Java 8 可选择在分层对象中进行空检查

Question

Using Optional to Null Check While Object Cloning different type of object.

class A{ C cObj; List<B> bList;}

class B{ C cObj; List<C> cList;}

class C { String label; String value;}

class D{ String name; String age; String addressCode;}

Mapping A -> D

d.setAddessCode(A.getBlist().get(0).getcList().get(0).getValue());

How can check null using java 8 optional

A.getBlist().get(0).getcList().get(0).getValue()

I tried

d.setAddessCode(Optional.ofNullable(A).map(A::getBList).map(Stream::of).orElseGet(Stream::empty).findFirst().map(B::getCList).map(Stream::of).orElseGet(Stream::empty).findFirst().map(C::getValue).orElse(null)));

How can i check null in List and value together.

Answer 1

There is no need to use Streams. Here is code with both null and empty checks:

d.setAddressCode(Optional.ofNullable(a)
                         .map(A::getbList)
                         .filter(bList -> ! bList.isEmpty())
                         .map(bList -> bList.get(0))
                         .map(B::getcList)
                         .filter(cList -> ! cList.isEmpty())
                         .map(cList -> cList.get(0))
                         .map(C::getValue)
                         .orElse(null));

That can be simplified with a little helper method:

public class MyUtils {
    public static <E> E getFirst(List<E> list) {
        return (list == null || list.isEmpty() ? null : list.get(0));
    }
}

d.setAddressCode(Optional.ofNullable(a)
                         .map(A::getbList)
                         .map(MyUtils::getFirst)
                         .map(B::getcList)
                         .map(MyUtils::getFirst)
                         .map(C::getValue)
                         .orElse(null));