java package a class CommonPrefix, the method comprising a return common to two string arguments prefix

Software NetBeans IDE 7.0.1, need to write the main class alone.

Packaging a class CommonPrefix, the method comprising a return of the two string arguments prefix shared public String getPerx (String sl, String s2). For example: "disappear, and" distance "of a total prefix" is, if not an empty string. Packaging executes the main class.

Main categories:

 CommonPrefix c=new CommonPrefix();
         c.getPrefix("aaaaff", "aaaava");

CommonPrefix categories:

public class CommonPrefix {
    public String getPrefix(String s1,String s2){
        char[] a=s1.toCharArray();
        char[] b=s2.toCharArray();
        int length1=a.length;
        
        int i;
        for(i=0;i<a.length;i++){
            if(a[i]==b[i]){
                System.out.print(a[i]);
            }
            else{
                return null;
                
            }
        }
        return " ";
        
    }
}