Question

我有两个来自Parse Query的ArrayLists，我正在与一个增强的for循环进行比较。我想检查2列表中是否有匹配的ObjectId，如果是这样，我想从“allDropList”中删除该Object。所有其他组件都正常工作，我相信我的增强for循环语法的问题。这是我的代码，谢谢！注意：我已经尝试过“if”和“while”，但没有成功。

public ArrayList<DropItem> filterDrops(ArrayList <DropItem> hasRelationList , ArrayList <DropItem> allDropsList){

    for(DropItem dropItemAll : allDropsList) {

        for(DropItem dropItemRelation  : hasRelationList) {

           /*if*/ while(dropItemAll.getObjectId().equals(dropItemRelation.getObjectId())) {
                  allDropsList.remove(dropItemAll);
            }
        }
    }
    return allDropsList;
}

Answer 1

问题是你无法迭代列表并使用for循环同时删除某些内容。请查看here以获得进一步说明。

Answer 2

以相反的顺序删除列表中的对象更安全：

  public ArrayList<DropItem> filterDrops(ArrayList <DropItem> hasRelationList ,ArrayList <DropItem> allDropsList){
    for(int i = allDropsList.size()-1; i >=0; i--) {
      for(int j = hasRelationList.size()-1; j >=0; j--) {
        if(allDropsList.get(i).getObjectId().equals(hasRelationList.get(j).getObjectId())) {
          allDropsList.remove(i);
        }
      }
    }
    return allDropsList;
  }

Answer 3

或者你可以简单地这样做：

    public ArrayList<DropItem> filterDrops(ArrayList <DropItem> hasRelationList , ArrayList <DropItem> allDropsList){

    ArrayList<DropItem> temp = new ArrayList<DropItem>(); //place the objects you want inside this arraylist
    for(DropItem dropItemAll : allDropsList) {
        int counter = 0;
        for(DropItem dropItemRelation  : hasRelationList) {

            if(dropItemAll.getId().equals(dropItemRelation.getId())) {
                break;
            }
            if(counter == hasRelationList.size()-1){
               temp.add(dropItemAll);
            }
            counter++;
        }
    }
    return temp;
}

Answer 4

如果您使用Iterator代替外部循环，则可以在迭代时从列表中删除项目，而无需借助其他＆＃39;。

import numpy as np
import matplotlib.pyplot as plt
from cartopy import crs

# a grid for the longitudes and latitudes
lats = np.linspace(-90, 90, 50)
longs = np.linspace(-180, 180, 50)
lats, longs = np.meshgrid(lats, longs)

# some data
data = lats[1:] ** 2 + longs[1:] ** 2

fig = plt.figure()

# create a new axes with a cartopy.crs projection instance
ax = fig.add_subplot(1, 1, 1, projection=crs.Mollweide())

# plot the date
ax.pcolormesh(
    longs, lats, data,
    cmap='hot',
    transform=crs.PlateCarree(),  # this means that x, y are given as longitude and latitude in degrees
)
fig.tight_layout()
fig.savefig('cartopy.png', dpi=300)

Answer 5

这里是使用Iterator的更正答案。这是一个编译和运行的完整示例：

import java.util.ArrayList;
import java.util.Iterator;

 public class DropTest {

    class DropItem {
       private String objectId;

       public String getObjectId() {
          return objectId;
       }

       public DropItem(String id) {
          this.objectId = id;
       }
    }

    public static void main(String[] args) {
        new DropTest().test();
    }

    public void test() {
        ArrayList<DropItem> allDropsList = new ArrayList<>();       
        allDropsList.add(new DropItem("one"));
        allDropsList.add(new DropItem("two"));
        allDropsList.add(new DropItem("three"));
        allDropsList.add(new DropItem("four"));
        allDropsList.add(new DropItem("five"));

        ArrayList<DropItem> hasReleationList = new ArrayList<>();
        hasReleationList.add(new DropItem("three"));
        hasReleationList.add(new DropItem("five"));

        System.out.println("before:");
        for(DropItem dropItem : allDropsList) {
           System.out.println(dropItem.getObjectId());
        }

        ArrayList<DropItem> result = filterDrops(hasReleationList, allDropsList);
        System.out.println("\n\nafter:");
        for(DropItem dropItem : result) {
           System.out.println(dropItem.getObjectId());
        }
   }

   public ArrayList<DropItem> filterDrops(ArrayList <DropItem> hasRelationList, ArrayList <DropItem> allDropsList) {
       Iterator<DropItem> allDropsIterator = allDropsList.iterator();

       while(allDropsIterator.hasNext()) {
          DropItem dropItemAll = allDropsIterator.next();

          for(DropItem dropItemRelation  : hasRelationList) {
              if(dropItemAll.getObjectId().equals(dropItemRelation.getObjectId())){
                  allDropsIterator.remove();
              }
          }
      }

      return allDropsList;
   }
}

增强了for循环以从ArrayList中删除对象

5 个答案: