Question

我有以下程序，我正在尝试打印null。我正在使用Java 8。以下是我实现它的代码：

            List<Person> personList = new ArrayList<>();                                
            personList.add(new Person("Alice", "alice@gmail.com", Gender.FEMALE, 15));
            personList.add(new Person("Bob", "bob@gmail.com", Gender.MALE, 16));           
            personList.add(new Person("Eric", "eric@gmail.com", Gender.MALE, 17));
            personList.add(new Person("Carol", "carol@gmail.com", Gender.FEMALE, 23));
            personList.add(new Person(null, "alice@gmail.com", Gender.FEMALE, 15));
            personList.add(new Person("Carol", "carol@gmail.com", Gender.FEMALE, 23));
            personList.add(new Person("David", "david@gmail.com", Gender.MALE, 19));
            personList.add(new Person("Bob", "bob@gmail.com", Gender.MALE, 16));

现在，下面是我编写的用于首先打印null名称的代码：

personList.stream().sorted(Comparator.nullsFirst(Comparator.comparing(Person::getName))).forEach(System.out::println);

但我收到以下异常，请告知如何克服这个问题？

Exception in thread "main" java.lang.NullPointerException
    at java.util.Comparator.lambda$comparing$77a9974f$1(Comparator.java:469)
    at java.util.Comparators$NullComparator.compare(Comparators.java:83)
    at java.util.TimSort.binarySort(TimSort.java:296)
    at java.util.TimSort.sort(TimSort.java:221)

Answer 1

这是因为您使用比较器错误。看到这个并且可以工作

import java.util.stream.*;
import static java.util.Comparator.*;
import java.util.*;
public class HelloWorld{

     public static void main(String []args){

        List<Person> personList = new ArrayList<>();                                
            personList.add(new Person("Alice", "alice@gmail.com"));
            personList.add(new Person("Bob", "bob@gmail.com"));           
            personList.add(new Person("Eric", "eric@gmail.com"));
            personList.add(new Person("Carol", "carol@gmail.com"));
            personList.add(new Person(null, "alice@gmail.com"));
            personList.add(new Person("Carol", "carol@gmail.com"));
            personList.add(new Person("David", "david@gmail.com"));
            personList.add(new Person("Bob", "bob@gmail.com"));
             System.out.println(personList);



            personList.stream().sorted(comparing(Person::getName, nullsFirst(naturalOrder()))).forEach(System.out::println);
     }
}
class Person
{
    private String name,email;
    Person(String name, String email)
    {
        this.name=name;
    }
    public String getName()
    {
        return name;
    }
    public String toString(){return name;}
}

Answer 2

NPE是将null值传递给Comparator.comparing

的结果

Javadoc说

如果参数为null，则@throws NullPointerException

也许尝试像

这样的东西

Comparator.comparing(Person::getName, Comparator.nullsFirst(String.CASE_INSENSITIVE_ORDER))

哪个产生

null
Alice
Bob
Bob
Carol
Carol
David
Eric

使用java 8的null第一个方法获得以下异常

2 个答案: