Using Stream to generate a list of POJOs... (just playing a bit)

[Claude Moore]

Need your help / advices.

I want to generate a list of POJOs using Stream. So, I defined a simple POJO :

public class MyPOJO {
	private int value;
	
	public MyPOJO(int value) {
		this.value = value;
	}
	
	public String toString() {
		return "POJO = "+this.value;
	}
}

and an helper class, Counter:

class Counter {

   private int x = 0;
   
    public int next() {
	  return ++x;
    }
}

the most concise way I found to generate a List of MyPOJO is the following:

Counter cnt = new Counter();
List<MyPOJO> list = Stream.<MyPOJO>generate(() -> new MyPOJO(cnt.next())).limit(100).collect(Collectors.toList());

It works. But how would judge it ? I'd say it's at least... convoluted.
Is there any idiomatic way to do the same ?

[Les Morgan]

Claude,

If I am correct in my understanding, then you want to know how many PoJo's there are (objects that have been created).  This seems like an excellent place to take advantage of a Class Variable.

public class MyPoJo{
 private static int iPoJo = 0;
  public MyPoJo(){
    iPoJo++;
  }
  public int count(){
    return iPoJo;
}

With each "new MYPoJo()", the creation of an object of type MyPoJo, then the counter gets incremented by 1.  This Class Varaible, iPoJo , is referenced in each instance of a MyPoJo object.

Les

[Claude Moore]

Well, not exactly.I used a constructor for MyPojo class taking an int as parameter just for simplicity, and to not be able to use default constructor using MyPOJO::new syntax in my stream expression.The goal wasn't to count created instances.

[Jeanne Boyarsky]

Claude,
Good question. Have a cow.

First Stream.<MyPOJO> is redundant. The compiler can infer the type so you don't need the generic.

Beyond that, I'd separate the two calls. Generate makes a list of numbers and mapToObject actually lets you create the POJO. I also added indentation to make it easier to see what it going on.

List<MyPOJO> list = IntStream.generate(() -> cnt.next())
	.mapToObj(MyPOJO::new)
	.limit(100)
	.collect(Collectors.toList());

[Claude Moore]

Jeanne,
first, thanks for the cow !  Second, your solution is very smart : I studied it a bit , and found another approach using iterate() method to generate the IntStream:

List<MyPOJO> list = IntStream.iterate(0,n -> ++n)
			    .mapToObj(x -> new MyPOJO(x))
			    .limit(100)
			    .collect(Collectors.toList());
		list.stream().forEach(System.out::println);

This way, I don't need my helper Counter class anymore.  Despite this, your solution it's simply great, thanks for sharing it !

[Jeanne Boyarsky]

Good idea getting rid of the counter! I wasn't sure if it was used laster which is why I didn't propose getting rid of it.

You can use a method reference for this to make it a tiny bit shorter.
.mapToObj(x -> new MyPOJO(x))

[Campbell Ritchie]

What's more, a counter like that counts hw many objects have been created, but there is no straightforward way to count how many objects have lost all their references and have become unreachable and eligible for garbage collection. Even if finalize() weren't deprecated, it wouldn't help much.

[Stephan van Hulst]

IntStream.range(0, 100).mapToObj(MyPojo::new)

[Jeanne Boyarsky]

Doh! Excellent point Stephen!

[Claude Moore]

Stephan van Hulst wrote:

IntStream.range(0, 100).mapToObj(MyPojo::new)

The only "problem" with this approach is that MyPojo doesn't have a default constructor.But I like your solution very much!!

[Stephan van Hulst]

You don't need a parameterless constructor. The class has a constructor that consumes an int, which counts as an IntFunction, and can be referrenced by MyPojo::new.

In fact, if your class only had a parameterless constructor, my approach wouldn't have worked.

[Claude Moore]

Stephan van Hulst wrote:You don't need a parameterless constructor. The class has a constructor that consumes an int, which counts as an IntFunction, and can be referrenced by MyPojo::new.

In fact, if your class only had a parameterless constructor, my approach wouldn't have worked.

You're right, Stephan !! Shame on me. I didn't realize that MyPOJO(int x) would have been called.