StackOverflowError异常，哪位好心人看看啊

Question

   打算用面向对象思想，写个求最小生成树问题。写了一半，就出现问题了import java.util.HashMap;

public class City {
    private String cityName; 

    private HashMap<City, Double> adjCities = new HashMap<City, Double>(); 

    public City(String cityName) {
        this.cityName = cityName;

    }

    public void addAjdCity(City city, double cost) {
        adjCities.put(city, cost);
        // System.out.println("test1");
        city.adjCities.put(this, cost);
        // System.out.println("test2");

    }

    public void removeAjdCity(City city) {

    }

    public void removeAjdCity(String cityName) {

    }

    public String toString() {
        return cityName + ":" + "\n" + adjCities;
    }

    public static void main(String[] args) {
        City c1 = new City("上海");
        City c2 = new City("杭州");

        c1.addAjdCity(c2, 100);
        // 怎么输出c1 c2时会出问题？StackOverflowError异常。如果把最后两句println注释掉，就不会报错？？
        System.out.println(c1);
        System.out.println(c2);

    }

}

Answer 1

你的toString方法里递归调用了,死循环出不来了...
这是AbstractMap的toString方法:
[code="java"]
public String toString() {
Iterator> i = entrySet().iterator();
if (! i.hasNext())
return "{}";

StringBuilder sb = new StringBuilder();
sb.append('{');
for (;;) {
    Entry<K,V> e = i.next();
    K key = e.getKey();
    V value = e.getValue();
    sb.append(key   == this ? "(this Map)" : key);
    sb.append('=');
    sb.append(value == this ? "(this Map)" : value);
    if (! i.hasNext())
    return sb.append('}').toString();
    sb.append(", ");
}
}

[/code]
由于你的Map里Key还是City对象,所以执行Map的toString时这一行 sb.append(key == this ? "(this Map)" : key);会自动调用key的toString方法,这样又进入了City的toString方法,然后就挂了.....