IndirectPQ.java

Below is the syntax highlighted version of IndirectPQ.java from §3.5 Searching Applications.

/******************************************************************************
 *  Compilation:  javac IndirectPQ.java
 *  Execution:    java IndirectPQ
 *  Dependencies: none
 *
 *  Indirect priority queue implementation with Java's TreeSet and
 *  HashMap. It assumes the priorities are integers and the values
 *  are strings, although it is easily modifiable for Comparable
 *  priorities and Object values.
 *
 *  % java IndirectPQ
 *  this
 *  is
 *  a
 *  test
 *
 *  Remarks
 *  -------
 *  All operations are efficient, but it could be improved by using
 *  a binary heap instead of the red-black tree.
 *
 ******************************************************************************/

import java.util.HashMap;
import java.util.TreeSet;

public class IndirectPQ {
    private TreeSet<Element> pq = new TreeSet<Element>();
    private HashMap<String, Element> st = new HashMap<String, Element>();

    private class Element implements Comparable<Element> {
        private String key;
        private int priority;

        public Element(String key, int priority) {
            this.key       = key;
            this.priority  = priority;
        }
        public int compareTo(Element that) {
            if (this.priority != that.priority)
                return this.priority - that.priority;
            return this.key.compareTo(that.key);
        }

        public boolean equals(Object other) {
            if (other == this) return true;
            if (other == null) return false;
            if (other.getClass() != this.getClass()) return false;
            Element that = (Element) other;
            return (this.priority == that.priority) && this.key.equals(that.key);
        }
    }


    public boolean isEmpty() { return pq.isEmpty(); }
    public int size()        { return pq.size();    }

    // insert a key with a given priority (changing the priority if the key is present)
    public void put(String key, int priority) {
        delete(key);
        Element element = new Element(key, priority);
        st.put(key, element);
        pq.add(element);
    }

    // does the key exist?
    public boolean exists(String key) {
        return st.get(key) != null;
    }

    // delete key
    void delete(String key) {
        Element element = st.get(key);
        if (element != null) {
            pq.remove(element);
            st.remove(key);
        }
    }

    // return the priority of a given key
    int get(String key) {
        Element element = st.get(key);
        return element.priority;
    }

    // return minimum priority, error if empty
    public int min() {
        Element min = pq.first();
        return min.priority;
    }

    // return minimum priority, error if empty
    public int max() {
        Element max = pq.last();
        return max.priority;
    }

    // delete and return the minimum value, error if empty
    public String delMin() {
        Element min = pq.first();
        pq.remove(min);
        st.remove(min.key);
        return min.key;
    }

    // delete and return the maximum value, error if empty
    public String delMax() {
        Element max = pq.last();
        pq.remove(max);
        st.remove(max.key);
        return max.key;
    }


    // test client
    public static void main(String[] args) {
        IndirectPQ pq = new IndirectPQ();
        pq.put("test", 31);
        pq.put("is",   55);
        pq.put("this", 25);
        pq.put("not",  65);
        pq.put("a",    36);
        pq.put("this", 61);      // changes the key
        pq.delete("not");

        while (!pq.isEmpty())
            StdOut.println(pq.delMax());
    }

}