索引越界异常

Question

I am working on a minecraft mod. The id system for EntityC4 is not working. The code is

public static List C4 = new ArrayList();  

public static EntityC4 getitemfromnumber(int num)
{
    EntityC4 entity = (EntityC4)C4.get(num);

    return entity;
}
public static void createdetonater(EntityC4 c4, int num)
{
    C4.add(num, c4);


}
public static int getnum(){
     int num = 0;
      for(boolean a = true; a != false;){
          EntityC4 c = (EntityC4)C4.get(num);
          System.out.print("current num : " + num);

            if(c != null)
            {
                a = false;
            }else{
                System.out.print("entity " + num + " is null" );
         num++;
            }
      }
      return num;
   }

And when I use getnum() an error shows up that says

java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
    at java.util.ArrayList.RangeCheck(Unknown Source)
    at java.util.ArrayList.get(Unknown Source)

EDIT:Ty and the loop was supposed the first non-existent space in the list.

Answer 1

You create a new ArrayList C4 , you never put anything in the List , and then you ask it for the element at index 0 . This element does not exists since the List is still empty, and it throws an exception.

You can avoid this exception by adjusting your method

public static EntityC4 getitemfromnumber(int num)
{
  EntityC4 entity = null;
  if ( num >=0 && num < C4.size ){
    entity = (EntityC4)C4.get(num);
  }
  return entity;
}

This will avoid the exception, but you will get stuck in your loop since you never find the element (which isn't available in the list)

Answer 2

C4 is obviously empty if you just call getNum() , thus the exception saying that the index you are accessing doesn't exist.

You might want to pre-populate the list by calling createdetonater(..) ?

Answer 3

Your error is coming from

EntityC4 c = (EntityC4)C4.get(num);

which states your C4 is empty, you would like to do a not null and notEmpty check on your list.

In order to check the reason why C4 is empty do a trace on createdetonater and see where its being called.

Answer 4

Your error message tells you everything: you're trying to get the zeroth element from a list that has no elements. You probably need to add something to C4 ,

Answer 5

If you rewrite your loop to use a counter then it will avoid this error. I am not sure exactly what your code is trying to do though (return first non-null instance?), so the loop below may be incorrect.

If you are trying to store a map of ID to entity, then you should probably use a HashMap<Integer,EntityC4> instead.

for(int num = 0; num < C4.size(); num++)
{
    EntityC4 c = (EntityC4)C4.get(num);
    System.out.print("current num : " + num);

    if(c != null)
    {
        return num;
    } else
    {
        System.out.print("entity " + num + " is null" );
    }
}
// throw an exception or whatever else your code should do here...