Consider the Following Method

Consider the following method:
public static int foo( String s1, String s2, int i )
{
if ( s2.length( ) > s1.length( ) )
return -1;
else if ( s1.substring( 0, s2.length( ) ).equals( s2 ) )
return i;
else
return foo( s1.substring( 1, s1.length( ) ), s2, i + 1 );
}
What is the output of the following code?
System.out.println( foo( "AB", "XYZ", 0 ) );
What is the output of the following code?
System.out.println( foo( "ABC", "A", 0 ) );
What is the output of the following code?
System.out.println( foo( "ABCDEFG", "DE", 0 ) );
What does foo( s1, s2, 0 ) return as a function of s1 and s2?