java在打印ArrayList multipe值时，仅显示>'[]'

Question

我在Java中显示ArrayList的元素时遇到问题。当从视图调用它时返回ArrayList时，返回现在包含伪值表单的bmiAnalyzer类。它显示
[] []
当java文件运行时

1. 观点。爪哇

   Switch(choice[0]){
       case 1:
   
           //Option 1 to display the data fo subject of given subject id.
   
           ArrayList<Records> arraylist = analyzier.find(choice[1]);
   
           System.out.println(ArrayList);
           Break;
       Case 2:
           //Option 2 to display the data fo subject from the range given of BMI.
   
           ArrayList<Records> arraylistList = analyzier.find(Double.parseDouble(choice[1]),Double.parseDouble(choice[2]));
   
           for (int i = 0; i < arraylistList.size(); i++){
   
               System.out.println((arraylistList.get(i)).toString());
           }
   
           Break;
   
       default:
           System.out.println("wrong input");
   }
   

2. BMI分析仪。爪哇

   public class BMIAnalyzier {
   
       public Records find(String sid) {
   
           System.out.println(new Records(sid, 0.0, 0.0, 0.0, "none"));
   
           return new Records(sid, 0.0, 0.0, 0.0, "none");
       }
   
       public ArrayList<Records> find(double bmi1, double bmi2) {
   
           ArrayList<Records> alr = new ArrayList<>();
   
           alr.add(new Records("S01", 0.0, 0.0, 0.0, "none"));
   
           alr.add(new Records("S02", 0.0, 0.0, 0.0, "none"));
   
           return alr;
       }
   }
   

3. 记录。爪哇

   public class Records extends ArrayList<Records> {
       private String SubjectId;
       private Double height;
       private Double width;
       private Double bmianalyzier;
       private String categories;
       public ArrayList <Records> list =new ArrayList<>();
       public Records(String sid, Double h, Double w, Double bmi, String c) {
       }
       //getter ans setter methods.
   }
   

输出：

Answer 1

我想你是java新手吧。你需要先了解基本知识。例如，在您的示例中，ArrayList是一个用于保存相同类型的多个值的集合。使用Records类，您可以扩展此处不需要的集合

public class Records extends ArrayList < Records > {

应该是

public class Records {

此外，您应该始终向类构造函数提供内容，否则不会为对象设置任何值

    public Records(String sid, Double h, Double w, Double bmi, String c) {
        this.SubjectId = sid;
       // set other values as well
    }

并且，find（stringsid）返回记录对象

ArrayList < Records > arraylist = analyzier.find(choice[1]);

应改为

Records record = analyzier.find(choice[1]);

要打印记录对象的值，请按照@Niklas的建议执行

Answer 2

主要的奥秘在于Records类扩展了ArrayList。我不知道您为什么要这样做，但是在这样的情况下，您继承了ArrayList的toString()方法，这是呈现[]的方法，因为数组是空的。您需要为Records类实现toString()

String toString() {
    return subjectId + " " + height + " " + width + " " + bmianalyzier + " " + categories;
}