diff --git a/src/main/java/com/thealgorithms/datastructures/trees/SegmentTree.java b/src/main/java/com/thealgorithms/datastructures/trees/SegmentTree.java
index 57b3edc163ca..af6acb0cbb2b 100644
--- a/src/main/java/com/thealgorithms/datastructures/trees/SegmentTree.java
+++ b/src/main/java/com/thealgorithms/datastructures/trees/SegmentTree.java
@@ -8,13 +8,18 @@ public class SegmentTree {
 
     /* Constructor which takes the size of the array and the array as a parameter*/
     public SegmentTree(int n, int[] arr) {
+        if (arr == null) {
+            throw new IllegalArgumentException("Input array must not be null");
+        }
+        if (n <= 0 || n > arr.length) {
+            throw new IllegalArgumentException("Size must be in the range [1, " + arr.length + "], but was " + n);
+        }
         this.n = n;
         int x = (int) (Math.ceil(Math.log(n) / Math.log(2)));
         int segSize = 2 * (int) Math.pow(2, x) - 1;
 
         this.segTree = new int[segSize];
         this.arr = arr;
-        this.n = n;
         constructTree(arr, 0, n - 1, 0);
     }
 
@@ -47,7 +52,8 @@ private void updateTree(int start, int end, int index, int diff, int segIndex) {
 
     /* A function to update the value at a particular index*/
     public void update(int index, int value) {
-        if (index < 0 || index > n) {
+        // Valid positions are 0..n-1; index == n is out of bounds and must not reach arr[index].
+        if (index < 0 || index >= n) {
             return;
         }
 
@@ -73,7 +79,8 @@ private int getSumTree(int start, int end, int qStart, int qEnd, int segIndex) {
 
     /* A function to query the sum of the subarray [start...end]*/
     public int getSum(int start, int end) {
-        if (start < 0 || end > n || start > end) {
+        // The last queryable position is n-1, so end == n is an out of range query.
+        if (start < 0 || end >= n || start > end) {
             return 0;
         }
         return getSumTree(0, n - 1, start, end, 0);
diff --git a/src/test/java/com/thealgorithms/datastructures/trees/SegmentTreeTest.java b/src/test/java/com/thealgorithms/datastructures/trees/SegmentTreeTest.java
new file mode 100644
index 000000000000..196c32575709
--- /dev/null
+++ b/src/test/java/com/thealgorithms/datastructures/trees/SegmentTreeTest.java
@@ -0,0 +1,96 @@
+package com.thealgorithms.datastructures.trees;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+import org.junit.jupiter.params.provider.ValueSource;
+
+class SegmentTreeTest {
+
+    private static SegmentTree treeOf(int... values) {
+        return new SegmentTree(values.length, values);
+    }
+
+    @ParameterizedTest
+    @CsvSource({"0, 4, 15", "0, 0, 1", "4, 4, 5", "1, 3, 9", "2, 4, 12"})
+    void testRangeSums(int start, int end, int expected) {
+        assertEquals(expected, treeOf(1, 2, 3, 4, 5).getSum(start, end));
+    }
+
+    @Test
+    void testSingleElementTree() {
+        SegmentTree tree = treeOf(42);
+        assertEquals(42, tree.getSum(0, 0));
+        tree.update(0, 7);
+        assertEquals(7, tree.getSum(0, 0));
+    }
+
+    @Test
+    void testUpdateIsReflectedInSubsequentQueries() {
+        SegmentTree tree = treeOf(1, 2, 3, 4, 5);
+        tree.update(2, 10);
+        assertEquals(22, tree.getSum(0, 4));
+        assertEquals(16, tree.getSum(1, 3));
+        tree.update(0, -1);
+        assertEquals(20, tree.getSum(0, 4));
+    }
+
+    @Test
+    void testNegativeValues() {
+        SegmentTree tree = treeOf(-5, 3, -2, 8);
+        assertEquals(4, tree.getSum(0, 3));
+        assertEquals(-4, tree.getSum(0, 2));
+    }
+
+    /**
+     * index == n is past the last element, so it must be rejected by the guard instead of reaching
+     * the backing array and throwing {@link ArrayIndexOutOfBoundsException}.
+     */
+    @ParameterizedTest
+    @ValueSource(ints = {5, 6, 100, -1})
+    void testUpdateOutOfRangeIndexIsIgnored(int index) {
+        SegmentTree tree = treeOf(1, 2, 3, 4, 5);
+        assertDoesNotThrow(() -> tree.update(index, 99));
+        assertEquals(15, tree.getSum(0, 4), "out of range update must not modify the tree");
+    }
+
+    @ParameterizedTest
+    @CsvSource({"0, 5", "0, 6", "3, 2", "-1, 3", "5, 5"})
+    void testOutOfRangeQueriesReturnZero(int start, int end) {
+        assertEquals(0, treeOf(1, 2, 3, 4, 5).getSum(start, end));
+    }
+
+    @Test
+    void testConstructorRejectsInvalidSize() {
+        assertThrows(IllegalArgumentException.class, () -> new SegmentTree(0, new int[] {1, 2, 3}));
+        assertThrows(IllegalArgumentException.class, () -> new SegmentTree(-1, new int[] {1, 2, 3}));
+        assertThrows(IllegalArgumentException.class, () -> new SegmentTree(4, new int[] {1, 2, 3}));
+    }
+
+    @Test
+    void testConstructorRejectsNullArray() {
+        assertThrows(IllegalArgumentException.class, () -> new SegmentTree(3, null));
+    }
+
+    @ParameterizedTest
+    @ValueSource(ints = {1, 2, 3, 4, 5, 6, 7, 8, 9, 16, 17})
+    void testMatchesBruteForceForVariousSizes(int size) {
+        int[] values = new int[size];
+        for (int i = 0; i < size; i++) {
+            values[i] = i * 3 - 4;
+        }
+        SegmentTree tree = new SegmentTree(size, values.clone());
+
+        for (int start = 0; start < size; start++) {
+            int expected = 0;
+            for (int end = start; end < size; end++) {
+                expected += values[end];
+                assertEquals(expected, tree.getSum(start, end), "sum of [" + start + ", " + end + "] with size " + size);
+            }
+        }
+    }
+}