使用Lambda创建带有空对象的ArrayList

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.

Answer 1

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
{
}

Answer 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());