将元素添加到数组列表时，未知的空指针异常

Question

What I am trying to do is add an element to an ArrayList each time a method is called.

public class Dice
{
private static int x3;
private static ArrayList<Integer> totals;

public Dice()
{
totals = new ArrayList<Integer>();
}

public static void Roll()
{
int x1 = (int)(Math.random()*6)+1;
int x2 = (int)(Math.random()*6)+1;
x3 = x1 + x2;
totals.add(x3);
}
}

Every time the Roll() method is called, I receive a Null Pointer Error for "totals.add(x3);"

Any ideas?

Answer 1

you have totals as static field and you are initializing it in constructor which is called when an instance is created, you need to put it in static initializer block

static {
    totals = new ArrayList<Integer>();
}

inside class body, and when you refer the fields from other class you need to specify class name because it is static fields, or make them non static and access them by creating instance

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• Understanding Class Members
• What is a NullPointerException, and how do I fix it?

Answer 2

You should make up your choice: either Roll() is static method or not. If it is static (as your code suggests) then you need to make sure totals is initialized when you call Dice.Roll() .

class Dice
{
private static int x3;
private static ArrayList<Integer> totals=new ArrayList<Integer>();


public static void Roll()
{
int x1 = (int)(Math.random()*6)+1;
int x2 = (int)(Math.random()*6)+1;
x3 = x1 + x2;
totals.add(x3);
}
}

Answer 3

if (totals != null) {
    totals.add(x3);
} else {
    totals = new ArrayList<Integer>();
    totals.add(x3);
}