My code :
static List<Object> data;
private void addItem(List<Object> list) {
try {
data = new ArrayList<Object>();
list.add("test");
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
ListTest test = new ListTest();
test.addItem(data);
}
Above code throws NullPointerException
. The code below does not throw NPE.
static List<Object> data = new Vector<Object>();
private void addItem(List<Object> list) {
try {
list.add("test");
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
ListTest test = new ListTest();
test.addItem(data);
}
Above code does not throw NullPointerException
. I don't understand the difference between both.
Even though you passed data
reference to the method, the moment you assign a new list to data
:
data = new ArrayList<Object>();
list
and data
reference are now pointing to 2 different objects. Before that assignment, data
was set to null
and so was list
. But after the assignment, only list
is set to null
. And thus calling list.add()
will result in NPE
.
In second case, data
was not null
to begin with.
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