为什么在Java中使用数组获取NullPointerException？

Question

public static byte[] main_Mem = new byte[2048];
public static SlotNode[] cache = new SlotNode[8];

Doesn't this create instances of the objects? Why would I be getting a NPE?

 //initialize main memory
           for (int i = 0; i<main_Mem.length; i++) {
                   main_Mem[i] = (byte) (0xFF & i);
                   System.out.printf("%X", 0xFF & i);
                   System.out.print("      " + i);
                   System.out.println(" ");

           }

           //initialize cache slots to 0
           for (int i = 0; i<cache.length; i++) {
                   cache[i].setValidBit(0);
                   cache[i].setTag(0);
                   cache[i].setData(0);
                   cache[i].setDirty(0);
           }

Answer 1

No, allocating memory for the arrays is not the same thing as allocating objects.

The array cache has 8 references of type SlotNode, all set to null until you assign them to point to a SlotNode instance.

Answer 2

You need to init the SlotNode, such as this:

   for (int i = 0; i<cache.length; i++) {
           cache[i] = new SlotNode(...); // Add the constructor parameters as needed
           cache[i].setValidBit(0);
           cache[i].setTag(0);
           cache[i].setData(0);
           cache[i].setDirty(0);
   }

By doing this:

public static SlotNode[] cache = new SlotNode[8];

you just initialize the array of 8 SlotNode instances - you need to initialize each of them separately.

Answer 3

This:

new SlotNode[8]

Only creates an array of 8 SlotNode references. They're all initialized to null by default, so you have to initialize them yourself later. For instance, in the loop:

for (int i = 0; i<cache.length; i++) {
    cache[i] = new SlotNode(); // initialize the object
    cache[i].setValidBit(0);
    cache[i].setTag(0);
    cache[i].setDirty(0);
}