Unable to add an element to an arraylist

Question

 public static ArrayList<ArrayList<Integer>> powerSet(ArrayList<Integer> originalList) {
 ArrayList<ArrayList<Integer>> sets = new ArrayList<>();
 ArrayList<Integer> list = new ArrayList<Integer>(originalList);
 int p=2;
 ArrayList<Integer> st=new ArrayList<>();
 while(p<=originalList.size())
 {
 for(int i=0;i<=originalList.size()-p;i++)
 {
 for(int j=0;j<p;j++)
 {
 st.add(list.get(i+j)); 
 }
 sets.add(st);// sets recieves a st that is empty which is not the case
 Iterator it=st.iterator();
 System.out.print("adding it to our sets ");
 while(it.hasNext())
 System.out.print(it.next()+" ");// prints the elements contained inside and proves that st is not empty
 System.out.println("");
 st.clear();
 }
 p+=1;
 }
 return sets;
 }

I have created above function to return the powerset of contiguous elements of a list and each set is of size greater than 2. But due to some unknown problem the list 'st' cannot be added to arraylist of arraylists 'sets' even though the arraylist 'st' prints out fine .

• UDKOX: What does cannot be added mean ?
• Vampire: And what // unable to print?
• vibhor vaish: i have made edits to the question for better clarity , please see the question again.

Answer 1

The problem is caused by the line sets.add(st); followed by calling 'st.clear()'. Since Java passes objects by reference, sets contains the exact object st. This means that any subsequent change to st will change the contents of sets. To avoid this problem, copy st to a new object before adding it to sets. Try replacing sets.add(st); with sets.add(new ArrayList<Integer>(st));

• vibhor vaish: That solved my problem, can we say this was like shadow cloning ?

Answer 2

Try this...

public static ArrayList<ArrayList<Integer>> powerSet(ArrayList<Integer> originalList) {
 ArrayList<ArrayList<Integer>> sets = new ArrayList<>();
 int p=2;
 while(p<=originalList.size()) {
 for(int i=0;i<=originalList.size()-p;i++) {
 ArrayList<Integer> st=new ArrayList<>();
 for(int j=0;j<p;j++) {
 st.add(list.get(i+j));
 } sets.add(st);
 Iterator it=st.iterator();
 System.out.print("adding it to our sets ");
 while(it.hasNext())
 System.out.print(it.next()+" ");
 System.out.println("");
 }
 p++;
 }
 return sets;
}

Answer 3

I would implement it this way:

public static ArrayList< ArrayList<Integer> > powerSet( ArrayList<Integer> originalList ) {
 ArrayList< ArrayList<Integer> > sets = new ArrayList<>();
 int p=2;
 while( p <= originalList.size() ){
 ArrayList<Integer> st; 
 for( int i=0 ; i<=originalList.size()-p ; i++ ){
 for( int j=0;j<p;j++ ){
 st = new ArrayList();
 st.add( originalList.get(i+j) ); 
 }
 sets.add(st);// sets recieves a st that is empty which is not the case
 Iterator it=st.iterator();
 System.out.print("adding it to our sets ");
 while( it.hasNext() )
 System.out.print(it.next()+" ");// prints the elements contained inside and proves that st is not empty
 System.out.println("");
 st.clear();
 }
 p+=1;
 }
 return sets;
}

• vibhor vaish: @JPichardo Your code won't serve the same goal as first of all: 1)each of my set will get over-written by a new value in the innermost for loop and I will get sets of only 1 length 2) it will ask me to initialize the 'st' as if in some case it doesn't go into the innermost for loop then I will get an error of an uninitialized set.