问题
Im a little confused on adding objects to an array of 7 objects.
I have a for loop and I want to add an item object with 3 arguments. I have used set and get for this. At the end of the for loop, id like to .add item object to the array list. When I try to do this I get an error:
Exception in thread "main" java.lang.NullPointerException at item.add(item.java:88) at homework3main.main(homework3main.java:38)
There are no errors flags on the lines 88 in item class and 38 in main, so I dont know how to correct.
public class item {
public static int x = 0;
public static String setName;
public static double setPrice;
public static int setPrioirty;
public int priority=-1;
public static double price;
public static String name;
private static item[] list;
item(){
this(-1,0, null);
priority=-1;
price=0;
name="No Name yet.";
}// default constructor.
public item(int i, double j, String k) {
setitem(i,j,k);//constructor with 3 arguments.
}
public void setitem (int i, double j, String k){// setting item with 3 attributes.
setPriority(i);
setPrice(j);
setName(k);
}
public void setName(String k) {//setting individual attributes in item.
// TODO Auto-generated method stub //page 378
name=k;
priority=-1;
price=0;
}
public void setPrice(double j) {//setting individual attributes in item.
// TODO Auto-generated method stub
if (j<0&&j>100){
System.out.println("Error: price is too low or high");
}
else
price=j;
}
public void setPriority(int i) {//setting individual attributes in item.
// TODO Auto-generated method stub
priority =((i>=0&&i<7)?i:0);
}
public double getPrice(){
return price;
}
public String getName(){
return name;
}
public double getPriority(){
return priority;
}
// eclipse made me create this method when I wanted to "add an itemObject to the list"
public static void add(item itemObject) {
System.out.println("Enter an item"+x);
if (x<7)
{
list[x]=itemObject;
System.out.println("Item added at index " + x);
x++;
}
}
}
---------------------------------main
public class homework3main extends item {
public static void main(String[] args) {
item list[]=new item [7]; // array object
Scanner keyboard= new Scanner(System.in);
for(int x=0; x<list.length;x++){
item itemObject=new item (setPrioirty,setPrice,setName);
//creating new object with 3 variables, name, price, priority
//list[x]=new item();// is this right?
System.out.println("Enter an item you want to add to your list");
list[x].setName=keyboard.next();
System.out.println("Enter a price");
Scanner pricedouble= new Scanner(System.in);
list[x].setPrice=pricedouble.nextDouble();
System.out.println("Enter the priority of the item");
Scanner priorityint= new Scanner(System.in);
list[x].setPrioirty=priorityint.nextInt();
//item itemObject=new item (setPrioirty,setPrice,setName);
list[x].add(itemObject);
}
回答1:
The main method is the wrong place to initialize an item's array. Here,
item list[]=new item [7];
you have declared a local array that is itself called list. Instead, initialize the list instance variable of item in its constructor or as part of the declaration.
Additionally, I don't see any reason why your class with the main method, homework3main, should extend item. Plus, the naming convention in Java is to name classes starting with an uppercase character, e.g. Item and Homework3Main.
来源：https://stackoverflow.com/questions/19232472/adding-an-object-to-an-array-in-for-loop-with-set-and-get