Effective Java: Programming Language Guide in Java

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Effective Java: Programming Language Guide
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To see why this for loop is preferable to the more obvious while loop, consider the following code fragment, which contains two while loops and one bug:
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Iterator i = citerator(); while (ihasNext()) { doSomething(inext()); } Iterator i2 = c2iterator(); while (ihasNext()) { // BUG! doSomethingElse(i2next()); }
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The second loop contains a cut-and-paste error: It initializes a new loop variable, i2, but uses the old one, i, which unfortunately is still in scope The resulting code compiles without error and runs without throwing an exception, but it does the wrong thing Instead of iterating over c2, the second loop terminates immediately, giving the false impression that c2 is empty Because the program errs silently, the error can remain undetected for a long time If the analogous cut-and-paste error were made in conjunction with the preferred for loop idiom, the resulting code wouldn't even compile The loop variable from the first loop would not be in scope at the point where the second loop occurred:
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for (Iterator i = citerator(); ihasNext(); ) { doSomething(inext()); } // Compile-time error - the symbol i cannot be resolved for (Iterator i2 = c2iterator(); ihasNext(); ) { doSomething(i2next()); }
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Moreover, if you use the for loop idiom, it's much less likely that you'll make the cut-andpaste error, as there's no incentive to use a different variable name in the two loops The loops are completely independent, so there's no harm in reusing the loop variable name In fact, it's stylish to do so The for loop idiom has one other advantage over the while loop idiom, albeit a minor one The for loop idiom is one line shorter, which helps the containing method fit in a fixed-size editor window, enhancing readability Here is another loop idiom for iterating over a list that minimizes the scope of local variables:
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// High-performance idiom for iterating over random access lists for (int i = 0, n = listsize(); i < n; i++) { doSomething(listget(i)); }
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Effective Java: Programming Language Guide
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This idiom is useful for random access List implementations such as ArrayList and Vector because it is likely to run faster than the preferred idiom above for such lists The important thing to notice about this idiom is that it has two loop variables, i and n, both of which have exactly the right scope The use of the second variable is essential to the performance of the idiom Without it, the loop would have to call the size method once per iteration, which would negate the performance advantage of the idiom Using this idiom is acceptable when you're sure the list really does provide random access; otherwise, it displays quadratic performance Similar idioms exist for other looping tasks, for example,
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for (int i = 0, n = expensiveComputation(); i < n; i++) { doSomething(i); }
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Again, this idiom uses two loop variables, and the second variable, n, is used to avoid the cost of performing redundant computation on every iteration As a rule, you should use this idiom if the loop test involves a method invocation and the method invocation is guaranteed to return the same result on each iteration A final technique to minimize the scope of local variables is to keep methods small and focused If you combine two activities in the same method, local variables relevant to one activity may be in the scope of the code performing the other activity To prevent this from happening, simply separate the method into two: one for each activity
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Item 30: Know and use the libraries
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Suppose you want to generate random integers between 0 and some upper bound Faced with this common task, many programmers would write a little method that looks something like this:
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static Random rnd = new Random(); // Common but flawed! static int random(int n) { return Mathabs(rndnextInt()) % n; }
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This method isn't bad, but it isn't perfect, either it has three flaws The first flaw is that if n is a small power of two, the sequence of random numbers it generates will repeat itself after a fairly short period The second flaw is that if n is not a power of two, some numbers will, on average, be returned more frequently than others If n is large, this flaw can be quite pronounced This is graphically demonstrated by the following program, which generates a million random numbers in a carefully chosen range and then prints out how many of the numbers fell in the lower half of the range:
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Effective Java: Programming Language Guide public static void main(String[] args) { int n = 2 * (IntegerMAX_VALUE / 3); int low = 0; for (int i = 0; i < 1000000; i++) if (random(n) < n/2) low++; } Systemoutprintln(low);
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If the random method worked properly, the program would print a number close to half a million, but if you run it, you'll find that it prints a number close to 666,666 Two thirds of the numbers generated by the random method fall in the lower half of its range! The third flaw in the random method is that it can, on rare occasion, fail catastrophically, returning a number outside the specified range This is so because the method attempts to map the value returned by rndnextInt() into a nonnegative integer with Mathabs If nextInt() returns IntegerMIN_VALUE, Mathabs will also return IntegerMIN_VALUE, and the remainder operator (%) will return a negative number, assuming n is not a power of two This will almost certainly cause your program to fail, and the failure may be difficult to reproduce To write a version of random that corrects these three flaws, you'd have to know a fair amount about linear congruential pseudorandom number generators, number theory, and two's complement arithmetic Luckily, you don't have to do this it's already been done for you It's called RandomnextInt(int), and it was added to the standard library package javautil in release 12 You don't have to concern yourself with the details of how nextInt(int) does its job (although you can study the documentation or the source code if you're morbidly curious) A senior engineer with a background in algorithms spent a good deal of time designing, implementing, and testing this method and then showed it to experts in the field to make sure it was right Then the library was beta tested, released, and used extensively by thousands of programmers for several years No flaws have yet been found in the method, but if a flaw were to be discovered, it would get fixed in the next release By using a standard library, you take advantage of the knowledge of the experts who wrote it and the experience of those who used it before you A second advantage of using the libraries is that you don't have to waste your time writing ad hoc solutions to problems only marginally related to your work If you are like most programmers, you'd rather spend your time working on your application than on the underlying plumbing A third advantage of using standard libraries is that their performance tends to improve over time, with no effort on your part Because many people use them and because they're used in industry-standard benchmarks, the organizations that supply these libraries have a strong incentive to make them run faster For example, the standard multiprecision arithmetic library, javamath, was rewritten in release 13, resulting in dramatic performance improvements Libraries also tend to gain new functionality over time If a library class is missing some important functionality, the developer community will make this shortcoming known The
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