What means ';' inside of for statement?

Question

I'm trying to understand the follow code below:

/**
   * Simple insertion sort.
   * @param a an array of Comparable items.
   */
  public static void insertionSort( Comparable [ ] a )
  {
      for( int p = 1; p < a.length; p++ )
      {
          Comparable tmp = a[ p ];
          int j = p;
          for( ; j > 0 && tmp.compareTo( a[j-1] ) < 0; j-- )
              a[ j ] = a[ j - 1 ];
          a[ j ] = tmp;
      }
  }

But i'm not sure what means for( ; ) so I need your help guys. Sorry if it's duplicated but I search here and in Google but nothing so far.

Answer 1

The first part of a for loop is what happens before the looping begins. Usually it's used for assigning a variable.

The ";" with nothing (apart from the bracket) before it merely says "There's nothing I want doing before the start of the loop". No variable needs to be assigned, etc.

Answer 2

It is simple: It basically means

for(..a.. ; ..bb.. ; ..c..){ // }.

..a.. : is what you initialize if any or leave it empty. ..b.. : Any check you want to carry once a loop is run once. ..c.. Any change to the variable after the loop is run once

Answer 3

I guess you're wandering about the second for loop syntax. It's just a regular for loop without any initialization part. j must be declared before the loop because it's used outside, after the loop has ended.

Answer 4

It's just a separator. If there's nothing in front of it, that means that part of the logic is empty. The format is

for(  <stuff to do before starting the loop>
    ;
      <stuff to do before each iteration, and maybe give a value of 0 to terminate it>
    ;
      <stuff to do at the end of each iteration>
   )

Frankly I find it very flexible, because not every loop is of the form "do this for each xxx". What you put in each of those sections is really arbitrary.