diff --git a/pom.xml b/pom.xml
index f38f7f7..5617dd0 100644
--- a/pom.xml
+++ b/pom.xml
@@ -13,4 +13,13 @@
         <maven.compiler.target>1.17</maven.compiler.target>
     </properties>
 
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <version>5.9.2</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
 </project>
\ No newline at end of file
diff --git a/src/main/java/net/berack/upo/ai/problem1/AStar.java b/src/main/java/net/berack/upo/ai/problem1/AStar.java
index 6dca0d5..5d49b9b 100644
--- a/src/main/java/net/berack/upo/ai/problem1/AStar.java
+++ b/src/main/java/net/berack/upo/ai/problem1/AStar.java
@@ -94,7 +94,7 @@ public class AStar<State, Action> {
 
         NodeState found = null;
         var list = new PriorityQueue<NodeState>();
-        list.add(new NodeState(null, initial, null, 0));
+        list.add(new NodeState(null, initial, null, 0, 0));
 
         while(list.size() > 0) {
             var current = list.poll();
@@ -106,19 +106,20 @@ public class AStar<State, Action> {
             for(var action : this.actions.apply(current.state)) try {
 
                 var next = this.transition.apply(current.state, action);
-
                 var cost = this.cost.apply(current.state, next, action);
                 var dist = this.heuristic.apply(next, goal);
 
-                list.add(new NodeState(current, next, action, current.cost + cost + dist));
+                list.add(new NodeState(current, next, action, current.cost + cost, dist));
 
-            } catch (Exception ignore) {}
+            } catch (Exception ignore) {
+                ignore.printStackTrace();
+            }
         }
 
         if(found == null) return null;
 
         var path = new ArrayList<Action>();
-        while(found != null) {
+        while(found != null && found.action != null) {
             path.add(found.action);
             found = found.parent;
         }
@@ -136,17 +137,19 @@ public class AStar<State, Action> {
         State state;
         Action action;
         int cost;
+        int total;
 
-        NodeState(NodeState parent, State state, Action action, int cost) {
+        NodeState(NodeState parent, State state, Action action, int cost, int heuristic) {
             this.parent = parent;
             this.state = state;
             this.action = action;
             this.cost = cost;
+            this.total = cost + heuristic;
         }
 
         @Override
         public int compareTo(NodeState other) {
-            return this.cost - other.cost;
+            return this.total - other.total;
         }
     }
 
diff --git a/src/main/java/net/berack/upo/ai/problem1/Puzzle8.java b/src/main/java/net/berack/upo/ai/problem1/Puzzle8.java
index 73f0a15..bed0cf0 100644
--- a/src/main/java/net/berack/upo/ai/problem1/Puzzle8.java
+++ b/src/main/java/net/berack/upo/ai/problem1/Puzzle8.java
@@ -16,6 +16,7 @@ import java.util.Random;
  */
 public class Puzzle8 implements Iterable<Integer> {
     public static final int LENGTH = 3;
+    public static final int[] DEFAULT_GOAL = new int[] {1, 2, 3, 4, 5, 6, 7, 8, 0};
 
     /**
      * Possibili movimenti della tessera "vuota"
@@ -34,18 +35,8 @@ public class Puzzle8 implements Iterable<Integer> {
      * Genera una nuova istanza del problema con le tessere posizionate in modo casuale.
      */
     public Puzzle8() {
-        var rand = new Random();
-        this.puzzle = new int[] {0, 1, 2, 3, 4, 5, 6, 7, 8};
-
-        for(var i = this.puzzle.length - 1; i > 0; i--) {
-            var j = rand.nextInt(i + 1);
-
-            var temp = this.puzzle[i];
-            this.puzzle[i] = this.puzzle[j];
-            this.puzzle[j] = temp;
-
-            if(this.puzzle[i] == 0) this.blank = i;
-        }
+        this.puzzle = Arrays.copyOf(DEFAULT_GOAL, DEFAULT_GOAL.length);
+        this.shuffle();
     }
 
     /**
@@ -58,7 +49,7 @@ public class Puzzle8 implements Iterable<Integer> {
      * @throws UnsupportedOperationException nel caso la mossa non sia disponibile.
      */
     public Puzzle8(Puzzle8 original, Move move) {
-        this.puzzle = Arrays.copyOf(original.puzzle, this.puzzle.length);
+        this.puzzle = Arrays.copyOf(original.puzzle, original.puzzle.length);
         this.blank = original.blank;
 
         this.move(move);
@@ -98,18 +89,6 @@ public class Puzzle8 implements Iterable<Integer> {
         }
     }
 
-
-    /**
-     * Indica la grandezza della tavoletta che contiene le tessere.
-     * Essa è una matrice 3x3.
-     * 
-     * @see #LENGTH
-     * @return la grandezza del lato della matrice
-     */
-    public int size() {
-        return LENGTH;
-    }
-
     /**
      * Permette di ricevere il valore della tessera posizionata nella zona richiesta.
      * La zona è indicata come una matrice 3x3 dove la prima zona si trova nelle coordinate
@@ -194,12 +173,83 @@ public class Puzzle8 implements Iterable<Integer> {
         this.blank = other;
     }
 
+    /**
+     * Questo metodo mette a caso le tessere del puzzle.
+     * É possibile che, una volta che le tessere sono state spostate a caso,
+     * non siano più raggiungibili alcuni stati.
+     * 
+     * @see #isSolvable()
+     * @see #isSolvable(Puzzle8)
+     */
+    public void shuffle() {
+        var rand = new Random();
+        for(var i = this.puzzle.length - 1; i > 0; i--) {
+            var j = rand.nextInt(i + 1);
+
+            var temp = this.puzzle[i];
+            this.puzzle[i] = this.puzzle[j];
+            this.puzzle[j] = temp;
+
+            if(this.puzzle[i] == 0) this.blank = i;
+        }
+    }
+
+    /**
+     * Indica se l'istanza del puzzle corrente sia risolvibile per il caso default.
+     * Con caso default si indica il puzzle composto da {@link #DEFAULT_GOAL}
+     * 
+     * @return true nel caso si possa risolvere, false altrimenti
+     */
+    public boolean isSolvable() {
+        return this.isSolvable(new Puzzle8(DEFAULT_GOAL));
+    }
+
+    /**
+     * Metodo per controllare la risolvibilità del puzzle corrente.
+     * 
+     * @param goal lo stato a cui si vuole arrivare
+     * @return true nel caso si possa risolvere, false altrimenti
+     */
+    public boolean isSolvable(Puzzle8 goal) {
+        var n = LENGTH * LENGTH;
+        var indexGoal = new int[n];
+
+        for(var i = 0; i < n; i++)
+            indexGoal[goal.puzzle[i]] = i;
+
+        var sum = 0;
+        for(var i = 0; i < n; i++) {
+            var curr = this.puzzle[i];
+            var iGoal = indexGoal[curr];
+
+            if(curr == 0) continue;
+
+            for(var j = n - 1; j > i ; j--) {
+                var val = this.puzzle[j];
+                if(val != 0 && indexGoal[val] < iGoal) sum += 1;
+            }
+        }
+
+        return (sum % 2) == 0;
+    }
+
+    /**
+     * Usa l'algoritmo A* per la soluzione del problema.
+     * Questo metodo cercherà di raggiungere lo stato goal indicato da {@link #DEFAULT_GOAL}
+     * @return una sequenza di mosse per poter risolvere il problema o null se non risolvibile
+     */
+    public List<Move> solve() {
+        return this.solve(new Puzzle8(DEFAULT_GOAL));
+    }
+
     /**
      * Usa l'algoritmo A* per la soluzione del problema
      * @param goal lo stato goal in cui arrivare
      * @return una sequenza di mosse per poter risolvere il problema o null se non risolvibile
      */
     public List<Move> solve(Puzzle8 goal) {
+        if(!this.isSolvable(goal)) return null;
+
         var aStar = new AStar<Puzzle8, Move>(Puzzle8::availableMoves, (p, m) -> new Puzzle8(p, m))
             .setHeuristic((p1, p2) -> {
                 var sum = 0;
@@ -219,6 +269,10 @@ public class Puzzle8 implements Iterable<Integer> {
         return aStar.solve(this, Objects.requireNonNull(goal));
     }
 
+    @Override
+    public String toString() {
+        return Arrays.toString(this.puzzle);
+    }
 
     @Override
     public boolean equals(Object obj) {
diff --git a/src/test/java/net/berack/upo/ai/problem1/TestPuzzle.java b/src/test/java/net/berack/upo/ai/problem1/TestPuzzle.java
new file mode 100644
index 0000000..f7ff407
--- /dev/null
+++ b/src/test/java/net/berack/upo/ai/problem1/TestPuzzle.java
@@ -0,0 +1,150 @@
+package net.berack.upo.ai.problem1;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static net.berack.upo.ai.problem1.Puzzle8.Move.*;
+
+import java.util.Arrays;
+
+import org.junit.jupiter.api.Test;
+
+public class TestPuzzle {
+
+    private int[] goalArray = new int[] {0,1,2,3,4,5,6,7,8};
+
+    @Test
+    public void testContructors() {
+        var puzzle = new Puzzle8(goalArray);
+
+        for(var i = 0; i < goalArray.length; i++)
+            assertEquals(goalArray[i], puzzle.get(i%3, i/3), "Error in initialization");
+
+        var puzzle2 = new Puzzle8(goalArray);
+        assertEquals(puzzle, puzzle2, "Error in equality");
+
+    }
+
+    @Test
+    public void testMoves() {
+        var puzzle = new Puzzle8(goalArray);
+
+        var moves = new Puzzle8.Move[] {RIGHT, RIGHT, DOWN, LEFT, LEFT, DOWN, RIGHT, RIGHT, UP};
+        var avail = new Puzzle8.Move[][] {
+            {DOWN, RIGHT},
+            {DOWN, LEFT, RIGHT},
+            {DOWN, LEFT},
+            {UP, DOWN, LEFT},
+            {UP, DOWN, LEFT, RIGHT},
+            {UP, DOWN, RIGHT},
+            {UP, RIGHT},
+            {UP, LEFT, RIGHT},
+            {UP, LEFT}
+        };
+        var after = new int[][] {
+            {1,0,2,3,4,5,6,7,8},
+            {1,2,0,3,4,5,6,7,8},
+            {1,2,5,3,4,0,6,7,8},
+            {1,2,5,3,0,4,6,7,8},
+            {1,2,5,0,3,4,6,7,8},
+            {1,2,5,6,3,4,0,7,8},
+            {1,2,5,6,3,4,7,0,8},
+            {1,2,5,6,3,4,7,8,0},
+            {1,2,5,6,3,0,7,8,4}
+        };
+
+        for(var i = 0; i < moves.length; i++) {
+            var valid = puzzle.availableMoves();
+
+            Arrays.sort(avail[i]);
+            Arrays.sort(valid);
+            assertArrayEquals(avail[i], valid, "Available moves not corrected at iteration " + i);
+
+            puzzle.move(moves[i]);
+            assertEquals(new Puzzle8(after[i]), puzzle, "Puzzle Equality not corrected at iteration " + i);
+        }
+    }
+
+    @Test
+    public void testMoveInit() {
+        var puzzle = new Puzzle8(goalArray);
+        var moves = new Puzzle8.Move[] {RIGHT, DOWN, LEFT, UP, DOWN, RIGHT, UP, LEFT};
+
+        for(var move : moves)
+            puzzle = new Puzzle8(puzzle, move);
+        assertEquals(new Puzzle8(goalArray), puzzle, "After useless moves the puzzle must be the same as before!");
+    }
+
+    @Test
+    public void testSolvable() {
+        var solvable = new Puzzle8(5,2,8,4,1,7,0,3,6);
+        var unsolvable = new Puzzle8(1,2,3,4,5,6,0,8,7);
+
+        assertTrue(solvable.isSolvable());
+        assertFalse(unsolvable.isSolvable());
+
+        var goal = new Puzzle8(0,1,2,3,4,5,6,7,8);
+
+        solvable = new Puzzle8(1,0,2,3,4,5,6,7,8);
+        unsolvable = new Puzzle8(2,1,0,3,4,5,6,7,8);
+
+        assertTrue(solvable.isSolvable(goal));
+        assertFalse(unsolvable.isSolvable(goal));
+    }
+
+    @Test
+    public void testSolveSimple() {
+        var goal = new Puzzle8(goalArray);
+
+        assertEquals(0, goal.solve(goal).size());
+
+        var puzzle = new Puzzle8(1,0,2,3,4,5,6,7,8);
+        var solution = new Puzzle8.Move[] {LEFT};
+        var actual = puzzle.solve(goal).toArray(new Puzzle8.Move[0]);
+
+        assertArrayEquals(solution, actual);
+
+        for(var move : actual) puzzle.move(move);
+        assertEquals(new Puzzle8(goalArray), puzzle);
+    }
+
+    @Test
+    public void testSolveSimple2() {
+        var puzzle = new Puzzle8(1,2,5,4,0,8,3,6,7);
+        var goal = new Puzzle8(goalArray);
+        var solution = new Puzzle8.Move[] {LEFT, DOWN, RIGHT, RIGHT, UP, UP, LEFT, LEFT};
+        var actual = puzzle.solve(goal).toArray(new Puzzle8.Move[0]);
+
+        assertArrayEquals(solution, actual);
+
+        for(var move : actual) puzzle.move(move);
+        assertEquals(new Puzzle8(goalArray), puzzle);
+    }
+
+    @Test
+    public void testSolve() {
+        var puzzle = new Puzzle8(3,5,6,1,2,4,0,7,8);
+        var puzzleGoal = new Puzzle8(1,2,3,4,5,6,7,8,0);
+        var solution = new Puzzle8.Move[] {RIGHT, UP, RIGHT, UP, LEFT, LEFT, DOWN, RIGHT, DOWN, RIGHT, UP, UP, LEFT, DOWN, RIGHT, DOWN};
+        var actual = puzzle.solve(puzzleGoal).toArray(new Puzzle8.Move[0]);
+
+        assertArrayEquals(solution, actual);
+
+        for(var move : actual) puzzle.move(move);
+        assertEquals(puzzleGoal, puzzle);
+    }
+
+    @Test
+    public void testSolveRand() {
+        var goal = new Puzzle8(Puzzle8.DEFAULT_GOAL);
+
+        var puzzle = new Puzzle8();
+        while(!puzzle.isSolvable(goal)) puzzle.shuffle();
+
+        var actions = puzzle.solve(goal);
+        for(var move : actions) puzzle.move(move);
+
+        assertEquals(puzzle, goal);
+    }
+}