Variables of Reference Type in Java

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4122 Variables of Reference Type
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A variable of type Object can hold a null reference or a reference to any object, whether class instance or array 41221 Heap Pollution It is possible that a variable of a parameterized type refers to an object that is not of that parameterized type This situation is known as heap pollution This situation can only occur if the program performed some operation that would give rise to an unchecked warning at compile-time
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DISCUSSION For example, the code:
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List l = new ArrayList<Number>(); List<String> ls = l; // unchecked warning
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gives rise to an unchecked warning, because it is not possible to ascertain, either at compile-time (within the limits of the compile-time type checking rules) or at run-time, whether the variable l does indeed refer to a List<String> If the code above is executed, heap pollution arises, as the variable ls, declared to be a List<String>, refers to a value that is not in fact a List<String> The problem cannot be identi ed at run-time because type variables are not rei ed, and thus instances do not carry any information at run-time regarding the actual type parameters used to create them In a simple example as given above, it may appear that it should be straightforward to identify the situation at compile-time and give a compilation error However, in the general (and typical) case, the value of the variable l may be the result of an invocation of a separately compiled method, or its value may depend upon arbitrary control ow The code above is therefore very atypical, and indeed very bad style Assignment from a value of a raw type to a variable of a parameterized type should only be used when combining legacy code which does not make use of parameterized types with more modern code that does If no operation that requires an unchecked warning to be issued takes place, heap pollution cannot occur Note that this does not imply that heap pollution only occurs if an unchecked warning actually occurred It is possible to run a program where some of the binaries were compiled by a compiler for an older version of the Java programming language, or by a compiler that allows the unchecked warnings to suppressed This practice is unhealthy at best Conversely, it is possible that despite executing code that could (and perhaps did) give rise to an unchecked warning, no heap pollution takes place Indeed, good programming practice requires that the programmer satisfy herself that despite any unchecked warning, the code is correct and heap pollution will not occur
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TYPES, VALUES, AND VARIABLES
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The variable will always refer to an object that is an instance of a class that implements the parameterized type
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For instance, the value of l in the example above is always a List
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4123 Kinds of Variables There are seven kinds of variables:
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2 An instance variable is a eld declared within a class declaration without using the keyword static ( 8311) If a class T has a eld a that is an instance variable, then a new instance variable a is created and initialized to a default value ( 4125) as part of each newly created object of class T or of any class that is a subclass of T ( 814) The instance variable effectively ceases to exist when the object of which it is a eld is no longer referenced, after any necessary nalization of the object ( 126) has been completed 3 Array components are unnamed variables that are created and initialized to default values ( 4125) whenever a new object that is an array is created ( 1510) The array components effectively cease to exist when the array is no longer referenced See 10 for a description of arrays 4 Method parameters ( 841) name argument values passed to a method For every parameter declared in a method declaration, a new parameter variable is created each time that method is invoked ( 1512) The new variable is initialized with the corresponding argument value from the method invocation The method parameter effectively ceases to exist when the execution of the body of the method is complete 5 Constructor parameters ( 881) name argument values passed to a constructor For every parameter declared in a constructor declaration, a new parameter variable is created each time a class instance creation expression ( 159) or
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1 A class variable is a eld declared using the keyword static within a class declaration ( 8311), or with or without the keyword static within an interface declaration ( 93) A class variable is created when its class or interface is prepared ( 1232) and is initialized to a default value ( 4125) The class variable effectively ceases to exist when its class or interface is unloaded ( 127)
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