Question

我是Java新手。这是一个错误。

private class applicantInfo {
    int Id;
    double quality;
}
private class allApplicants {
    applicantInfo[] applicantArr = new applicantInfo[20];
}
public void newGame {
    allApplicants applicants = new allApplicants();
    applicants.applicantArr[0].Id = 5;
}

我在applicants.applicantArr[0].Id = 5;时收到错误。

我想要做的就是在C：

中与此相似

typedef struct _applicantInfo{
    int Id;
    double quality;
} applicantInfo;

typedef struct _allApplicants {
    applicantInfo applicantArr[20];
} allApplicants;

int main () {
    allApplicants applicants;
    applicants.applicantArr[0].Id = 5;  
}

我怎么能用Java做到这一点？

Answer 1

Java和C数组之间的区别在于C初始化Array中的所有值，而Java将它们设置为null。所以当你打电话时

applicants.applicantArr[0].Id = 5;

您将获得NullPointerException，因为applicants.applicantArr [0]为null。您需要创建一个新的applicantInfo并在访问它之前将其放入数组中：

allApplicants applicants = new allApplicants();
applicants.applicantArr[0] = new applicantInfo();
applicants.applicantArr[0].Id = 5;

Answer 2

你需要在newGame（）中执行此操作：

applicantInfo item = new applicantInfo();//first create a applicantInfo object
item.Id= 5;//set the object properties
applicants.applicantArr[0]= item;//assign the object to the array

这是因为数组在Java中的工作方式与C中的方式不同。看看this

此处还有一个tutorial，可以帮助您入门。

Answer 3

我建议使用注释和TODO为您的代码建立一个高级结构，您可以填写详细信息。最后一部分，我建议newGame方法的结构，将帮助你摆脱你得到的错误。

ApplicantInfo类的结构：

public class ApplicantInfo {
    private int ID;
    private double quality;

    // Constructor to create an instance with the specified ID value
    public ApplicantInfo(int id){
        // TODO: Initialize the value for ID field
    }

    // Method to get the value for ID
    public int getID(){
        // TODO: return value of ID field
    }

    // Method to set the value for ID
    public void setID(int id){
        // TODO: set the value for ID field
    }

    // Getter and setter methods for "quality" 
    // on the lines of the above methods
}

AllApplicants类的结构：

public class AllApplicants {
    private ApplicantInfo[] applicantArr = new ApplicantInfo[20];

    // Method to get the applicant info at a given index
    public ApplicantInfo getApplicant(int index){
        // TODO: Get the applicant from the array present at the specified index
    }

    // Method to add an applicant info at a given index
    public boolean addApplicant(ApplicantInfo applicant, int index){
        // TODO: Try to add the specified applicant to the array at the specified index
        // Return true to indicate that the applicant was successfully added, 
        // Return false to indicate that an applicant is already present at the specified index
    }
}

因为这只是一个骨架 newGame方法的结构：

public void newGame {
    AllApplicants applicants = new AllApplicants();

    // In order to achieve doing "applicants.applicantArr[0].Id = 5;", you
    // need to do the following.

    // Create a new applicant info with ID as 5
    ApplicantInfo applicant = new ApplicantInfo(5);

    // Add the applicant to the applicant array at index 0
    applicants.addApplicant(applicant, 0);
}

除了阅读有关数组的内容，如@codeman所述，您可能还想看看Java Naming Convention

对象变量的Java数组更改

3 个答案: