Java Learning Path (v)--thinking in Java

Writing procedures to be careful, encounter bugs to be good at troubleshooting.

Today in Java to write a few small programs, because of the pursuit of optimization and efficacy, ignoring a small detail problem. But finally found the bug and solved it.

We all know that when using a for loop, for example:

for (int i = 0; i < n; i++) {...}

We usually write here. But this is not enough, because this statement can also be optimized. such as I<n, it is better to judge the i!=n directly, so that in the computer can save computing time. Also, i++ can also be changed to ++i, because i++ need the computer to save the value before I +1,++i directly let i+1 and no temporary variable. However, because of this, a small imperceptible bug happened.

The original topic is very simple, is to find the whole number of integers a, b. At first I wrote this:

Import Java.io.*;import java.util.*;p ublic class Number2 {public static void main (string[] args) throws Ioexception{int be Gin, end; String str; BufferedReader br = new BufferedReader (new InputStreamReader (system.in)); str = Br.readline (); begin = new Integer (str); BufferedReader BR1 = new BufferedReader (new InputStreamReader (system.in)); str = Br1.readline (); end = new Integer (str); for (; begin! = end; ++begin) {Boolean b = true;for (int i = 2; I! = math.sqrt (begin); ++i) {if (0 = = begin% i) {b = False;break;}} if (b) System.out.println (begin);}}}

Pay attention to the code coloring section, at first glance nothing wrong, the pursuit of performance. But the result of the operation was a surprise. For a prime number from 101 to 200, 121,169 two numbers appear. After careful investigation I found that the original bug is here: Math.sqrt () is a double,i is an int, in the comparison with double is never equal. Or we'll cast the coloring statement to (int), or we'll rewrite it as I<MATH.SQRT (begin).

After the modification, it runs successfully with the following code:

1 ImportJava.io.*;2 ImportJava.util.*;3  Public classnumber2 {4      Public Static voidMain (string[] args)throwsioexception{5         intbegin, end;6 String str;7BufferedReader br =NewBufferedReader (NewInputStreamReader (system.in));8str =br.readline ();9Begin =NewInteger (str);TenBufferedReader BR1 =NewBufferedReader (NewInputStreamReader (system.in)); Onestr =br1.readline (); AEnd =NewInteger (str); -          for(; begin! = end; + +)begin) { -             Booleanb =true; the              for(inti = 2; I < math.sqrt (begin); ++i) { -                 if(0 = = begin%i) { -b =false; -                      Break; +                 } -             } +             if(b) A System.out.println (begin); at         } -     } -}

The input and output streams that appear in the code are discussed later.

Java Learning Path (v)--thinking in Java