software element that is based on a type definition. When created, an occurrence is said to have been instantiated, and both the creation process and the result of creation are called instantiation.
A Java typecast behaves similarly; if the object being cast is not actually an instance of the target type, and cannot be converted to one by a language-defined method, an instance of java.lang.ClassCastException will be thrown. [9]
A popular example of a badly considered design is containers of top types, [citation needed] like the Java containers before Java generics were introduced, which requires downcasting of the contained objects so that they can be used again.