basic graph implementation

Graph.java

import java.util.HashSet;
import java.util.Set;

public class Graph {

  public class Node {
		private final Set<Branch> branches = new HashSet<>();
		
		public final String name;
		
		public Node(String name) {
			this.name = name;
		}
		public Branch addBranchTo(Node to)
		{
			return new Branch(this, to);
		}
		public void addBranch(Branch b)
		{
			if (!equals(b.from) && !equals(b.to))
				throw new IllegalArgumentException();
			branches.add(b);
		}
	}
	
	public class Branch {
		public final Node from;
		public final Node to;
		
		public Branch(Node from, Node to) {
			this.from = from;
			this.to = to;
			to.addBranch(this);
			from.addBranch(this);
		}
	}
}