added a reasonable coverage

src/main/java/ll/Attempt2.java

@@ -4,6 +4,7 @@ import java.io.File;
 import java.io.FilenameFilter;
 import java.io.IOException;
 import java.util.AbstractMap;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.Comparator;
@@ -60,7 +61,8 @@ public abstract class Attempt2<R> extends AbstractMap<Integer,R> implements Sort
     private void loadIdOnDisk() {
         String[] buildDirs = dir.list(createDirectoryFilter());
         if (buildDirs==null)    buildDirs=new String[0];
-        idOnDisk = new SortedStringList(Arrays.asList(buildDirs));
+        // wrap into ArrayList to enable mutation
+        idOnDisk = new SortedStringList(new ArrayList<String>(Arrays.asList(buildDirs)));
     }
 
     public Comparator<? super Integer> comparator() {

src/main/java/ll/SortedStringList.java

@@ -36,6 +36,16 @@ public class SortedStringList extends AbstractList<String> {
         return data.size();
     }
 
+    @Override
+    public String remove(int index) {
+        return data.remove(index);
+    }
+
+    @Override
+    public boolean remove(Object o) {
+        return data.remove(o);
+    }
+
     /**
      * Finds the index of the entry lower than v.
      */

src/test/java/ll/Attempt2Test.java

@@ -6,20 +6,26 @@ import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
 
+import java.io.IOException;
+import java.util.Map;
 import java.util.NoSuchElementException;
 
 /**
  * @author Kohsuke Kawaguchi
  */
 public class Attempt2Test extends Assert {
+    // A=1, B=3, C=5
     @Rule
     public FakeMapBuilder aBuilder = new FakeMapBuilder();
     private FakeMap a;
 
+    // empty map
     @Rule
     public FakeMapBuilder bBuilder = new FakeMapBuilder();
     private FakeMap b;
 
+    @Rule
+    public FakeMapBuilder localBuilder = new FakeMapBuilder();
 
     @Before
     public void setUp() throws Exception {
@@ -47,6 +53,13 @@ public class Attempt2Test extends Assert {
             a.get(1).asserts(1,"A");
     }
 
+    @Test
+    public void lookupWithBogusKeyType() {
+        assertNull(a.get(null));
+        assertNull(a.get("foo"));
+        assertNull(a.get(this));
+    }
+
     @Test
     public void firstKey() {
         assertEquals(1, a.firstKey().intValue());
@@ -76,4 +89,24 @@ public class Attempt2Test extends Assert {
         assertNull(a.search( 99, Direction.ASC));
         assertNull(a.search(-99, Direction.DESC));
     }
+
+    /**
+     * If load fails, search needs to gracefully handle it
+     */
+    @Test
+    public void unloadableData() throws IOException {
+        FakeMap m = localBuilder.add(1, "A").addUnloadable("B").add(5, "C").make();
+
+        assertNull(m.search(3,Direction.EXACT));
+        m.search(3,Direction.DESC).asserts(1,"A");
+        m.search(3,Direction.ASC ).asserts(5,"C");
+    }
+
+    @Test
+    public void eagerLoading() throws IOException {
+        Map.Entry[] b = a.entrySet().toArray(new Map.Entry[3]);
+        ((Build)b[0].getValue()).asserts(1,"A");
+        ((Build)b[1].getValue()).asserts(3,"B");
+        ((Build)b[2].getValue()).asserts(5,"C");
+    }
 }

src/test/java/ll/FakeMapBuilder.java

@@ -31,6 +31,12 @@ public class FakeMapBuilder implements TestRule {
         return this;
     }
 
+    public FakeMapBuilder addUnloadable(String id) throws IOException {
+        File build = new File(dir,id);
+        build.mkdir();
+        return this;
+    }
+
     public FakeMap make() {
         return new FakeMap(dir);
     }