Field Declarations in Java

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1348 Field Declarations
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In particular, no linkage error will occur in the case where a class could no longer be recompiled because a eld access previously referenced a eld of a superclass with an incompatible type The previously compiled class with such a reference will continue to reference the eld declared in a superclass Thus compiling and executing the code:
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class Hyper { String h = "hyper"; } class Super extends Hyper { String s = "super"; } class Test { public static void main(String[] args) { Systemoutprintln(new Super()h); } }
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produces the output:
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hyper Changing Super to be de ned as: class Super extends Hyper { String s = "super"; int h = 0; }
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recompiling Hyper and Super, and executing the resulting new binaries with the old binary of Test produces the output:
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hyper The eld h of Hyper is output by the original binary of main While this may
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seem surprising at rst, it serves to reduce the number of incompatibilities that occur at run time (In an ideal world, all source les that needed recompilation would be recompiled whenever any one of them changed, eliminating such surprises But such a mass recompilation is often impractical or impossible, especially in the Internet And, as was previously noted, such recompilation would sometimes require further changes to the source code) As an example, if the program:
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class Hyper { String h = "Hyper"; } class Super extends Hyper { } class Test extends Super { public static void main(String[] args) { String s = new Test()h; Systemoutprintln(s); } }
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is compiled and executed, it produces the output: Suppose that a new version of class Super is then compiled: 346
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nal Fields and Constants
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class Super extends Hyper { char h = 'h'; }
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If the resulting binary is used with the existing binaries for Hyper and Test, then the output is still:
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Hyper
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even though compiling the source for these binaries:
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class Hyper { String h = "Hyper"; } class Super extends Hyper { char h = 'h'; } class Test extends Super { public static void main(String[] args) { String s = new Test()h; Systemoutprintln(s); } }
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1349 final Fields and Constants
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If a eld that was not final is changed to be final, then it can break compatibility with pre-existing binaries that attempt to assign new values to the eld For example, if the program:
class Super { static char s; } class Test extends Super { public static void main(String[] args) { s = 'a'; Systemoutprintln(s); } }
is compiled and executed, it produces the output: Suppose that a new version of class Super is produced:
would result in a compile-time error, because the h in the source code for main would now be construed as referring to the char eld declared in Super, and a char value can t be assigned to a String Deleting a eld from a class will break compatibility with any pre-existing binaries that reference this eld, and a NoSuchFieldError will be thrown when such a reference from a pre-existing binary is linked Only private elds may be safely deleted from a widely distributed class For purposes of binary compatibility, adding or removing a eld f whose type involves type variables ( 44) or parameterized types ( 45) is equivalent to the addition (respectively, removal) of a eld of the same name whose type is the erasure ( 46) of the type of f
1349 nal Fields and Constants
class Super { final static char s = b ; }
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class Flags { final static boolean debug = true; } class Test { public static void main(String[] args) { if (Flagsdebug) Systemoutprintln("debug is true"); } }
is compiled and executed, it produces the output:
debug is true
Suppose that a new version of class Flags is produced:
class Flags { final static boolean debug = false; } If Flags is recompiled but not Test, then running the new binary with the existing binary of Test produces the output: debug is true because the value of debug was a compile-time constant, and could have been used in compiling Test without making a reference to the class Flags
This result is a side-effect of the decision to support conditional compilation, as discussed at the end of 1421 This behavior would not change if Flags were changed to be an interface, as in the modi ed example:
interface Flags { boolean debug = true; } class Test { public static void main(String[] args) { if (Flagsdebug) Systemoutprintln("debug is true"); } }
(One reason for requiring inlining of constants is that switch statements require constants on each case, and no two such constant values may be the same The
If Super is recompiled but not Test, then running the new binary with the existing binary of Test results in a IllegalAccessError Deleting the keyword final or changing the value to which a eld is initialized does not break compatibility with existing binaries If a eld is a constant variable ( 4124), then deleting the keyword final or changing its value will not break compatibility with pre-existing binaries by causing them not to run, but they will not see any new value for the usage of the eld unless they are recompiled This is true even if the usage itself is not a compiletime constant expression ( 1528) If the example: