Java allows the use of these forms of comment:

/* a multiline
   comment */
// a single-line comment

Prior to C99, C does not permit the single-line form.

In both C and Java, you might disable large sections of temporarily unwanted code by using comments, although that can be problematic because comments do not nest. However, as C uses a preprocessor, it has a more robust method:

  /* enabled code */
#if 0
  /* disabled code */
#endif
  /* enabled code */