Java 获取然后从列表中删除

Question

在我的代码中，我有一个 class 的实例列表。我想获得 1 个实例的属性，即 ArrayList。在这个 class 中，我实现了 getter 和 setter。所以我打电话给

listofinstances.get(i).getArrayList().remove(0);

为了删除此列表的第一项。这有效吗？？或者我必须首先获取列表，将其存储到一个临时变量，删除我想要的项目，最后用 set 方法刷新它？？

例子

tmp = listofinstances.get(i).getArrayList();
tmp.remove(0);
listofinstances.get(i).setArrayList(tmp);

Answer 1

两个代码片段将产生相同的结果，因为 listofinstances.get(i).getArrayList().remove(0) 更新了 listofinstances.get(i).getArrayList() 返回的 List 实例，并且第二个代码片段做同样的事情，仅在两行中。

第二个片段中的 listofinstances.get(i).setArrayList(tmp); 行是多余的，因为 listofinstances 的第 i 元素已经包含对 tmp 引用的同一列表的引用。

Answer 2

删除

listOfInstances.get(1).getArrayList().remove(1);

足够有效。

为了演示这一点，我为您编写了一个测试代码。我刚刚删除了第二个对象的 ArrayList 的第二个 String 元素，您可以比较初始状态和更新后的状态；

import java.util.ArrayList;
import java.util.Arrays;

public class TestQuestion {

    public static void main(String[] args) {
        // inital objects to be stored
        ArrayList<String> arr1 = new ArrayList<String>(
                Arrays.asList((new String[] { "ins1: test string1",
                        "ins1: test string2", "ins1: test string3" })));
        PlaceHolderObject<String> pho1 = new PlaceHolderObject<String>();
        pho1.setArrayList(arr1);

        ArrayList<String> arr2 = new ArrayList<String>(
                Arrays.asList((new String[] { "ins2: test string1",
                        "ins2: test string2", "ins2: test string3" })));
        PlaceHolderObject<String> pho2 = new PlaceHolderObject<String>();
        pho2.setArrayList(arr2);

        ArrayList<String> arr3 = new ArrayList<String>(
                Arrays.asList((new String[] { "ins3: test string1",
                        "ins3: test string2", "ins3: test string3" })));
        PlaceHolderObject<String> pho3 = new PlaceHolderObject<String>();
        pho3.setArrayList(arr3);

        // gather up all instance in one
        ArrayList<PlaceHolderObject<String>> listOfInstances = new ArrayList<PlaceHolderObject<String>>();

        // assignments
        listOfInstances.add(pho1);
        listOfInstances.add(pho2);
        listOfInstances.add(pho3);

        // print contents of listOfInstances objects
        System.out.println("Contents of the 'listOfInstances' list");
        System.out.println("**************************************");

        for (int i = 0; i < listOfInstances.size(); i++)
            System.out.println(listOfInstances.get(i).getArrayList());

        System.out.println();

        // print references of the contents of listOfInstances objects
        System.out.println("References of the 'listOfInstances' list");
        System.out.println("****************************************");

        for (int i = 0; i < listOfInstances.size(); i++) {
            System.out.println(i+1 + "th Object: " + listOfInstances.get(i) );
            System.out.println("   ArrayList hashcode: " + listOfInstances.get(i).getArrayList().hashCode()  );
        }

        // Remove second item of the second object
        listOfInstances.get(1).getArrayList().remove(1);
        System.out.println();
        System.out.println();

        // print contents of listOfInstances objects
        System.out.println("Contents of the 'listOfInstances' updated list");
        System.out.println("**********************************************");

        for (int i = 0; i < listOfInstances.size(); i++)
            System.out.println(listOfInstances.get(i).getArrayList());

        System.out.println();

        // print references of the contents of updated listOfInstances objects
        System.out.println("References of the 'listOfInstances' updated list");
        System.out.println("************************************************");

        for (int i = 0; i < listOfInstances.size(); i++) {
            System.out.println(i+1 + "th Object: " + listOfInstances.get(i) );
            System.out.println("   ArrayList hashcode: " + listOfInstances.get(i).getArrayList().hashCode()  );
        }

    }

    // A POJO class that only stores an arrayList
    public static class PlaceHolderObject<T> {
        private ArrayList<T> arrayList;

        // no-arg default constructor
        public PlaceHolderObject() {
        }

        // parametric constructor
        public PlaceHolderObject(ArrayList<T> arrayList) {
            this.arrayList = arrayList;
        }

        public ArrayList<T> getArrayList() {
            return arrayList;
        }

        public void setArrayList(ArrayList<T> arrayList) {
            this.arrayList = arrayList;
        }

    }

}

输出结果如下；

Contents of the 'listOfInstances' list
**************************************
[ins1: test string1, ins1: test string2, ins1: test string3]
[ins2: test string1, ins2: test string2, ins2: test string3]
[ins3: test string1, ins3: test string2, ins3: test string3]

References of the 'listOfInstances' list
****************************************
1th Object: TestQuestion$PlaceHolderObject@5058431c
   ArrayList hashcode: 1200611515
2th Object: TestQuestion$PlaceHolderObject@529e0c79
   ArrayList hashcode: -744028452
3th Object: TestQuestion$PlaceHolderObject@645064f
   ArrayList hashcode: 1606298877


Contents of the 'listOfInstances' updated list
**********************************************
[ins1: test string1, ins1: test string2, ins1: test string3]
[ins2: test string1, ins2: test string3]
[ins3: test string1, ins3: test string2, ins3: test string3]

References of the 'listOfInstances' updated list
************************************************
1th Object: TestQuestion$PlaceHolderObject@5058431c
   ArrayList hashcode: 1200611515
2th Object: TestQuestion$PlaceHolderObject@529e0c79
   ArrayList hashcode: 828096323
3th Object: TestQuestion$PlaceHolderObject@645064f
   ArrayList hashcode: 1606298877

希望对您有所帮助。

Java 获取然后从列表中删除

Java Get and then remove from a list

java

getter

setter

list

instances