Can anyone help me with the AlphaNumericChooser class? I'm still unsure.

package com.teamtreehouse.vending;

import org.junit.Before;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;


import static org.junit.Assert.*;

public class AlphaNumericChooserTest {

    private AlphaNumericChooser chooser;

    // I added this rule setup code for you, thank me later. ;)
    @Rule
    public ExpectedException thrown = ExpectedException.none();

    @Before
    public void setUp() throws Exception {
        chooser = new AlphaNumericChooser(26, 10);
    }

    @Test
    public void validInputReturnsProperLocation() throws Exception {
        AlphaNumericChooser.Location loc = chooser.locationFromInput("B4");

        assertEquals("proper row", 1, loc.getRow());
        assertEquals("proper column", 3, loc.getColumn());
    }

    @Test(expected = InvalidLocationException.class)
    public void choosingWrongInputIsNotAllowed() throws Exception {
        chooser.locationFromInput("WRONG");
    }

    @Test(expected = InvalidLocationException.class)
    public void choosingLargerThanMaxIsNotAllowed() throws Exception {
        chooser.locationFromInput("B52");
    }

    @Test
    public void constructingLargerThanAlphabetNotAllowed() throws Exception {
      thrown.expect(IllegalArgumentException.class);
      thrown.expectMessage("Maximum rows supported is 26");
        new AlphaNumericChooser(27, 10);
    }
}

package com.teamtreehouse.vending;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class AlphaNumericChooser extends AbstractChooser {
    private int ALPHA_FIRST = (int) 'A';
    private int ALPHA_LAST = (int) 'Z';

    public AlphaNumericChooser(int maxRows, int maxColumns) {
        super(maxRows, maxColumns);
        int maxAllowedAlpha = (ALPHA_LAST - ALPHA_FIRST) + 1;
        if (maxRows > maxAllowedAlpha) {
            throw new IllegalArgumentException("Maximum rows supported is " + maxAllowedAlpha);
        }
    }

    @Override
    public Location locationFromInput(String input) throws InvalidLocationException {
        Pattern pattern = Pattern.compile("^(?<row>[a-zA-Z]{1})(?<column>[0-9]+)$");
        Matcher matcher = pattern.matcher(input);
        if (!matcher.matches()) {
            throw new InvalidLocationException("Invalid buttons");
        }
        int row = inputAsRow(matcher.group("row"));
        int column = inputAsColumn(matcher.group("column"));
        return new Location(row, column);

    }

    private int inputAsRow(String rowInput) {
        rowInput = rowInput.toUpperCase();
        return (int) rowInput.charAt(0) - ALPHA_FIRST;

    }

    private int inputAsColumn(String columnInput) {
        int columnAsInt = Integer.parseInt(columnInput);
        return columnAsInt - 1;
    }
}

package com.teamtreehouse.vending;

public abstract class AbstractChooser {
    private final int maxRows;
    private final int maxColumns;

    public class Location {
        private final int row;
        private final int column;

        protected Location(int row, int column) throws InvalidLocationException {
            if (row > maxRows || column > maxColumns) {
                throw new InvalidLocationException("Invalid Location");
            }
            this.row = row;
            this.column = column;
        }

        public int getRow() {
            return row;
        }

        public int getColumn() {
            return column;
        }
    }

    public AbstractChooser(int maxRows, int maxColumns) {
        this.maxRows = maxRows;
        this.maxColumns = maxColumns;
    }

    public abstract Location locationFromInput(String input) throws InvalidLocationException;
}

package com.teamtreehouse.vending;

public class InvalidLocationException extends Exception {
    public InvalidLocationException(String s) {
        super(s);
    }
}