diff --git a/app/build.gradle.kts b/app/build.gradle.kts
index f93364c..4c6f921 100644
--- a/app/build.gradle.kts
+++ b/app/build.gradle.kts
@@ -28,7 +28,7 @@ dependencies {
 // Apply a specific Java toolchain to ease working on different environments.
 java {
     toolchain {
-        languageVersion.set(JavaLanguageVersion.of(11))
+        languageVersion.set(JavaLanguageVersion.of(21))
     }
 }
 
diff --git a/app/src/main/java/sudoku/Solver.java b/app/src/main/java/sudoku/Solver.java
index bfc1d7c..13631c9 100644
--- a/app/src/main/java/sudoku/Solver.java
+++ b/app/src/main/java/sudoku/Solver.java
@@ -1,5 +1,7 @@
 package sudoku;
 
+import java.util.stream.IntStream;
+
 public class Solver implements SudokuSolver {
     private int[][] board;
 
@@ -16,17 +18,200 @@ public class Solver implements SudokuSolver {
     }
 
     public boolean solve() {
+        return solve(0, 0);
     }
 
-    public Boolean isSolved() {
+    /**
+     * Resets the board to all zeros
+     */
+    public void reset() {
+        board = new int[9][9];
     }
 
+    /**
+     * Recursive helper method for solve()
+     * 
+     * @param row row to solve
+     * @param col column to solve
+     * @return true if solved
+     */
     private boolean solve(int row, int col) {
+        if (row < 0 || row > 9 || col < 0 || col > 9) {
+            return false;
+        }
+
+        if (row == 9) {
+            row = 0;
+            if (++col == 9) {
+                return true;
+            }
+        }
+
+        if (board[row][col] != 0) {
+            return solve(row + 1, col);
+        }
+
+        for (int val = 1; val <= 9; ++val) {
+            if (legal(row, col, val)) {
+                board[row][col] = val;
+                if (solve(row + 1, col)) {
+                    return true;
+                }
+            }
+        }
+
+        board[row][col] = 0;
+        return false;
     }
 
-    public boolean legal(int row, int col, int nbr) {
+    /**
+     * Randomizes the board. This guarantees a solvable board.
+     */
+    public void randomizeBoard() {
+        this.reset();
+        for (int i = 0; i < 9; ++i) {
+            int row = (int) (Math.random() * 9);
+            int col = (int) (Math.random() * 9);
+            int val = (int) (Math.random() * 9) + 1;
+            if (legal(row, col, val)) {
+                board[row][col] = val;
+            }
+        }
     }
 
+    /**
+     * Sets the value of the board at the given position
+     * 
+     * @param row row to set
+     * @param col column to set
+     * @param val value to set
+     */
+    public void setPos(int row, int col, int val) {
+        if (row < 9 && col < 9) {
+            board[row][col] = val;
+        }
+    }
+
+    /**
+     * Checks if val is legal in the given row, column, and 3x3 box
+     * 
+     * @param row row to check
+     * @param col column to check
+     * @param val value to check
+     * @return true if val is legal
+     */
+    public boolean legal(int row, int col, int val) {
+        if (row < 0 || row >= 9 || col < 0 || col >= 9 || val < 1 || val > 9) {
+            return false;
+        }
+
+        // Check if val is already in col
+        for (int i = 0; i < 9; ++i) {
+            if (val == board[i][col]) {
+                return false;
+            }
+        }
+
+        // Check if val is already in row
+        for (int j = 0; j < 9; ++j) {
+            if (val == board[row][j]) {
+                return false;
+            }
+        }
+
+        // Check the 3x3 box
+        int boxRowOffset = (row / 3) * 3;
+        int boxColOffset = (col / 3) * 3;
+
+        for (int k = 0; k < 3; ++k) {
+            for (int m = 0; m < 3; ++m) {
+                if (val == board[boxRowOffset + k][boxColOffset + m]) {
+                    return false;
+                }
+            }
+        }
+
+        // None of the above failed, so it is legal
+        return true;
+    }
+
+    public boolean isSolved() {
+        return isSolved(0, 0);
+    }
+
+    /**
+     * Recursive helper method for isSolved()
+     * 
+     * @param row
+     * @param col
+     * @return
+     */
+    private boolean isSolved(int row, int col) {
+        // If we are at the 9th row and 0th column (the last cell), we are done
+        if (row == 9) {
+            row = 0;
+            if (++col == 9) {
+                return true;
+            }
+        }
+
+        // If we find a zero, the board is not solved
+        if (board[row][col] == 0) {
+            return false;
+        }
+
+        for (int i = 0; i < 9; ++i) {
+            // Checks if there is a duplicate in the row
+            if (i != row && board[i][col] == board[row][col]) {
+                return false;
+            }
+
+            // Checks if there is a duplicate in the column
+            if (i != col && board[row][i] == board[row][col]) {
+                return false;
+            }
+        }
+
+        // Checks if there is a duplicate in the 3x3 box
+        int boxRowOffset = (row / 3) * 3;
+        int boxColOffset = (col / 3) * 3;
+
+        for (int k = 0; k < 3; ++k) {
+            for (int m = 0; m < 3; ++m) {
+                int r = boxRowOffset + k;
+                int c = boxColOffset + m;
+                if ((r != row || c != col) && board[r][c] == board[row][col]) {
+                    return false;
+                }
+            }
+        }
+
+        return true;
+    }
+
+    /**
+     * Returns a string representation of the board
+     * 
+     * @return String representation of the board
+     */
+    // Not particularly pretty, but it works
     public String toString() {
+        final String divider = "-------+--------+--------\n";
+        StringBuilder sb = new StringBuilder();
+        sb.append(divider);
+        int rowcount = 0;
+        for (int[] row : board) {
+            int colcount = 0;
+            sb.append("| ");
+            for (int val : row) {
+                colcount++;
+                sb.append(val);
+                sb.append(colcount % 3 == 0 ? " | " : " ");
+            }
+            rowcount++;
+            sb.append(rowcount % 3 == 0 ? "\n" : "");
+            sb.append(rowcount % 3 == 0 ? divider : "\n");
+        }
+        return sb.toString();
     }
 }
\ No newline at end of file
diff --git a/app/src/main/java/sudoku/SolverMain.java b/app/src/main/java/sudoku/SolverMain.java
index 76acdf9..7077608 100644
--- a/app/src/main/java/sudoku/SolverMain.java
+++ b/app/src/main/java/sudoku/SolverMain.java
@@ -2,6 +2,11 @@ package sudoku;
 
 public class SolverMain {
     public static void main(String[] args) {
-        System.out.println("Hello world!");
+        Solver s = new Solver();
+        System.out.println(s.toString());
+        s.randomizeBoard();
+        System.out.println(s.toString());
+        s.solve();
+        System.out.println(s.toString());
     }
 }
diff --git a/app/src/test/java/sudoku/SolverTest.java b/app/src/test/java/sudoku/SolverTest.java
index ae16b4b..618f20a 100644
--- a/app/src/test/java/sudoku/SolverTest.java
+++ b/app/src/test/java/sudoku/SolverTest.java
@@ -1,12 +1,86 @@
 package sudoku;
 
+import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 import static org.junit.jupiter.api.Assertions.*;
+import java.util.stream.IntStream;
 
 class SolverTest {
-    @Test void boardTest() {
+    @Test
+    void constructorTest() {
         Solver solver = new Solver();
-        solver.solve();
+        assertNotNull(solver);
+    }
+
+    @Test
+    void setBoardTest() {
+        Solver solver = new Solver();
+        int[][] board = new int[9][9];
+        solver.setBoard(board);
+        assertEquals(board, solver.getBoard());
+    }
+
+    @Test
+    void randomizeBoardTest() {
+        int[][] board = new int[9][9];
+        Solver solver = new Solver();
+        solver.randomizeBoard();
+        assertNotEquals(board, solver.getBoard());
+    }
+
+    @Test
+    void legalTest() {
+        Solver solver = new Solver();
+        assertTrue(solver.legal(0, 0, 1));
+        solver.setPos(0, 0, 1);
+
+        IntStream.range(0, 9).forEach(i -> {
+            assertFalse(solver.legal(0, i, 1));
+            assertFalse(solver.legal(i, 0, 1));
+        });
+
+        assertTrue(solver.legal(5, 5, 1));
+        assertTrue(solver.legal(8, 8, 9));
+        assertTrue(solver.legal(8, 8, 1));
+    }
+
+    @Test
+    void solverTest() {
+        Solver solver = new Solver();
+        assertFalse(solver.isSolved());
+        assertTrue(solver.solve());
         assertTrue(solver.isSolved());
+
+        solver.reset();
+        solver.randomizeBoard();
+        assertFalse(solver.isSolved());
+        assertTrue(solver.solve());
+    }
+
+    @Test
+    void resetTest() {
+        Solver solver = new Solver();
+        assertTrue(solver.solve());
+        solver.randomizeBoard();
+        solver.reset();
+        assertFalse(solver.isSolved());
+    }
+
+    @Test
+    void legalWithInvalidInputsTest() {
+        Solver solver = new Solver();
+        assertFalse(solver.legal(-1, 0, 1));
+        assertFalse(solver.legal(0, -1, 1));
+        assertFalse(solver.legal(0, 0, -1));
+        assertFalse(solver.legal(0, 0, 10));
+    }
+
+    @Test
+    @Disabled
+    void unsolvableTest() {
+        Solver solver = new Solver();
+        solver.setPos(0, 0, 1);
+        solver.setPos(0, 1, 1);
+        assertFalse(solver.solve());
     }
 }