Add tests for AStar.java, enhance documentation (#5603)

src/test/java/com/thealgorithms/datastructures/graphs/AStarTest.java

@@ -0,0 +1,46 @@
+package com.thealgorithms.datastructures.graphs;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class AStarTest {
+
+    private AStar.Graph graph;
+    private int[] heuristic;
+
+    @BeforeEach
+    public void setUp() {
+        // Initialize graph and heuristic values for testing
+        graph = new AStar.Graph(5);
+        ArrayList<Integer> graphData = new ArrayList<>(Arrays.asList(0, 1, 1, null, 0, 2, 2, null, 1, 3, 1, null, 2, 3, 1, null, 3, 4, 1, null));
+        AStar.initializeGraph(graph, graphData);
+
+        heuristic = new int[] {5, 4, 3, 2, 0}; // Heuristic values for each node
+    }
+
+    @Test
+    public void testAStarFindsPath() {
+        AStar.PathAndDistance result = AStar.aStar(0, 4, graph, heuristic);
+        assertEquals(3, result.getDistance(), "Expected distance from 0 to 4 is 3");
+        assertEquals(Arrays.asList(0, 1, 3, 4), result.getPath(), "Expected path from 0 to 4");
+    }
+
+    @Test
+    public void testAStarPathNotFound() {
+        AStar.PathAndDistance result = AStar.aStar(0, 5, graph, heuristic); // Node 5 does not exist
+        assertEquals(-1, result.getDistance(), "Expected distance when path not found is -1");
+        assertNull(result.getPath(), "Expected path should be null when no path exists");
+    }
+
+    @Test
+    public void testAStarSameNode() {
+        AStar.PathAndDistance result = AStar.aStar(0, 0, graph, heuristic);
+        assertEquals(0, result.getDistance(), "Expected distance from 0 to 0 is 0");
+        assertEquals(Arrays.asList(0), result.getPath(), "Expected path should only contain the start node");
+    }
+}