java - 在 Java 中初始化类实例中的变量数组

Question

我来自 C 背景，在 Java 中遇到了问题。目前，我需要在对象数组中初始化变量数组。

我知道在 C 中它类似于malloc-ing数组中int的数组，structs如：

typedef struct {
  char name;
  int* times;
} Route_t

int main() {
  Route_t *route = malloc(sizeof(Route_t) * 10);
  for (int i = 0; i < 10; i++) {
    route[i].times = malloc(sizeof(int) * number_of_times);
  }
...

到目前为止，在 Java 中我有

public class scheduleGenerator {

class Route {
        char routeName;
    int[] departureTimes;
}

    public static void main(String[] args) throws IOException {
      /* code to find number of route = numRoutes goes here */
      Route[] route = new Route[numRoutes];

      /* code to find number of times = count goes here */
      for (int i = 0; i < numRoutes; i++) {
        route[i].departureTimes = new int[count];
...

但它吐出一个NullPointerException. 我做错了什么，有没有更好的方法来做到这一点？

Answer 1

当你初始化你的数组

Route[] route = new Route[numRoutes];

有numRoutes插槽都填充了它们的默认值。对于引用数据类型，默认值为null，因此当您尝试Route在第二个循环中访问对象时，for它们都是null，您首先需要像这样初始化它们：

public static void main(String[] args) throws IOException {
      /* code to find number of route = numRoutes goes here */
      Route[] route = new Route[numRoutes];

      // Initialization:
      for (int i = 0; i < numRoutes; i++) {
           route[i] = new Route();
      }

      /* code to find number of times = count goes here */
      for (int i = 0; i < numRoutes; i++) {
        // without previous initialization, route[i] is null here 
        route[i].departureTimes = new int[count];

Answer 2

Route[] route = new Route[numRoutes];

在 java 中，当您创建对象数组时，所有插槽都使用默认值声明如下 Objects = null 原语 int = 0 boolean = false

这些 numRoutes 插槽都填充了它们的默认值，即 null。当您尝试访问循环中的 Route 对象时，数组引用指向 null，您首先需要像这样初始化它们：

  // Initialization:
  for (int i = 0; i < numRoutes; i++) {
       route[i] = new Route();
       route[i].departureTimes = new int[count];
  }

Answer 3

for (int i = 0; i < numRoutes; i++) {
  route[i] = new Route();
  route[i].departureTimes = new int[count];