CLASSES in Java

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CLASSES
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Examples of Field Declarations
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}; } int w = x = 3; // ok - x at left hand side of assignment int p = x; // ok - instance initializers may access static elds static int u = (new Object(){int bar(){return x;}})bar(); // ok - occurs in a different class static int x; int m = j = 4; // ok - j at left hand side of assignment int o = (new Object(){int bar(){return j;}})bar(); // ok - occurs in a different class int j; }
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833 Examples of Field Declarations
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The following examples illustrate some (possibly subtle) points about eld declarations 8331 Example: Hiding of Class Variables The example:
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class Point { static int x = 2; }
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47 2
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class Test extends Point { static double x = 47; public static void main(String[] args) { new Test()printX(); } void printX() { Systemoutprintln(x + " " + superx); } }
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because the declaration of x in class Test hides the de nition of x in class Point, so class Test does not inherit the eld x from its superclass Point Within the declaration of class Test, the simple name x refers to the eld declared within class Test Code in class Test may refer to the eld x of class Point as superx (or, because x is static, as Pointx) If the declaration of Testx is deleted:
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class Point { static int x = 2; }
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Examples of Field Declarations
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CLASSES
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class Test extends Point { public static void main(String[] args) { new Test()printX(); } void printX() { Systemoutprintln(x + " " + superx); } }
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8332 Example: Hiding of Instance Variables
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This example is similar to that in the previous section, but uses instance variables rather than static variables The code:
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class Point { int x = 2; }
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class Test extends Point { double x = 47; void printBoth() { Systemoutprintln(x + " " + superx); } public static void main(String[] args) { Test sample = new Test(); sampleprintBoth(); Systemoutprintln(samplex + " " + ((Point)sample)x); } }
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47 2 47 2
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because the declaration of x in class Test hides the de nition of x in class Point, so class Test does not inherit the eld x from its superclass Point It must be noted, however, that while the eld x of class Point is not inherited by class Test, it is nevertheless implemented by instances of class Test In other words, every instance of class Test contains two elds, one of type int and one of type double Both elds bear the name x, but within the declaration of class Test, the simple name x always refers to the eld declared within class Test Code in
then the eld x of class Point is no longer hidden within class Test; instead, the simple name x now refers to the eld Pointx Code in class Test may still refer to that same eld as superx Therefore, the output from this variant program is:
CLASSES
Examples of Field Declarations
instance methods of class Test may refer to the instance variable x of class Point as superx Code that uses a eld access expression to access eld x will access the eld named x in the class indicated by the type of reference expression Thus, the expression samplex accesses a double value, the instance variable declared in class Test, because the type of the variable sample is Test, but the expression ((Point)sample)x accesses an int value, the instance variable declared in class Point, because of the cast to type Point If the declaration of x is deleted from class Test, as in the program:
class Point { static int x = 2; }
then the eld x of class Point is no longer hidden within class Test Within instance methods in the declaration of class Test, the simple name x now refers to the eld declared within class Point Code in class Test may still refer to that same eld as superx The expression samplex still refers to the eld x within type Test, but that eld is now an inherited eld, and so refers to the eld x declared in class Point The output from this variant program is:
2 2 2 2
8333 Example: Multiply Inherited Fields A class may inherit two or more elds with the same name, either from two interfaces or from its superclass and an interface A compile-time error occurs on any attempt to refer to any ambiguously inherited eld by its simple name A quali ed name or a eld access expression that contains the keyword super ( 15112) may be used to access such elds unambiguously In the example:
interface Frob { float v = 20f; } class SuperTest { int v = 3; }
class Test extends Point { void printBoth() { Systemoutprintln(x + " " + superx); } public static void main(String[] args) { Test sample = new Test(); sampleprintBoth(); Systemoutprintln(samplex + " " + ((Point)sample)x); } }