decompiling - How do Java decompilers discern a for-loop from a while-loop? -

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consider these 2 methods:

public static void forloop(int start, int limit) {     (int = start; < limit; i++) {      } }  public static void whileloop(int start, int limit) {     int = start;     while (i < limit) {         i++;     } }

when compiled, produce bytecode (this verbose output of javap):

  public static void forloop(int, int);     descriptor: (ii)v     flags: acc_public, acc_static     code:       stack=2, locals=3, args_size=2          0: iload_0          1: istore_2          2: iload_2          3: iload_1          4: if_icmpge     13          7: iinc          2, 1         10: goto          2         13: return       linenumbertable:         line 6: 0         line 9: 13       localvariabletable:         start  length  slot  name   signature             2      11     2                   0      14     0 start               0      14     1 limit      public static void whileloop(int, int);     descriptor: (ii)v     flags: acc_public, acc_static     code:       stack=2, locals=3, args_size=2          0: iload_0          1: istore_2          2: iload_2          3: iload_1          4: if_icmpge     13          7: iinc          2, 1         10: goto          2         13: return       linenumbertable:         line 12: 0         line 13: 2         line 14: 7         line 16: 13       localvariabletable:         start  length  slot  name   signature             0      14     0 start               0      14     1 limit               2      12     2

as can see, code section both of these methods exactly same. however, when decompile class using jd, correctly produces:

public static void forloop(int start, int limit) {   (int = start; < limit; i++) {} }  public static void whileloop(int start, int limit) {   int = start;   while (i < limit) {     i++;   } }

how able this? bytecode of these methods same! despite fact linenumbertable , localvariabletable attributes different each method, reluctant believe reason since not required attributes code attribute of method contain (per section 4.7 of java language specification, java se 8 edition).

the line numbers , local variable scope.

for loop:

 linenumbertable:     line 6: 0     line 9: 13   localvariabletable:     start  length  slot  name   signature         2      11     2               0      14     0 start           0      14     1 limit

while loop:

linenumbertable:         line 12: 0         line 13: 2         line 14: 7         line 16: 13   localvariabletable:     start  length  slot  name   signature         0      14     0 start           0      14     1 limit           2      12     2

the for loop has fewer distinct lines of code - makes sense because wraps initialization , increment in 1 line.


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