java - 使用数组打印时出现空指针异常

Question

似乎无法消除该错误，查看了消除它的不同方法，但无法真正理解。

static Holding[]holding = new Holding[15];

public static void main(String[] args){
    sampleData();   //Sample Data from Assignment

    int options;
    do{ 
        scanner = new Scanner(System.in);

        System.out.println("1. Add Holding");
        System.out.println("7. Print all Holdings");
        System.out.println("13. Exit");

        options = scanner.nextInt();
        switch(options){

        case 1:
            addHolding();
            break;
        case 7:
            printHolding();
            break;

        default:
            System.out.println("Please");
        }

    }while(options != 13);
    System.out.println("I'm out");

}

public static void sampleData(){
    holding[0] = new Book("b000001", "Intro to Java");
    holding[1] = new Book("b000002", "Learning UML");
    holding[2] = new Book("b000003", "Design Patterns");
    holding[3] = new Book("b000004", "Advanced Java");
    holding[4] = new Video("v000001", "Java 1", 4);
    holding[5] = new Video("v000002", "Java 2", 6);
    holding[6] = new Video("v000003", "UML 1", 6);
    holding[7] = new Video("v000004", "UML 2", 4);

}

    public static void printHolding(){
    int option;
    do{
        for(Holding h : holding){   
            h.print();
            System.out.println();
        }

        System.out.println("To exit press '0'");
        option = input.nextInt();
    }while(option != 0);


}

Exception in thread "main" java.lang.NullPointerException
    at s3599741_A2.LibraryMenu.printHolding(LibraryMenu.java:307)
    at s3599741_A2.LibraryMenu.main(LibraryMenu.java:71)

错误是

h.print();

和

printHolding();

在 switch 语句中。 正如您所看到的，该数组是由另一个对象组成的，例如

holding[i] = new Book("Insert ID", "Insert Title");

我写了一个这样的问题，但被标记为重复，我查看了链接

What is a NullPointerException, and how do I fix it?

但是找不到我遇到的问题或者我还没有了解的知识。有人可以解释我应该做什么吗？

Holding 的限制必须是 15，我不能使用 ArrayList 或 List。

控股类(Class):

public abstract class Holding{

private String holdingId;
private String title;

public Holding(String holdingId, String title){

    this.holdingId = holdingId;
    this.title = title;         
}

public String getId() {

    return holdingId;
}

public String getTitle(){

    return title;
}

public boolean getStatus(){

    return active;  
}   

public void print(){
    System.out.println("ID: " + getId());
    System.out.println("Title: " + getTitle());     
}

}

Answer 1

您的holding数组开始时填充有null。 sampleData 仅将 holding 的八个条目(条目 0 - 7，包含在内)设置为非 null 值；这意味着条目 8 - 14(含)仍然为 null。

这意味着这段代码将抛出:

for (Holding h : holding) {
    h.print();                 // Here
    System.out.println();
}

要修复此问题，请添加 null 检查:

for (Holding h : holding) {
    if (h != null) {
        h.print();
        System.out.println();
    }
}

...和/或跟踪您通过实例字段添加了多少 Holding(因为您显然不允许为此使用正确的数据结构，列表);

static int holdingCount = 0;

...那么 sampleData 看起来像:

public static void sampleData() {
    addSampleData(new Book("b000001", "Intro to Java"));
    addSampleData(new Book("b000002", "Learning UML"));
    addSampleData(new Book("b000003", "Design Patterns"));
    addSampleData(new Book("b000004", "Advanced Java"));
    addSampleData(new Video("v000001", "Java 1", 4));
    addSampleData(new Video("v000002", "Java 2", 6));
    addSampleData(new Video("v000003", "UML 1", 6));
    addSampleData(new Video("v000004", "UML 2", 4));
}

然后

private static void addSampleData(Holding h) {
    if (holdingCount < holding.length) {
        holding[holdingCount++] = h;
    } else {
        throw new IllegalStateException("Tried to add too many holdings to array");
    }
}

addHolding 会做类似的事情。

但是，这只是因为您说过不允许为此使用正确的数据结构。