问如何正确使用Map.Entry接口，为什么Map.Entry的方法已经实现了？

EN

import java.util.HashMap;
import java.util.*;

class Dog {
    String color;

    Dog(String c)  color = c;
    public boolean equals(Object o) 
        return ((Dog) o).color.equals(this.color);  
    public int hashCode() 
        return color.length();
    public String toString()
        return color + " dog";  
}

public class TestHashMap {
  public static void main(String[] args) {
    HashMap<Dog, Integer> hashMap = new HashMap<Dog, Integer>();
    Dog d1 = new Dog("red");
    Dog d2 = new Dog("black");
    Dog d3 = new Dog("white");
    Dog d4 = new Dog("white");

    hashMap.put(d1, 10);
    hashMap.put(d2, 15);
    hashMap.put(d3, 5);
    hashMap.put(d4, 20);

    //print size
    System.out.println(hashMap.size());

    //loop HashMap
    for (Map.Entry<Dog, Integer> entry : hashMap.entrySet()) {
      System.out.println(entry.getKey().toString() + " - "+      entry.getValue());
    }
  }
}