import java.util.*;
import java.util.stream.*;

class Untitled {


    public static void main(String[] args) {
        ArrayList<A> doItemList = new ArrayList<A>();
        doItemList.add(new A("a", 1));
        doItemList.add(new A("b", 2));
        doItemList.add(new A("a", 3));
        Map<String, List<A>> result = doItemList.stream().collect(  
                    Collectors.groupingBy(  
                            x -> x.name));
        for (String s : result.keySet())
            System.out.println(s);
    }
}

class A
{
    public String name;
    public int age;
    public A(String s, int n) { name = s; age = n; }
}

结果
a
b

昨天的问题不知道你搞清楚了没有，如果有疑问，请追问，如果问题解决，麻烦点下我回答右边的采纳，谢谢。

过滤老王：

 import java.util.*;
import java.util.stream.*;

class Untitled {


    public static void main(String[] args) {
        ArrayList<A> doItemList = new ArrayList<A>();
        doItemList.add(new A("a", 1));
        doItemList.add(new A("b", 2));
        doItemList.add(new A("a", 3));
        doItemList.add(new A("老王", 3));
        Map<String, List<A>> result = doItemList.stream().filter(x -> !x.name.equals("老王")).collect( 
                    Collectors.groupingBy(x -> x.name));
        for (String s : result.keySet())
            System.out.println(s);
    }
}

class A
{
    public String name;
    public int age;
    public A(String s, int n) { name = s; age = n; }
}