Symbol.java

public class Symbol implements Comparable<Symbol>{
        
        String letter;//Inner node's letter is null.
        int frequency;
        double low;
        double high;
        double probability;
        
        Symbol(String letter, int frequency){
                this.letter = letter;
                this.frequency = frequency;
        }

        Symbol(String letter, double p, double low, double high){
                this.letter = letter;
                this.probability = p;
                this.low = low;
                this.high = high;
        }
        
        
        @Override
        /**
         * It's a method of interface Comparable. We can use method Collections.sort(List<T>) then.
         */
        public int compareTo(Symbol arg0) {
                // TODO Auto-generated method stub
                return letter.charAt(0) - arg0.letter.charAt(0);
        }
        
        public String toString(){
                return letter + " " + String.valueOf(frequency);
        }
}