Create an ArrayList with empty objects using lambda

Question

I want to create an ArrayList of objects that have an id and an empty list using streams. I tried different ways but none seams to work. If someone can give me some hits will help me a lot. Here is the code that I want to convert to Java 8:

    this.registers = new ArrayList<Supplier>();
    for (int i = 0; i < this.numberOfSuppliers; i++) {
        Supplier supplier = new Supplier();
        supplier.setSupplierNumber(i);
        supplier.setMaterials(new ArrayList<Warehouse>());
        this.registers.add(supplier);
    }

Thank you in advance.

Unless this is just an exercise in Streams, don't convert to Streams. The for loop is good as it is. You will gain nothing by converting to Streams, not even clarity or simplicity of code. In my opinion, it'll actually be less clean/simple (see answer). — Andreas
– Andreas, Commented Mar 4, 2017 at 23:01

Donald Raab · Accepted Answer · 2017-03-05 03:54:06Z

You might consider adding a constructor to Supplier that takes the id as a parameter, and initializes the list of materials, so you don't have to expose a setter. Then the solution using either the loop or streams becomes much simpler.

public void buildSuppliersWithLoop()
{
    ArrayList<Supplier> registers = new ArrayList<>()
    int numberOfSuppliers = 100;
    for (int i = 0; i < numberOfSuppliers; i++)
    {
        registers.add(new Supplier(i));
    }
}

public void buildSuppliersWithStream()
{
    int numberOfSuppliers = 100;
    List<Supplier> registers = IntStream.range(0, numberOfSuppliers)
            .mapToObj(Supplier::new)
            .collect(Collectors.toList());
}

public class Supplier
{
    private int number;
    private List<Warehouse> materials;

    public Supplier(int number)
    {
        this.number = number;
        this.materials = new ArrayList<>();
    }
}

public class Warehouse
{
}

Boschi · Accepted Answer · 2017-03-04 22:54:21Z

2

It is not my preferred approach (I think the for loop is fine here), but in streams you can do this:

this.registers = IntStream.range(0, this.numberOfSuppliers)
         .map(i -> {
                       Supplier supplier = new Supplier();
                       supplier.setSupplierNumber(i);
                       supplier.setMaterials(new ArrayList<Warehouse>());
                       return supplier;
         })
         .collect(Collectors.toList());

answered Mar 4, 2017 at 22:54

Boschi

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3 Comments

Andreas Over a year ago

Caveat: That creates a List<Supplier>, not an ArrayList<Supplier>. If an ArrayList is required, replace Collectors.toList() with Collectors.toCollection(ArrayList<Supplier>::new).

Donald Raab Over a year ago

I believe you need to use mapToObj here, instead of map, as map returns an IntStream, not a Stream.

marius_neo Over a year ago

You can also extract the creation of the Supplier object to a different method to improve readability.

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