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Field Access Using a Primary 15111
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static String when(String name, Object t) { return " when " + name + " holds a " + tgetClass() + " at run time"; } }
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produces the output:
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tx=1 when t holds a class T at run time sx=0 when s holds a class S at run time sx=0 when s holds a class T at run time
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The last line shows that, indeed, the eld that is accessed does not depend on the run-time class of the referenced object; even if s holds a reference to an object of class T, the expression sx refers to the x eld of class S, because the type of the expression s is S Objects of class T contain two elds named x, one for class T and one for its superclass S This lack of dynamic lookup for eld accesses allows programs to be run ef ciently with straightforward implementations The power of late binding and overriding is available, but only when instance methods are used Consider the same example using instance methods to access the elds:
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Now the output is:
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tz()=1 when t holds a class T at run time sz()=0 when s holds a class S at run time sz()=1 when s holds a class T at run time
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The last line shows that, indeed, the method that is accessed does depend on the run-time class of referenced object; when s holds a reference to an object of class
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class S { int x = 0; int z() { return x; } } class T extends S { int x = 1; int z() { return x; } } class Test { public static void main(String[] args) { T t = new T(); Systemoutprintln("tz()=" + tz() + when("t", t)); S s = new S(); Systemoutprintln("sz()=" + sz() + when("s", s)); s = t; Systemoutprintln("sz()=" + sz() + when("s", s)); } static String when(String name, Object t) { return " when " + name + " holds a " + tgetClass() + " at run time"; }
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15112 Accessing Superclass Members using super
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EXPRESSIONS
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T, the expression sz() refers to the z method of class T, despite the fact that the type of the expression s is S Method z of class T overrides method z of class S
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The following example demonstrates that a null reference may be used to access a class (static) variable without causing an exception:
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class Test { static String mountain = "Chocorua"; static Test favorite(){ Systemoutprint("Mount "); return null; }
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public static void main(String[] args) { Systemoutprintln(favorite()mountain); } }
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It compiles, executes, and prints:
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Mount Chocorua
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15112 Accessing Superclass Members using super The special forms using the keyword super are valid only in an instance method, instance initializer or constructor, or in the initializer of an instance variable of a class; these are exactly the same situations in which the keyword this may be used ( 1583) The forms involving super may not be used anywhere in the class Object, since Object has no superclass; if super appears in class Object, then a compile-time error results Suppose that a eld access expression supername appears within class C , and the immediate superclass of C is class S Then supername is treated exactly as if it had been the expression ((S)this)name ; thus, it refers to the eld named name of the current object, but with the current object viewed as an instance of the superclass Thus it can access the eld named name that is visible in class S , even if that eld is hidden by a declaration of a eld named name in class C The use of super is demonstrated by the following example:
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interface I { int x class T1 implements class T2 extends T1 class T3 extends T2 = 0; } I { int x = 1; } { int x = 2; } {
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Even though the result of favorite() is null, a NullPointerException is not thrown That Mount is printed demonstrates that the Primary expression is indeed fully evaluated at run time, despite the fact that only its type, not its value, is used to determine which eld to access (because the eld mountain is static)