Nullpointer exception when modifying string through another class - Java -

in main method, doing this:

rtalgorithm rt = new rtalgorithm(); string s = "looooolasdasdl"; rt.encrypt(s); system.out.println(s); 

here rt class:

package rt.encrypt;  public class rtalgorithm { public static string encrypt(string s) {     alg_flip(s);     return s; }  private static string alg_flip(string s) {     string s1 = "";     for(int = s.length() - 1; >= 0; i++) {         s1 = s1 + s.charat(i);     }     return s1; } } 

however, giving me nullpointerexception on rt.encrypt(s) line in main method have

if return value in method in .modify have value back:

you're doing:

string s = "looooolasdasdl"; rt.modify(s); system.out.println(s); 

you need

string s = "looooolasdasdl"; string s2 = rt.modify(s); system.out.println(s2); 

and in modify method need return of alg_flip() call

public string modify(string s) {     return alg_flip(s); } 

edit

if use lot of algorithms should consider strategy pattern

the main problem of attempt still call method parameter, don't save happend return statements.

edit 2:

i got main problem, although return part problem:

for(int = s.length() - 1; >= 0; i++) {     s1 = s1 + s.charat(i); } 

you want chars @ , loop reverse add variable

for(int = s.length() - 1; >= 0; i--) {     s1 = s1 + s.charat(i); } 

this should do

this works me:

public class main {      public static void main(string[] args) throws parseexception {          rtalgorithm rta = new rtalgorithm();         string s = "looooolasdasdl";         s = rta.encrypt(s);         system.out.println(s);      } }  class rtalgorithm {     public static string encrypt(string s) {         return alg_flip(s);     }      private static string alg_flip(string s) {         string s1 = "";         (int = s.length() - 1; >= 0; i--) {             s1 = s1 + s.charat(i);         }         return s1;     } } 

but if write static methods, don't need create instance.

instead of rt.encrypt(s); rtalgorithm.encrypt(s);