我有两个包含此类对象的列表:

public class SchoolObj
{
    private String name;
    private String school;

    public SchoolObj()
    {
        this(null, null);
    }

    public SchoolObj(String nameStr, String schoolStr)
    {
        this.setName(nameStr);
        this.setSchool(schoolStr);
    }

    public String getName()
    {
        return this.name;
    }

    public void setName(String name)
    {
        this.name = name;
    }

    public String getSchool()
    {
        return this.school;
    }

    public void setSchool(String school)
    {
        this.school = school;
    }

    @Override
    public String toString()
    {
        return this.getName() + ' ' + this.getSchool();
    }
}


我想通过nameschool比较这两个列表中的对象。如果它们相等,则需要创建一个新的List,其中包含在两个列表中都找到的SchoolObj对象。

我知道我们可以使用两个for循环,并在下面的createSharedListViaLoop方法中进行。

我的问题是,如何用Java流完成同一件事?

我在下面尝试了createSharedListViaStream,但是它没有按预期工作。

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class StreamTest
{
    public static void main(String[] args)
    {
        List<SchoolObj> listOne = new ArrayList<SchoolObj>();
        // TODO: Add sample data to listOne.
        listOne.add(new SchoolObj("nameA", "schoolX"));
        listOne.add(new SchoolObj("nameC", "schoolZ"));

        List<SchoolObj> listTwo = new ArrayList<SchoolObj>();
        // TODO: Add sample data to listTwo.
        listTwo.add(new SchoolObj("nameA", "schoolX"));
        listTwo.add(new SchoolObj("nameB", "schoolY"));

        // Print results from loop method.
        System.out.println("Results from loop method:");
        List<SchoolObj> resultsViaLoop = StreamTest.createSharedListViaLoop(listOne, listTwo);
        for (SchoolObj obj : resultsViaLoop)
        {
            System.out.println(obj);
        }

        // Print results from stream method.
        System.out.println("Results from stream method:");
        List<SchoolObj> resultsViaStream = StreamTest.createSharedListViaStream(listOne, listTwo);
        for (SchoolObj obj : resultsViaStream)
        {
            System.out.println(obj);
        }
    }

    public static List<SchoolObj> createSharedListViaLoop(List<SchoolObj> listOne, List<SchoolObj> listTwo)
    {
        List<SchoolObj> result = new ArrayList<SchoolObj>();

        for (SchoolObj one : listOne)
        {
            for (SchoolObj two : listTwo)
            {
                if (one.getName().equals(two.getName()) && one.getSchool().equals(two.getSchool()))
                {
                    result.add(one);
                }
            }
        }

        return result;
    }

    public static List<SchoolObj> createSharedListViaStream(List<SchoolObj> listOne, List<SchoolObj> listTwo)
    {
        List<SchoolObj> listOneList = listOne.stream().filter(two -> listTwo.stream()
              .anyMatch(one -> one.getName().equals(two.getName()) && two.getSchool().equals(one.getSchool())))
              .collect(Collectors.toList());

        return listOneList;
    }
}

最佳答案

让我们遍历代码的每个部分。首先,createSharedListViaStream

public static List<SchoolObj> createSharedListViaStream(List<SchoolObj> listOne, List<SchoolObj> listTwo)
{
    // We create a stream of elements from the first list.
    List<SchoolObj> listOneList = listOne.stream()
    // We select any elements such that in the stream of elements from the second list
    .filter(two -> listTwo.stream()
    // there is an element that has the same name and school as this element,
        .anyMatch(one -> one.getName().equals(two.getName())
            && two.getSchool().equals(one.getSchool())))
    // and collect all matching elements from the first list into a new list.
    .collect(Collectors.toList());
    // We return the collected list.
    return listOneList;
}


在运行完代码后,它会完全按照您的要求进行操作。现在,让我们来看一下createSharedListViaLoop

public static List<SchoolObj> createSharedListViaLoop(List<SchoolObj> listOne, List<SchoolObj> listTwo)
{
    // We build up a result by...
    List<SchoolObj> result = new ArrayList<SchoolObj>();
    // going through each element in the first list,
    for (SchoolObj one : listOne)
    {
    // going through each element in the second list,
        for (SchoolObj two : listTwo)
        {
    // and collecting the first list's element if it matches the second list's element.
            if (one.getName().equals(two.getName()) && one.getSchool().equals(two.getSchool()))
            {
                result.add(one);
            }
        }
    }
    // We return the collected list
    return result;
}


到目前为止,一切都很好...对吗?实际上,您在createSharedListViaStream中的代码从根本上是正确的。而是,您的createSharedListViaLoop可能导致输出差异。

考虑以下一组输入:
List1 = [SchoolObj("nameA","SchoolX"), SchoolObj("nameC","SchoolZ")]
List2 = [SchoolObj("nameA","SchoolX"), SchoolObj("nameA","SchoolX"), SchoolObj("nameB","SchoolY")]

在这里,createSharedListViaStream将返回出现在两个列表中的第一个列表的唯一元素:SchoolObj("nameA","SchoolX")。但是,createSharedListViaLoop将返回以下列表:[SchoolObj("nameA","SchoolX"),SchoolObj("nameA","SchoolX")]。更准确地说,createSharedListViaLoop将收集正确的对象,但是它将收集两次。我怀疑这是基于与createSharedListViaStream的输出进行比较而导致createSharedListViaLoop的输出“不正确”的原因。

createSharedListViaLoop进行此复制的原因是由于缺少内部for循环的终止。尽管我们遍历第一个列表的所有元素以检查第二个列表中是否存在这些元素,但是找到单个匹配项即可将元素添加到结果中。我们可以通过将内部循环更改为以下内容来避免添加多余的元素:

for (SchoolObj one : listOne)
    {
    for (SchoolObj two : listTwo)
    {
        if (one.getName().equals(two.getName()) && one.getSchool().equals(two.getSchool()))
        {
            result.add(one);
            break;
        }
    }
}


此外,如果您不想在列表中重复对象(按内存中的位置),则可以使用distinct,如下所示:

List<SchoolObj> result = ...;
result = result.stream().distinct().collect(Collectors.toList());


最后请注意,在以下情况下,以上内容将使结果与众不同:

List<SchoolObj> list = new ArrayList<>();
SchoolObj duplicate = new SchoolObj("nameC", "schoolD");
listOne.add(duplicate);
listOne.add(duplicate);
list.stream().distinct().forEach(System.out::println);
// prints:
// nameC schoolD


但是,除非您为SchoolObj覆盖equals方法,否则它在以下情况下将不起作用:

List<SchoolObj> list = new ArrayList<>();
listOne.add(new SchoolObj("nameC", "schoolD"));
listOne.add(new SchoolObj("nameC", "schoolD"));
list.stream().distinct().forEach(System.out::println);
// prints (unless Object::equals overridden)
// nameC schoolD
// nameC schoolD

09-10 13:33
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