The ubiquitous bug of "Java FAQ"

The code is as follows:

Public class example023 {public static void main (String[] args)  { Errormethod (); Rightmethod ();} Private static void errormethod ()  {StringBuffer word = null; Random rnd = new random ();switch  (Rnd.nextint (2))  {case 1:word =  New stringbuffer (' P '); Case 2:word = new stringbuffer (' G ');d efault:word =  New stringbuffer (' M ');} Word.append (' a '); Word.append (' I '); Word.append (' n '); SYSTEM.OUT.PRINTLN (word);} Private static void rightmethod ()  {StringBuffer word = null; Random rnd = new random ();switch  (Rnd.nextint (3))  { // error1case  1:word = new stringbuffer ("P");// error2break;// error3case 2:word =  new stringbuffer ("G");// error2break;// error3default:word = new  StringBuffer ("M");// error2break; error3}word.append (' a '); Word.append (' I '); Word.append (' n '); SYSTEM.OUT.PRINTLN (Word);}}

Output Result:

The first line of output is Ain, the second line output achieves the desired effect, and outputs a pain,gain,main random.

Code parsing:

The first line of the output corresponding function is Errormethod, the second line corresponding to the function is Rightmethod. In Errormethod, there are three bugs.

•   The first bug is that the selected random number causes the switch statement to reach only two of its three cases. The specification of Random.nextint (int) describes the word: "Returns a pseudo-random, evenly distributed int value from 0 (inclusive) to a specified numeric value (not included)." This means that the expression Rnd.nextint (2) possible values only 0 and 1,switch statements will never reach the case 2 branch, which means that the program will never print gain. The Nextint parameter should be 3 instead of 2.

• last and most subtle. The bug is that the expression new StringBuffer (' m ') may not be doing what you want it to do. You might not be familiar with the StringBuffer (Char) constructor, which is easy to explain: it doesn't exist at all. The StringBuffer has a parameterless constructor, a constructor that takes a string as the initial content of the string buffer, and a constructor that accepts an int as the initial capacity of the buffer. In this example, the compiler chooses the constructor that accepts int to convert the character value ' m ' to an int value of 77 by widening the original type conversion. In other words, new StringBuffer (' m ') returns an empty string buffer with an initial capacity of 77. The remainder of the program adds the characters a, I, and N to the empty string buffer and prints out the string buffer that is always ain content. The point to remember here is that char is not a string, it is more like an int.

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Source code Address: Https://github.com/rocwinger/java-disabuse

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The ubiquitous bug of "Java FAQ"