Search a String Vector for String

Question

I can't figure out why this won't work. It is supposed to search the id vector for stringValue. It does not return any compilation errors, it just simply won't match stringValue with the id.get(i) method.
        for ( int i = 0; i < id.size(); i++ )
          if ( stringValue == id.get(i) )
            JOptionPane.showMessageDialog( null, "Item already exist.", "Error", JOptionPane.ERROR_MESSAGE );
            stringValue = null;
        }

Answer 1

"==" tests for object equality (i.e., that two variables contain the exact same object)
"equals()" tests for object equivalence (i.e., that two objects have the same value, whether or not they are the same object)
For strings, it's possible to have two different strings to contain the same sequence of characters. In this case, "==" returns false, while "equals()" returns true.