java 进阶专用。

				// Page 36
				
				import java.util.*;
				
				public final class PhoneNumber {
				    private final short areaCode;
				    private final short exchange;
				    private final short extension;
				
				    public PhoneNumber(int areaCode, int exchange,
				                       int extension) {
				        rangeCheck(areaCode,   999, "area code");
				        rangeCheck(exchange,   999, "exchange");
				        rangeCheck(extension, 9999, "extension");
				        this.areaCode  = (short) areaCode;
				        this.exchange  = (short) exchange;
				        this.extension = (short) extension;
				    }
				
				    private static void rangeCheck(int arg, int max,
				                                   String name) {
				        if (arg < 0 || arg > max)
				           throw new IllegalArgumentException(name +": " + arg);
				    }
				
				    public boolean equals(Object o) {
				        if (o == this)
				            return true;
				        if (!(o instanceof PhoneNumber))
				            return false;
				        PhoneNumber pn = (PhoneNumber)o;
				        return pn.extension == extension &&
				               pn.exchange  == exchange  &&
				               pn.areaCode  == areaCode;
				    }
				
				    // No hashCode method!
				
				/*
				    // Page 39
				    public int hashCode() {
				        int result = 17;
				        result = 37*result + areaCode;
				        result = 37*result + exchange;
				        result = 37*result + extension;
				        return result;
				    }
				*/
				
				/* COMMENTED OUT
				    // Lazily initialized, cached hashCode - page 40
				    private volatile int hashCode = 0;  // (See Item 48)
				
				    public int hashCode() {
				        if (hashCode == 0) {
				            int result = 17;
				            result = 37*result + areaCode;
				            result = 37*result + exchange;
				            result = 37*result + extension;
				            hashCode = result;
				        }
				        return hashCode;
				    }
				*/
				
				    // ... // Remainder omitted
				
				    public static void main(String[] args) {
				        Map m = new HashMap();
				        m.put(new PhoneNumber(408, 867, 5309), "Jenny");
				        System.out.println(m.get(new PhoneNumber(408, 867, 5309)));
				    }
				}