How to change String Array to ArrayList in java

This is not how you use an ArrayList.

First, you need to write:

List<String> strArrEmailIds = new ArrayList<>();

So, program to the interface and use the Java 7 diamond operator.

Next, remove the index i. You don't need this.

Finally, just do:

emailAddress.add(rs.getString("EmailAddress")); 

To convert it back to an String[] you can then do:

String[] arr = emailAddress.toArray(new String[emailAddress.size()]);

Here is my suggestion for you final code:

public String[] getemailAddr(String strAccountnbr) throws Exception {
    final List<String> emailAddress = new ArrayList<>();
    final String strQuery2 = "SELECT c.EmailAddress AS EmailAddress"
            + " FROM customeremailid c "
            + "WHERE c.AccountNbr = ? ";
    try (final PreparedStatement ps = getDBConn().prepareStatement(strQuery2)) {
        ps.setString(1, strAccountnbr);
        try (final ResultSet rs = ps.executeQuery()) {
            while (rs.next()) {
                emailAddress.add(rs.getString("EmailAddress"));
            }
        }
    }
    return emailAddress.toArray(new String[emailAddress.size()]);
}

I have removed your pointless assignments to null. I have added try-with-resources blocks to close your external resources, you code was one massive memory leak.

If you have a ArrayList, then you dont need a array again, indeed a ArrayList is backed by Array itself and its dynamic in size.

List<String> emailAddress= new ArrayList<String>(); // dynamic array
...
while(rs.next()){
    emailAddress.add((rs.getString("EmailAddress"));
    ...
}
return emailAddress.toArray(new String[emailAddress.size()]); // creating array of String type

And ArrayList#toArray converts List to Array which has done at last in the code.

declare it as

ArrayList<String> emailAddress= new ArrayList<String>();
...
emailAddress.add((rs.getString("EmailAddress")); 

convert it to String[]:

return emailAddress.toArray(new String[emailAddress.size()]);

You use ArrayList here wrongly in your code. When you define

ArrayList<String> strArrEmailIds = new ArrayList<String>();
String[] emailAddress= new String[strArrEmailIds.size()];
strArrEmailIds.toArray(emailAddress);

strArrEmailIds by default has a size of 0, so the generated emailAddress array also gets a length of 0. Later in the while loop, you are trying to assign the value to the emailAddress[0], it will throw ArrayIndexOutOfBoundsException.

Instead, the correct way is :

ArrayList<String> strArrEmailIds = new ArrayList<String>();
//....
while(rs.next()){
    //....
    strArrEmailIds.add(rs.getString("EmailAddress"));
}
//....
String[] emailAddress = strArrEmailIds.toArray(new String[strArrEmailIds.size()]);

java.lang.ArrayIndexOutOfBoundsException: 0 if your result set goes beyond 25 itteration.

How to convert array to ArrayList ?

Arrays.asList(myArray)

in your case you can have a list and in the resulset itteration you can add them to the list like

List<String> emails = new ArrayList<String>();
while(...){
emails.add(rs.getString("EmailAddress"));
}