当我尝试从对象数组调用方法时，我的 Java 代码崩溃

Question

I'm just learning Java and trying to have an array of a class. When a call a methed from the array it crashes. Works fine if it is not an array

public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  cDate test=new cDate();
  test.setDay(0);
  mAppoitments = new cDate[24];
  // crashes why?????
  mAppoitments[0].setDay(0); 

Answer 1

You haven't filled your array with objects. You have to:

  cDate[0] = test;

Otherwise you have null at index 0, and you cannot invoke anything on null .

And next time you ask a question, give all needed details:

• what is the exception message and stacktrace. "crashes" means almost nothing
• tell us what are your variables that are not initialized in the code snippet. You can see one answer that is telling you to fix a local declaration, which is probably an instance variable.

Answer 2

You have an array of 24 objects, each of which is set to null . You need to initialize each one before you can call methods on it.

Answer 3

You have initialized the array but not the objects in the array. Try initializing these elements before using them.

mAppoitments = new cDate[24];
for (int i = 0; i < mAppoitments.length; i++)
    mAppoitments[i] = new cDate();
mAppoitments[0].setDay(0);

Answer 4

final int COUNT= 24;

mAppoitments = new cDate[COUNT];
for(int i = 0 ; i < COUNT ; ++i) {
   mAppoitments[i] = new cDate();
   mAppoitments[i].setDay(0); 
}

Answer 5

cDate myAppointments = new cDate[24];

try declaring the variable type