java - How to update an element in an array? -

how can observation method increase single element in numberofobs array 1, every time use it?

i error: exception in thread "main" java.lang.arrayindexoutofboundsexception: 0

public class database {     private arraylist <string> databaseofbirds = new arraylist<string>();     private int[] numberofobs = new int[databaseofbirds.size()];      public void add(string name, string latinname) {         databaseofbirds.add(name + "(" + latinname + ")");      }      public string observation(string birdname){          for(int = 0; < databaseofbirds.size(); i++) {             if(databaseofbirds.get(i).contains(birdname)) {                 numberofobs[i]++;                  return "";             }            }         return "is not bird!";     } } 

when create numberofobs array it's length zero, since @ point databaseofbirds empty. unless you're growing array in unlisted code you'll nullpointerexception first time find bird.

you fix making numberofobs list:

private list<integer> numberofobs = new arraylist<>(); 

then in add method add line:

numberofobs.add(0); 

finally, change

numberofobs[i]++; 

to

numberofobs.set(i, 1 + numberofobs.get(i));