1) Implement a test case for a class that manipulates graphs:

package graph;
import junit.framework.TestCase;
public class TestGraph extends TestCase {
  public static void main(String[] args) {
    junit.textui.TestRunner.main(new String[] { "graph.TestGraph" });
  }
  public TestGraph(final String name) {
    super(name);
  }
  public final void testAddNode() throws Exception {
    Graph<String> g = new GraphImpl<String>();
    g.add("abelha");
    assertEquals(g.contains("abelha"), true);
  }
  public final void testAddLink() throws Exception {
    Graph<String> g = new GraphImpl<String>();
    g.add("abelha");
    g.add("vespa");
    g.link("abelha", "vespa");
    assertEquals(g.contains("abelha"), true);
    assertEquals(g.contains("vespa"), true);
    assertEquals(g.neighbors("abelha", "vespa"), true);
    assertEquals(g.neighbors("vespa", "abelha"), true);
  }
}

2) What is junit?
2.1) How does it work?

3) What about the class that passes the tests?
package graph;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
public class GraphImpl<T> implements Graph<T> {
  private Map<T, List<T>> nodes;
  public GraphImpl() {
    nodes = new HashMap<T, List<T>>();
  }
  public void add(T e) {
    if (!nodes.containsKey(e)) {
      nodes.put(e, new LinkedList<T>());
    }
  }
  public boolean contains(T e) {
    return nodes.containsKey(e);
  }
  public void link(T e1, T e2) {
    List<T> l1 = nodes.get(e1);
    l1.add(e2);
    List<T> l2 = nodes.get(e2);
    l2.add(e1);
  }
  public boolean neighbors(T e1, T e2) {
    List<T> l1 = nodes.get(e1);
    List<T> l2 = nodes.get(e2);
    return l1.contains(e2) && l2.contains(e1);
  }
}