Graph.java

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


public class Graph<T> {
    private HashMap<T, Set<T>> adjacencyList;

    public Graph() {
        this.adjacencyList = new HashMap<>();
    }

    public void addVertex(T v) {
        if (this.adjacencyList.containsKey(v)) {
            throw new IllegalArgumentException("Vertex exists.");
        }

        this.adjacencyList.put(v, new HashSet<T>());
    }

    public void addEdge(T v, T u) {
        if (!(this.adjacencyList.containsKey(v) && this.adjacencyList.containsKey(u))) {
            throw new IllegalArgumentException();
        } else {
            this.adjacencyList.get(v).add(u);
            this.adjacencyList.get(u).add(v);
        }
    }

    public Iterable<T> getNeighbors(T v) {
        return this.adjacencyList.get(v);
    }

    public Iterable<T> getAllVertices() {
        return this.adjacencyList.keySet();
    }
}