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while (len > 0) { if (ca[--len] == ch) { ++count; } }
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This nal tweak runs in a time of 2877 milliseconds So from our starting point to now we ve improved the method s performance by about 300 percent It still isn t blazingly fast, but certainly the improvement is well worth the small amount of additional code The charCounter example was of course just used as a simple vehicle to discuss techniques for optimizing code within time-consuming loops If we were actually interested in the functionality provided by the charCounter method, we might consider the implementation shown in Listing 1111 At rst glance, it seems reasonable that this approach would be more ef cient than the original technique used in Listing 117 but somewhat less ef cient than our fastest attempt shown in Listing 1110 Listing 1111 Using indexOf
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int count = 0; int index = 0; while (true) { index = sindexOf(ch, index); if (index != -1) { ++count; ++index; } else { break; } }
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The results obtained using the above implementation of charCounter on the test string are in the neighborhood of 630 milliseconds However, only the fact that indexOf is implemented as a native method on TINI makes it faster than Listing 1110 What wasn t mentioned in the previous discussion was the possible bene t of unrolling loops to further reduce the impact of the overhead of the loop structure This isn t speci c to TINI or Java It is a time-honored tradition usually employed out of true desperation Whether unrolling a loop is bene cial really depends on the time required testing for loop termination versus the time required for the sum of the operations performed inside the loop If the loop structure is lean and mean when compared with the operations performed inside the loop, there is little practical bene t to unrolling the loop On the other hand, if the loop terminator requires a large amount of time (perhaps due to necessary method invocations)
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compared to the time required to execute the code within the loop, it may be worthwhile In the previous example, we achieved substantial performance gains by replacing unnecessary method invocations with fast local variable accesses Under certain circumstances we can also improve performance by caching class or instance elds in local variables Accessing an instance variable is somewhat expensive because it requires the JVM to parse the object s internal structure and extract the speci ed eld Consider the array compare example shown in Listing 1112 In this case the arrays a and b are private instance variables If the arrays are of identical length, the isEqual method iterates through the elements of the array, checking for equality If the array elements are unequal at any index, isEqual aborts immediately and returns false Listing 1112 Array compare
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private byte[] a; private byte[] b; private boolean isEqual() { if (alength != blength) { return false; } for (int i = 0; i < alength; i++) { if (a[i] != b[i]) { return false; } } return true; }
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The worst-case execution time results when the arrays are identical If is run on arrays 16 kilobytes in length with identical contents, it requires 7390 milliseconds to compare all of the elements If we modify the method slightly and cache the array references, a and b, in local variables as shown in Listing 1113, the resulting run time is 4431 milliseconds In this case three instance variable accesses are replaced by local variable accesses for each loop iteration: two used in the comparison of the array elements and one used in extracting the array length Caching instance elds in local variables
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