Multidimentional Arraylist (Index error)

Question

I want to use multidimentional arraylist for my work. Please help me to solve this problem. My code is:

public class newType {
  String name;
  int id;

  public newType() {
    name="";
    id=0;
  }

  public newType(String name,int id) {
    this.name=name;
    this.id=id;
  }
}

public class MainClass {
  public static void main (String[] args) throws Exception {    

   HashMap<String, ArrayList<ArrayList<newType>>> temp=new HashMap<String,   ArrayList<ArrayList<newType>>>();

    ArrayList<ArrayList<newType>> arrObj=new ArrayList<ArrayList<newType>>();

    arrObj.clear();
    arrObj.add(new ArrayList<ValueType>());
    arrObj.add(new ArrayList<ValueType>());
    arrObj.add(new ArrayList<ValueType>());
    arrObj.get(0).add(new newType("Jhon",1));
    arrObj.get(1).add(new newType("get",1));
    arrObj.get(1).add(new newType("book",1));
    arrObj.get(2).add(new newType("coma",1));
    arrObj.get(2).add(new newType("brother",1));
    temp.put("Jhon", arrObj);

    arrObj.clear();
    arrObj.add(new ArrayList<newType>());
    arrObj.add(new ArrayList<newType>());
    arrObj.add(new ArrayList<newType>());
    arrObj.get(0).add(new newType("Mikel",1));
    arrObj.get(0).add(new newType("son",,2));
    arrObj.get(1).add(new newType("puts",2));
    arrObj.get(1).add(new newType("his",1));
    arrObj.get(2).add(new newType("when",1));
    arrObj.get(2).add(new newType("hich",1));
    temp.put("Mikel", arrObj);

    arrObj.clear();
    arrObj=temp.get("Mikel");
    System.out.println(arrObj.get(0).get(0).name);
  }
 }

When I run this program, compiler give me following error:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0 at java.util.ArrayList.RangeCheck(Unknown Source) at java.util.ArrayList.get(Unknown Source) at MainClass.main(MainClass.java:47)

The error is related to arrObj.get(0).get(0).name

How can I solve it?

@RomanC I am not sure what you mean. Isn't it same rule as with arrays? Two-dimensional array is just one dimensional array of other one dimensional arrays. Same rule can be applied to Lists (OP is using one ArrayList to store other ArrayLists). — Pshemo
– Pshemo, Commented Jul 14, 2014 at 19:25
@Pshemo The list is not an array and name things their own names. — Roman C
– Roman C, Commented Jul 14, 2014 at 19:28

rgettman · Accepted Answer · 2014-07-14 19:21:40Z

4

When you put to your HashMap, it doesn't add a copy of your 2D ArrayList, it adds the reference to it. But you are clearing the 2D ArrayList each time you add it, so you wind up with 2 references to the same empty 2D ArrayList by the time you query its contents.

Create a new 2D ArrayList each time, so you can add separate objects each time. Instead of

arrObj.clear();

Try

arrObj = new ArrayList<ArrayList<newType>>();

(There's no need to clear() the last time.)

answered Jul 14, 2014 at 19:21

rgettman

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