Java 8流-使用自定义收集器时发生NullPointerException

我已经通过实现Collector接口并重写其方法来实现了一个自定义收集器。Collector实现如下：

class MyCustomCollector implements Collector<Person, StringJoiner, String>{

    @Override
    public Supplier<StringJoiner> supplier() {
        // TODO Auto-generated method stub
        return () -> new StringJoiner("|");
    }

    @Override
    public BiConsumer<StringJoiner, Person> accumulator() {
        // TODO Auto-generated method stub
        return (joiner,person) -> joiner.add(person.name.toUpperCase());
    }

    @Override
    public BinaryOperator<StringJoiner> combiner() {
        // TODO Auto-generated method stub
        return (joiner1, joiner2) -> joiner1.merge(joiner2);
    }

    @Override
    public Function<StringJoiner, String> finisher() {
        // TODO Auto-generated method stub
        return StringJoiner::toString;
    }

    @Override
    public Set<java.util.stream.Collector.Characteristics> characteristics() {
        // TODO Auto-generated method stub
        return null;
    }
}

这是我的Person类：

class Person implements Comparable<Object>{
    String name;
    int age;

    Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return name;
    }

    public int compareTo(Object obj){
        int returnValue;
        if(age ==((Person) obj).age)
            returnValue=0;
        else
            if(age >((Person) obj).age)
                returnValue = 1;
            else
                returnValue =-1;

        return returnValue;
    }

    public boolean equals(Object obj)
    {
        if(!(obj instanceof Person))
            return false;

        return (age == ((Person) obj).age); 
    }

    public int hashCode()
    {  
        return name.hashCode();  
    } 

}

这是我的电话声明...

List<Person> persons =
        Arrays.asList(
            new Person("Max", 18),
            new Person("Peter", 23),
            new Person("Pamela", 23),
            new Person("David", 12),
            new Person("Pam", 23));
String names2 = persons.stream()
            .collect(new MyCustomCollector());

当上面的语句被执行时，我得到了一个N​​ullPointerException，如下所示：

java.util.stream.ReduceOps $ 3.getOpFlags（ReduceOps.java:185）处的线程“ main”中的java.lang.NullPointerException，java.util处的java.util.stream.AbstractPipeline.evaluate（AbstractPipeline.java:234） .stream.ReferencePipeline.collect（ReferencePipeline.java:499）位于com.my.j8.TestStreams.main（TestStreams.java:231）

有人可以告诉我我在哪里犯错吗？