how to make a Set to be empty

Question

I want to initialize my set to be empty (not null), can anyone help me? Here is my code

public class Graph {
private Set<Vertex> vertices;

public Graph() {
    vertices = {};
}

I know that this is a wrong way to do that, but couldn't do anything else

vertices = new HashSet();?? – PermGenError Feb 25 '13 at 13:40 — PermGenError, Feb 25 '13 at 13:40

score 4 · Answer 1 · edited May 23 '17 at 10:24

Set is an interface. You need to decide on the implementation required, and then construct using a no-args constructor. e.g.

vertices = new HashSet<Vertex>();

or

vertices = new TreeSet<Vertext>();

See this SO question/answer for more info on TreeSet vs HashSet. Given that Set is an interface, any number of implementations could exist (you could even write your own) but I suspect you'll want one of these two to begin with.

score 1 · Answer 2 · answered Feb 25 '13 at 13:40

You need to execute a constructor:

public Graph() {
  vertices = new HashSet<>();
}

Set is an interface definition, so you will need to pick a concrete implementation of Set. HashSet is one such implementation, but TreeSet is another (TreeSet is actually an implementation of a SortedSet).

score 1 · Answer 3 · answered Feb 25 '13 at 13:40

1

public Graph(){  
  vertices = new HashSet<Vertex>();  
}

or

public Graph(){  
  vertices = new TreeSet<Vertex>();  
}

answered Feb 25 '13 at 13:40

Cratylus

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or `new HashSet<>();` in Java 7 – Reimeus Feb 25 '13 at 13:41
@Reimeus:I am still "stuck" in Java 6 – Cratylus Feb 25 '13 at 13:41

score 1 · Answer 4 · answered Feb 25 '13 at 13:40

1

Replace

vertices = {};

With

vertices = new HashSet<Vertex>();

This will initialize an empty HashSet for your Set interface

answered Feb 25 '13 at 13:40

Achrome

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how to make a Set to be empty

4 Answers4