Null pointer exception in arraycopy

Question

I am currently learning Java and am trying to work through learning generics. I have the class Holder which takes varargs. I am trying to place these into an array myArray for future use. However I get a NullPointerException on the arraycopy line and I can't seem to figure out why.

class Holder<T> {

  T[] myArray;

  public Holder(T... args) {
    System.arraycopy(args, 0, myArray, 0, args.length);  //null pointer exception
  }

This is what I am using to create the instance:

Holder<Integer> mySample = new Holder<>(1,2,3);

Add a breakpoint at this line and see which of (args or myArray is null). — Andrius Naruševičius
– Andrius Naruševičius, Commented Apr 30, 2013 at 6:08
Pro tip: You can clone arrays, so you could ditch that complicated arraycopy call and just say myArray = args.clone(); — gustafc
– gustafc, Commented Apr 30, 2013 at 6:12
myArray = args.clone() fixed it. Just curious how would I initialise myArray without getting a "generic array creation" error? — user2334659
– user2334659, Commented Apr 30, 2013 at 6:19

MadProgrammer · Accepted Answer · 2013-04-30 06:20:59Z

3

it is because myarray is not initialized.

T[] myArray; // `myarray` is uninitialized, as of now.

public Holder(T... args) {
    if(args != null && args.length > 0) {
        myArray = new t[args.length]; // `myarray` is initialized here.
        System.arraycopy(args, 0, myArray, 0, args.length);
    }
}

edited Apr 30, 2013 at 6:20

MadProgrammer

348k22 gold badges243 silver badges379 bronze badges

answered Apr 30, 2013 at 6:07

Ankit

6,6226 gold badges51 silver badges72 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

MadProgrammer Over a year ago

Make a null and 0 length check of args and you have yourself a nice answer ;)

Ankit Over a year ago

@MadProgrammer 0 length array shouldn't cause any problem

MadProgrammer Over a year ago

No, but why waste a call to arrayCopy :P

gustafc Over a year ago

@MadProgrammer why null check? This code should break if you pass it null instead of a proper array. If you let myArray be null you're just deferring the problem to a later occasion, which makes it harder to find the error.

MadProgrammer Over a year ago

Generally, checking for null is good practice. NullPointerException is an unchecked exception, meaning it won't be caught by normal try-catch blocks. Better to simply allow the myArray to remain null (which matches the requirements of the method) then crash the program - IMHO. Programming by exception is never a good idea ;)

|

Suresh Atta · Accepted Answer · 2013-04-30 06:07:46Z

0

 System.arraycopy(args, 0, myArray, 0, args.length);

1)myArray is not intialized .

2)and possibility of args to be null

answered Apr 30, 2013 at 6:07

Suresh Atta

122k38 gold badges206 silver badges315 bronze badges

Collectives™ on Stack Overflow

Null pointer exception in arraycopy

2 Answers 2

6 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related