使用Java的空第一方法获取以下例外8

Question

我有下面的程序，我试图在其中打印null。我正在使用Java 8。下面是我的代码来实现它：

  List<Person> personList = new ArrayList<>();         
      personList.add(new Person("Alice", "[email protected]", Gender.FEMALE, 15)); 
      personList.add(new Person("Bob", "[email protected]", Gender.MALE, 16));   
      personList.add(new Person("Eric", "[email protected]", Gender.MALE, 17)); 
      personList.add(new Person("Carol", "[email protected]", Gender.FEMALE, 23)); 
      personList.add(new Person(null, "[email protected]", Gender.FEMALE, 15)); 
      personList.add(new Person("Carol", "[email protected]", Gender.FEMALE, 23)); 
      personList.add(new Person("David", "[email protected]", Gender.MALE, 19)); 
      personList.add(new Person("Bob", "[email protected]", Gender.MALE, 16)); 

现在下面是我写的，首先打印null名称代码：

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

，但我得到了下面的异常，请告知如何克服这一点？

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

的NPE是传递一个null值Comparator.comparing

的Javadoc的结果表示

抛出：NullPointerException - 如果参数为null

也许你可以试试

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

将会产生

null 
Alice 
Bob 
Bob 
Carol 
Carol 
David 
Eric 

Answer 2

这是因为你使用的比较是错误的。看到这个，并应该工作

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", "[email protected]")); 
      personList.add(new Person("Bob", "[email protected]"));   
      personList.add(new Person("Eric", "[email protected]")); 
      personList.add(new Person("Carol", "[email protected]")); 
      personList.add(new Person(null, "[email protected]")); 
      personList.add(new Person("Carol", "[email protected]")); 
      personList.add(new Person("David", "[email protected]")); 
      personList.add(new Person("Bob", "[email protected]")); 
      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;} 
}