用contains()方法，需要重寫 equals與 hashCode方法
eg: public class Authority
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((url == null) ? 0 : url.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Authority other = (Authority) obj;
if (url == null) {
if (other.url != null)
return false;
} else if (!url.equals(other.url))
return false;
return true;
}