toArray() vs. toArray(new Object[0])

Question

I have an ArrayList<Clause> called listtable. For some reason Clause[] whatever = listtable.toArray() gives an incompatible types error but Clause[] whatever = listtable.toArray(new Clause[0]) works just fine. Why is this so? What is the difference between the two calls? The javadoc says they are "identical in function".

Here is my full code (the relevant statement is right before the end):

public static Clause[] readCNF(String name,Boolean print) throws IOException 
    {
        BufferedReader file = new BufferedReader(new FileReader("./" + name));

        ArrayList<Clause> listtable = new ArrayList<Clause>();
        String line = null; 
        while ((line = file.readLine()) != null) {

            if(line.charAt(0) == 'p')
            {
                 Scanner scanner = new Scanner(line); 
                 scanner.next(); scanner.next(); Clause.NumVars = scanner.nextInt(); Clause.NumClauses = scanner.nextInt(); 

            } else if(line.charAt(0) != 'c') {  

                 ArrayList<Integer> lits = new ArrayList<Integer>();
                 Scanner scanner = new Scanner(line);

                 while(scanner.hasNext())
                 {
                     int var = scanner.nextInt();
                     if(var != 0){ lits.add(var);}
                 }

                 listtable.add(new Clause(lits));

            } 
        }

        if(print) {
            for(Clause clause : listtable)
            {
                clause.print();
            }
        }

       return(listtable.toArray(new Clause[0])); //since the return type is Clause[] this is the same as the statements in the question
    }

Continue reading the javadoc: _the runtime type of the returned array is that of the specified array._ — Sotirios Delimanolis, Oct 27 '14 at 00:16
Still, why would they have a toArray() method if it doesn't work? — Elliot Gorokhovsky, Oct 27 '14 at 00:17
What do you mean _it doesn't work_? Did you put a cast? Is that a safe cast? What is the return type specified by the javadoc? It works very well and as intended. — Sotirios Delimanolis, Oct 27 '14 at 00:19
I mean it doesn't compile without the new Object[0] inside the parentheses — Elliot Gorokhovsky, Oct 27 '14 at 00:20
The javadoc very clearly states that the return type of the method is `Object[]`. So assign it to an `Object[]`. — Sotirios Delimanolis, Oct 27 '14 at 00:21

score 3 · Accepted Answer · answered Oct 27 '14 at 08:18

toArray() returns an array of Object. You have to cast every element of the array to your desired type.

The toArray(T[]) accepts a generic type and returns an array of the specific type. No need to cast the return value and/or the elements of the array.

As stated in the comments above, the toArray() method came pre-generics.

    List<String> list = new ArrayList<String>();
    list.add("Alice");
    list.add("Bob");

    String[] strArray = list.toArray(new String[0]);
    for (String str : strArray) {
        System.out.println(str);
    }

    Object[] objArray = list.toArray();
    for (Object obj : objArray) {
        String str = (String) obj;
        System.out.println(str);
    }

toArray() vs. toArray(new Object[0])

1 Answers1