java - 如果从另一个类中调用它，为什么我的ArrayList为空？

我是Java的新手，将为您提供一点帮助。

有关我的代码的注意事项

我有两节课。

第一类称为EvidenceBox.class

第二类称为Police.class

在EvidenceBox类中，我有一个名为getParticularEvidence()的方法。

    public ArrayList <Evidence> getParticularEvidence(int caseNumber, String evidenceType) {
      EvidenceBox e = new EvidenceBox(caseNumber, evidenceType);
      return e.getEvidenceByType(caseNumber, evidenceType);
    }

该方法正在做的是将caseNumber和证据类型作为参数。在此方法内部，我们正在创建EvidenceBox类的Object实例，以便我可以调用EvidenceBox类中的另一个名为getEvidenceByType()的方法。该方法的重点是将上面的两个参数传递给另一个类中的另一个方法，该类在for循环内具有if语句，该语句检查参数是否等于称为证据框列表的ArrayList中的某个值，如果相等则则返回相等的证据列表。

当我在Main类中调用此方法时:

System.out.println(crimeBoxOne.getEvidenceByType(2005000381, "S.Avery"));

它的作用是转到getEvidenceByType方法，然后调用Evidence类中的另一个方法:

    public ArrayList < Evidence > getEvidenceByType(int keyNumber, String keyName) {
      ArrayList < Evidence > setOfEvidence = new ArrayList < > (evidenceBoxList);
      for (Evidence e: setOfEvidence) {
        if (this.getCaseNumber() == keyNumber) {
          if (this.getCaseName().equals(keyName)) {
            System.out.println("Pieces of DNA Evidence in Evidence Box number " + this.getCaseNumber() + " found by " + this.getCaseName() + ": " + this.getEvidence());
            getEvidence().add(e);
          } else {
            System.out.println("Not found!");
          }
        } else {
          System.out.println("Not Found!");
        }
      }
      return getEvidence();
    }

getCaseNumber是EvidenceBox的编号，而getCaseName是证据框的名称

我的问题

我的问题是，即使不是因为我将EvidenceBox添加到ArrayList，列表的返回值始终为空。我该如何解决这个问题？

getEvidence()是返回证据框列表的getter方法。

这是我的EvidenceBox类:

package looselycoupled;

import java.util.*;

public class EvidenceBox {
  private int caseNumber;
  private String caseName;
  private ArrayList evidenceBoxList;

  public EvidenceBox(int caseNumber, String caseName) {
    this.evidenceBoxList = new ArrayList < Evidence > ();
    this.caseNumber = caseNumber;
    this.caseName = caseName;
  }

  public ArrayList < Evidence > getEvidenceByType(int keyNumber, String keyName) {
    ArrayList < Evidence > setOfEvidence = new ArrayList < > (evidenceBoxList);
    for (Evidence e: setOfEvidence) {
      if (this.getCaseNumber() == keyNumber) {
        if (this.getCaseName().equals(keyName)) {
          System.out.println("Pieces of DNA Evidence in Evidence Box number " + this.getCaseNumber() + " found by " + this.getCaseName() + ": " + this.getEvidence());
          getEvidence().add(e);
        } else {
          System.out.println("Not found!");
        }
      } else {
        System.out.println("Not Found!");
      }
    }
    return getEvidence();
  }

  public void add(Evidence getTypeFromEvidenceClass) {
    evidenceBoxList.add(getTypeFromEvidenceClass);
  }


  public ArrayList < Evidence > getEvidence() {
    return evidenceBoxList;
  }



  public void printRecordsOfTheEvidence() {
    Set < Evidence > setOfEvidence = new HashSet < > (evidenceBoxList);
    for (Evidence e: setOfEvidence) {
      System.out.println(Collections.frequency(evidenceBoxList, e) + "x " + e);
    }
  }

  public int getCaseNumber() {
    return caseNumber;
  }

  public String getCaseName() {
    return caseName;
  }

  @
  Override
  public String toString() {
    return caseNumber + " " + caseName;
  }
}

和警察课:

package looselycoupled;

import java.util.ArrayList;

public class Cop {
    private String name;
    //private ArrayList collectionOfEvidences = new ArrayList<>();
    private ArrayList<EvidenceBox> collectionOfEvidences = new ArrayList<>();
    public Cop(String name) {
        this.name = name;
    }

    public void copTakesAnEvidenceBox(EvidenceBox box) {
        collectionOfEvidences.add(box);
    }

    public ArrayList<Evidence> getParticularEvidence(int caseNumber, String evidenceType) {
        EvidenceBox e = new EvidenceBox(caseNumber, evidenceType);
        return e.getEvidenceByType(caseNumber, evidenceType);
    }

    public ArrayList<EvidenceBox> getEvidence() {
        return collectionOfEvidences;
    }

    public String toString() {
        return name;
    }
}

最佳答案

evidenceBoxList对EvidenceBox类是私有的。

每当您执行EvidenceBox e = new EvidenceBox(caseNumber, evidenceType);时，它都会创建此列表的新Blank实例，不带任何值。

接下来，您要遍历此列表，并尝试使用以下代码来添加值:

ArrayList < Evidence > setOfEvidence = new ArrayList < > (evidenceBoxList);
    for (Evidence e: setOfEvidence) {
      if (this.getCaseNumber() == keyNumber) {
        if (this.getCaseName().equals(keyName)) {
          System.out.println("Pieces of DNA Evidence in Evidence Box number " + this.getCaseNumber() + " found by " + this.getCaseName() + ": " + this.getEvidence());
          getEvidence().add(e);
        } else {
          System.out.println("Not found!");
        }
      } else {
        System.out.println("Not Found!");
      }
    }
    return getEvidence();
  }

现在，由于您最初在evidenceBoxList中没有任何元素，因此它将永远不会进入for循环，也永远不会调用add()方法。

然后，getEvidence()方法将该空白列表返回给您。

在for循环之前添加getEvidence().add(new Evidence(keyNumber,keyName));以验证问题。

ArrayList < Evidence > setOfEvidence = new ArrayList < > (evidenceBoxList);
    /* Add Here */
    getEvidence().add(new Evidence(keyNumber,keyName));
    for (Evidence e: setOfEvidence) {
        /* Rest of the code */

现在，它应该返回带有一个对象的列表。