Stack in Constructor

Question

Card karte;
Stack <Card> kartenstapel;

 public Standard52() {

     /** this works*/
     this.karte = new Card(Suit.HEARTS,13);                       

     /** this throws a 0-pointer exception when I try to initial*/

     this.kartenstapel.addElement(new Card(Suit.HEARTS, 13));
     this.kartenstapel.addElement(new Card(Suit.HEARTS, 13));
     this.kartenstapel.addElement(new Card(Suit.HEARTS, 13));
     this.kartenstapel.addElement(new Card(Suit.HEARTS, 13));
 }

Why i get a null pointer exception when i try to inital it. Its more theorie then practice but i do not undersstand my failure maybe some of you can explain me where my mistake is.

Does this answer your question? [What is a NullPointerException, and how do I fix it?](https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — leopal, Jan 03 '20 at 11:14
You have not initialized `kartenstapel` which is why it is throwing an exception. Initialize it by `Stack kartenstapel = new Stack<>();` and then proceed to run the code. It should work just fine. — Mohit Dodhia, Jan 03 '20 at 11:16

score 3 · Answer 1 · answered Jan 03 '20 at 11:13

3

You need to initialize the Stack as well. So, it would be

public Standard52() {
    this.kartenstapel = new Stack<>();
    // now add elements
 }

answered Jan 03 '20 at 11:13

Richard Robinson

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score 1 · Answer 2 · answered Jan 03 '20 at 11:14

You have to initialize the Stack collection:

Card karte;
Stack <Card> kartenstapel = new Stack<>(); // initialization to an empty stack

public Standard52() {

     /** this works*/
     this.karte = new Card(Suit.HEARTS,13);                       

     /** this should work as the kartenstapel is initialized */

     this.kartenstapel.addElement(new Card(Suit.HEARTS, 13));
     this.kartenstapel.addElement(new Card(Suit.HEARTS, 13));
     this.kartenstapel.addElement(new Card(Suit.HEARTS, 13));
     this.kartenstapel.addElement(new Card(Suit.HEARTS, 13));
 }

score 0 · Answer 3 · answered Jan 03 '20 at 11:15

0

You need to initialize kartenstapel.

Stack <Card> kartenstapel = new Stack<>();

answered Jan 03 '20 at 11:15

Vikas

6,868
4
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Stack in Constructor

3 Answers3