Recursion is a technique in which a method calls itself until a base (or ending) condition is hit.
If no base (or ending) condition is hit, then the recursion does not stop; this is called infinite recursion.

Here is an example:

public void mystery (int x)
{
   if (x <= 0)  //the base or ending condition
      return; //a void method can simply end by executing return
   else {
      System.out.print( x + " "); //notice it is print so cursor stays on same line
      mystery( x - 2);  //recursion the method is calling itself
   }

}

What is printed when mystery(4) is executed?

How to solve ..
develop a method trace table...

mystery(4)
        prints 4 and a space then calls               [output: 4  ]
                   mystery(2)
                            prints 2 and a space then calls  [output: 4  2 ]
                                 mystery(0)
                                         base condition of if (x <= 0) is true: recursion stops


output & answer is:
4 2