Question

我来自C背景，并且遇到了Java问题。目前，我需要初始化对象数组中的变量数组。

我在C中知道它类似于malloc-ing int数组中的structs数组，类似于：{/ p>

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NullPointerException。我做错了什么，有更好的方法吗？

Answer 1

初始化数组时

Route[] route = new Route[numRoutes];

有numRoutes个插槽全部填充了默认值。对于参考数据类型，默认值为null，因此当您尝试访问第二个Route循环中的for个对象时，它们都是null，您首先需要初始化他们有点像这样：

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];

在创建对象数组的java中，所有插槽都使用默认值声明，如下所示 Objects = null 原语 int = 0 boolean = false

这些numRoutes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，您首先需要以某种方式初始化它们：

  // 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];

在Java中的类实例中初始化变量数组

3 个答案: