在我对DB的请求中，我使用Laravel的模型和函数interface MyFunc<T> {
boolean func(T v1, T v2);
}
class HighTemp {
private int hTemp;
HighTemp(int ht) { hTemp = ht; }
boolean sameTemp(HighTemp ht2) {
return hTemp == ht2.hTemp;
}
boolean lessThanTemp(HighTemp ht2) {
return hTemp < ht2.hTemp;
}
}
public class InstanceMethObjectRefDemo {
static<T> int Counter(T[] vals, MyFunc<T> f, T v) {
int count = 0;
for (int i = 0; i < vals.length; i++)
if (f.func(vals[i], v)) count++;
return count;
}
public static void main(String[] args) {
int count;
HighTemp[] weekDayHighs = {
new HighTemp(89), new HighTemp(82),
new HighTemp(90), new HighTemp(89),
new HighTemp(89), new HighTemp(91),
new HighTemp(84), new HighTemp(83)
};
count = Counter(weekDayHighs, HighTemp::sameTemp, new HighTemp(89));
System.out.println(count + " days had a high of 89");
HighTemp[] weekDayHighs2 = {
new HighTemp(32), new HighTemp(12),
new HighTemp(24), new HighTemp(19),
new HighTemp(18), new HighTemp(12),
new HighTemp(-1), new HighTemp(13)
};
count = Counter(weekDayHighs2, HighTemp::sameTemp, new HighTemp(12));
System.out.println(count + " days had a high of 12");
count = Counter(weekDayHighs, HighTemp::lessThanTemp, new HighTemp(89));
System.out.println(count + " days had a high less than 89");
count = Counter(weekDayHighs2, HighTemp::lessThanTemp, new HighTemp(19));
System.out.println(count + " days had a high of less then 19");
}
}
，如：
with问题是如果要在模板中执行以下操作：
$user = User::with("city")->get();
只有当用户指定了city时才会起作用，因此数据库表中存在value。否则返回错误：
如何摆脱多余的支票，如：
{{$user->city()->name}}
这太棒了！
答案 0 :(得分：5)
在模型上定义关系时，请使用withDefault方法：
class User extends Model
{
public function city()
{
return $this->hasOne(City::class)->withDefault();
}
}
有了这个，$user->city将始终返回City模型。