Java Printing ArrayList of ArrayList

Question

I am trying to print my arraylist but i don't know why my printing does not print line by line of IntegerPair in each index of Adjlist:

 private ArrayList<ArrayList<IntegerPair>> AdjList; //outer arraylist 
 private ArrayList<IntegerPair> storeNeighbour; //inner arraylist
 private IntegerPair pair;

This is my snippet:

for (ArrayList<IntegerPair> l1 : AdjList) {
  for (IntegerPair n : l1) {
    System.out.println( n + "# ");
  }     
}

Answer 1

Here :

for (ArrayList<IntegerPair> l1 : AdjList) {
    for (IntegerPair n : l1) {
        System.out.println( n + "# ");
    }       
}

You don't differentiate each printed List.
As a result, you will have a series of output without knowing those associated to a same list.

A more readable print would be :

for (ArrayList<IntegerPair> l1 : AdjList) {
    System.out.println("ArrayList with :");
    for (IntegerPair n : l1) {
        System.out.println( n + "# ");
    }       
}

You don't specify your output. So I don't suppose toString() is or not overridden. If it is not overridden you should either override it to render the String expected here : System.out.println( n + "# ");, or you should specify the content to render here :

System.out.println( n.getOne() + "," + n.getOther()  + "# ");

As a side note, toString() is designed for debugging/logging, not for displaying functional messages as an object could be rendered in a way for a case and in another way for other cases.

Answer 2

The default behavior of ArrayList.toString() is to return a single string containing a (somewhat) beautified list of calls to toString() on each element in the list.

So, long story short: you are almost there; the one thing that is missing:

@Override 
public String toString() {
...

within your class IntegerPair.

Like:

public class IntegerPair {
  private final Integer first;
  private final Integer second;
  ...
  @Override 
  public String toString() { 
    return "(" + first + "/" + second ")";
  }

or something alike. Without overriding toString() your class will fall back on the default implementation given in java.lang.Object; and that method returns class name + hashcode number (and is thus not so human-readable).