diff --git a/algorithm-exercises-java/src/main/java/ae/hackerrank/DiagonalDifference.java b/algorithm-exercises-java/src/main/java/ae/hackerrank/DiagonalDifference.java
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+++ b/algorithm-exercises-java/src/main/java/ae/hackerrank/DiagonalDifference.java
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+package ae.hackerrank;
+
+import java.util.List;
+
+/**
+ * Diagonal Difference.
+ *
+ * @link Problem definition [[docs/hackerrank/warmup/diagonal_difference.md]]
+ */
+public class DiagonalDifference {
+
+  private DiagonalDifference() {}
+
+  static java.util.logging.Logger logger = util.CustomLogger.getLogger();
+
+  /**
+   * diagonalDifference.
+   */
+  public static int diagonalDifference(List<List<Integer>> matrix) {
+    int diag1 = 0;
+    int diag2 = 0;
+    int last = matrix.size() - 1;
+
+    for (int i = 0; i < matrix.size(); i++) {
+      diag1 += matrix.get(i).get(i);
+      diag2 += matrix.get(last - i).get(i);
+    }
+
+    return Math.abs(diag1 - diag2);
+  }
+}
diff --git a/algorithm-exercises-java/src/test/java/ae/hackerrank/DiagonalDifferenceTest.java b/algorithm-exercises-java/src/test/java/ae/hackerrank/DiagonalDifferenceTest.java
new file mode 100644
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+++ b/algorithm-exercises-java/src/test/java/ae/hackerrank/DiagonalDifferenceTest.java
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+package ae.hackerrank;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.Arrays;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+
+class DiagonalDifferenceTest {
+
+  @Test void testDiagonalDifference() {
+
+    List<List<Integer>> matrix = Arrays.asList(
+        Arrays.asList(11, 2, 4),
+        Arrays.asList(4, 5, 6),
+        Arrays.asList(10, 8, -12)
+    );
+    Integer expected = 15;
+    Integer resultFound = DiagonalDifference.diagonalDifference(matrix);
+
+    assertEquals(expected, resultFound,
+        String.format("DiagonalDifference.diagonalDifference answer must be: %d", expected)
+    );
+  }
+}
diff --git a/docs/hackerrank/warmup/diagonal_difference.md b/docs/hackerrank/warmup/diagonal_difference.md
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+++ b/docs/hackerrank/warmup/diagonal_difference.md
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+# [Diagonal Difference](https://www.hackerrank.com/challenges/diagonal-difference)
+
+Difficulty: #easy
+Category: #warmup
+
+Given a square matrix, calculate the absolute difference between the sums
+of its diagonals.
+For example, the square matrix $ arr $ is shown below:
+
+```text
+1 2 3
+4 5 6
+9 8 9
+```
+
+The left-to-right $ diagonal = 1 + 5 + 9 = 15 $.
+The right to left $ diagonal = 3 + 5 + 9 = 17 $.
+Their absolute difference is $ |15 - 17| = 2 $.
+
+## Function description
+
+Complete the $ diagonalDifference $ function in the editor below.
+diagonalDifference takes the following parameter:
+
+- int ` arr[n][m] `: an array of integers
+
+## Return
+
+- int: the absolute diagonal difference
+
+## Input Format
+
+The first line contains a single integer, n, the number of
+rows and columns in the square matrix arr.
+Each of the next n lines describes a row, arr[i], and consists of
+space-separated integers ` arr[i][j] `.
+
+## Constraints
+
+$ -100 \leq $ ` arr[i][j] ` $ \leq 100 $
+
+## Output Format
+
+Return the absolute difference between the sums of the matrix's
+two diagonals as a single integer.
+
+## Sample Input
+
+```text
+3
+11 2 4
+4 5 6
+10 8 -12
+```
+
+Sample Output
+
+```text
+15
+```
+
+## Explanation
+
+The primary diagonal is:
+
+```text
+11
+   5
+     -12
+```
+
+Sum across the primary diagonal: 11 + 5 - 12 = 4
+The secondary diagonal is:
+
+```text
+     4
+   5
+10
+```
+
+Sum across the secondary diagonal: $ 4 + 5 + 10 = 19 $
+Difference: $ |4 - 19| = 15 $
+
+*Note*: $ |x| $ is the
+[absolute value](https://www.mathsisfun.com/numbers/absolute-value.html)
+of $ x $