如果在谓词中给出了所有条件，则获取所有匹配的非null元素

Question

如何获取id = 2的所有元素？

下面的代码只给出了最后一个，即channelMetadataDetail3：

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


public class APICheck {
    public static void main(String[] args) {

        List<ChannelMetadataDetails> channelMetadataDetails = 
        new ArrayList<ChannelMetadataDetails>();
        ChannelMetadataDetails channelMetadataDetail1 = 
        new ChannelMetadataDetails();
        channelMetadataDetail1.setId("2");
        channelMetadataDetail1.setUnitOfMeasureId("3");

        ChannelMetadataDetails channelMetadataDetail2 = 
        new ChannelMetadataDetails();
        channelMetadataDetail2.setId("2");
        channelMetadataDetail2.setUnitOfMeasureId("3");
        channelMetadataDetail2.setMeasureTypeId("5");

        ChannelMetadataDetails channelMetadataDetail3 = 
        new ChannelMetadataDetails();
        channelMetadataDetail3.setId("2");

        channelMetadataDetails.add(channelMetadataDetail1);
        channelMetadataDetails.add(channelMetadataDetail2);
        channelMetadataDetails.add(channelMetadataDetail3);

        channelMetadataDetails =channelMetadataDetails.
        stream().filter(cm -> cm.getId().equals("2")).
        filter(cm -> cm.getUnitOfMeasureId().equals(null)).
        filter(cm -> cm.getMeasureTypeId().equals(null)).
        collect(Collectors.toList());

    }   

}

注意： - 如果cm.getUnitOfMeasureId()为null，那么我将获得NullPointerException

更新：

简而言之，我只想要那些元素非空的对象。

例如使用for循环：

for (ChannelMetadataDetails cmEach : channelMetadataDetails) {
            if(cmEach.getId() != null && cmEach.getUnitOfMeasureId() == null && cmEach.getMeasureTypeId() == null) {
                channelMetadataDetails.stream().filter(cm -> cm.getId().equals("2")).collect(Collectors.toList());
            } else if(cmEach.getId() == null && cmEach.getUnitOfMeasureId() != null && cmEach.getMeasureTypeId() == null) {
                channelMetadataDetails.stream().filter(cm -> cm.getUnitOfMeasureId().equals("4")).collect(Collectors.toList());
            } else if(cmEach.getId() == null && cmEach.getUnitOfMeasureId() == null && cmEach.getMeasureTypeId() != null) {
                channelMetadataDetails.stream().filter(cm -> cm.getMeasureTypeId().equals("5")).collect(Collectors.toList());
            } else if(cmEach.getId() != null && cmEach.getUnitOfMeasureId() != null && cmEach.getMeasureTypeId() == null) {
                channelMetadataDetails.stream().filter(cm -> cm.getId().equals("2") && cm.getUnitOfMeasureId().equals("4")).collect(Collectors.toList());
            } else if(cmEach.getId() != null && cmEach.getUnitOfMeasureId() == null && cmEach.getMeasureTypeId() != null) {
                channelMetadataDetails.stream().filter(cm -> cm.getId().equals("2") && cm.getMeasureTypeId().equals("5")).collect(Collectors.toList());
            } else if(cmEach.getId() != null && cmEach.getUnitOfMeasureId() != null && cmEach.getMeasureTypeId() != null) {
                channelMetadataDetails.stream().filter(cm -> cm.getId().equals("2") && cm.getUnitOfMeasureId().equals("4") && cm.getMeasureTypeId().equals("5")).collect(Collectors.toList());
            }
        }

Answer 1

你只得到最后一个元素的原因是你的第二个和第三个过滤器要求cm.getUnitOfMeasureId()和cm.getMeasureTypeId()为空。这只适用于最后一个元素。如果您只想按getId()过滤，请删除其他过滤条件：

channelMetadataDetails =
    channelMetadataDetails.stream()
                          .filter(cm -> cm.getId() != null && cm.getId().equals("2"))
                          .collect(Collectors.toList());

编辑：

根据您的编辑，您希望3个属性中至少有一个具有值，并且您想要检查所有非空值：

channelMetadataDetails =
    channelMetadataDetails.stream()
                          .filter(cm -> cm.getId() != null || cm.getUnitOfMeasureId() != null || cm.getMeasureTypeId() != null)
                          .filter(cm -> cm.getId() == null || cm.getId().equals("2"))
                          .filter(cm -> cm.getUnitOfMeasureId() == null || cm.getUnitOfMeasureId().equals("4"))
                          .filter(cm -> cm.getMeasureTypeId() == null || cm.cm.getMeasureTypeId().equals("5"))
                          .collect(Collectors.toList());