ArrayList<Object>的JAVA屬性不回傳值

類醫院

package app;

import java.util.ArrayList;
import app.Department;

public class Hospital {
    String hospitalId;
    String hospitalName;
    int departmentCount;
    ArrayList<Department> departmentList = new ArrayList<Department>();

    public Hospital(String hospitalId, String hospitalName, ArrayList<Department> departmentList, int departmentCount) {
        this.hospitalId = hospitalId;
        this.hospitalName = hospitalName;
        this.departmentList = departmentList;
        this.departmentCount = departmentCount;
    }
    
    public ArrayList<Department> getDepartmentList() {
        System.out.println("Returning "   departmentList.size());
        return departmentList;
    }

}

班級部

package app;

public class Department {
    String departmentId;
    private String hospitalDeptName;
    String hospitalDeptType;
    String subtypeBitFlag;
    String status;

    public Department(String departmentId, String hospitalDeptName, String hospitalDeptType, String subtypeBitFlag, String status) {
        this.departmentId = departmentId;
        this.hospitalDeptName = hospitalDeptName;
        this.hospitalDeptType = hospitalDeptType;
        this.subtypeBitFlag = subtypeBitFlag;
        this.status = status;
    }
    
    public String getDepartmentName() {
        return hospitalDeptName;
    }
}

我正在創建一個物件 Department 并將它們存盤在 ArrayList

ArrayList<Department> departmentList = new ArrayList<Department>();
departmentList.add(new Department(departmentId, hospitalDeptName, hospitalDeptType, subtypeBitFlag, status));

我正在創建一個物件 Hospital 并將上面的 departmentList 作為部門串列傳遞給建構式

ArrayList<Hospital> hospitalList = new ArrayList<Hospital>();
Hospital hospital = new Hospital(hospitalId, hospitalName, departmentList, departmentList.size());

遍歷醫院物件并提取資訊

            for (Hospital hospital : hospitalList) {
                System.out.println("The number of departments is: " hospital.departmentList.size()   " for Hospital: "   hospital.hospitalId );
                
                for (Department department : hospital.departmentList) {
                    System.out.println("Hospital ID: "  hospital.hospitalId   ", Hospital Name: "  hospital.hospitalName   ", Department Name: "   department.getDepartmentName());
                }
            }

日志如下所示。我無法找回部門，甚至無法找回他們的價值觀。但是...我能夠從物件 Hospital 中檢索除 ArrayList 之外的任何內容。計數 0 證明好像物件 Hospital 沒有填充 Department 的陣列串列

The number of departments is: 0 for Hospital: q
The number of departments is: 0 for Hospital: a
The number of departments is: 0 for Hospital: s
The number of departments is: 0 for Hospital: 1
The number of departments is: 0 for Hospital: 2
The number of departments is: 0 for Hospital: 3

uj5u.com熱心網友回復：

您只創建了一個陣列串列，所有醫院都只參考了同一個串列，您稍后將其清除，因此，所有部門都被洗掉了。

讓我們一步一步來：

ArrayList<Department> departmentList = new ArrayList<Department>();

這會創建一個新的 arraylist 物件，并將對它的參考分配給一個名為 的新宣告的區域變數departmentList。new ArrayList()就像“蓋房子”，ArrayList<Department> departmentList;就像“在地址簿中打開一個新頁面并將其命名為“departmentList”。departmentList = new...是：在地址簿上寫下那個房子的地址。至關重要departmentList的一點：與串列不同——它是對它的參考——它是地址簿中的頁面，你可以用來到達房子，而不是房子本身。

departmentList.add(new Department(departmentId, hospitalDeptName, > hospitalDeptType, subtypeBitFlag, status));

.是取消參考運算子：這相當于“在地址簿中獲取標題為“departmentList”的頁面，然后走到房子本身。打開門并對它大喊“添加！”。事情可能會發生在你做這個。

Hospital hospital = new Hospital(hospitalId, hospitalName, departmentList, departmentList.size());

專注于departmentList部分：這說：“拿一張紙，把地址復制到上面，然后把那張紙交給醫院建設者”。它不會克隆串列。

public Hospital(String hospitalId, String hospitalName, ArrayList<Department> departmentList, int departmentCount) {
    this.departmentList = departmentList;
}

更多地址簿頁面的復制。從來沒有人在任何地方復制房子。

現在有許多筆記本頁面在上面浮動，所有的地址都相同。如果有頁面的任何人決定訪問一所房子并通過窗戶扔一塊磚，其他人也會看到。這不好 - 現在有人可以更改您醫院的科室串列。您可能想克隆該陣列串列。

您的醫院建構式應該有：

this.departmentList = List.copyOf(departmentList);

反而。

標籤：爪哇班级目的数组列表

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