Biggest day-to-day warm-up

public class Reading
{
    /** Returns the label for the day, such as "Mon" */
    public String getDay()
    { /* implementation not shown */ }
 
    /** Returns the day's high temperature, in whole degrees */
    public int getHigh()
    { /* implementation not shown */ }
 
    // There may be instance variables, constructors, and methods that are not shown.
}

public class WeatherLog
{
    /**
     * Readings for consecutive days, oldest first.
     * Contains at least 2 readings and no null elements.
     */
    private ArrayList<Reading> readings;
 
    /**
     * Returns the label of the day whose high rose the most compared
     * with the day before it, as described in the question.
     * Postcondition: readings is unchanged.
     */
    public String biggestWarmUp()
    { /* to be implemented */ }
 
    // There may be instance variables, constructors, and methods that are not shown.
}