add test cases and readme file

Author: wsw-stack

README-M4.md

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+# Milestone 4 – Stream support for **JSONObject**
+
+## What is Added
+
+| Item | Description |
+|------|-------------|
+| `JSONObject.JSONNode` | Immutable leaf wrapper containing an absolute `path` and its `value`. |
+| `Stream<JSONNode> JSONObject.toStream()` | Depth-first, **lazy** flattening of any `JSONObject/JSONArray` into a `Stream`. Only leaf nodes are emitted (low memory footprint). |
+
+Path conventions
+* Object keys: `/parent/child`
+* Array items:  `/array[0]/child`
+
+---
+
+## Run the test class
+```bash
+mvn -Dtest=org.json.junit.milestone4.tests.JSONObjectStreamTest test

src/test/java/org/json/junit/milestone4/tests/JSONObjectStreamTest.java

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+package org.json.junit.milestone4.tests;
+
+import org.json.JSONObject;
+import org.json.JSONObject.JSONNode;
+import org.json.XML;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.io.InputStream;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+import java.util.Optional;
+import java.util.Scanner;
+import java.util.stream.Collectors;
+
+import static org.junit.Assert.*;
+
+/**
+ * Unit tests for the JSONObject.toStream() extension (Milestone 4) – JUnit 4 / Java 8.
+ */
+public class JSONObjectStreamTest {
+
+    private static JSONObject catalog;
+
+    @BeforeClass
+    public static void loadXml() throws Exception {
+        try (InputStream in = JSONObjectStreamTest.class.getResourceAsStream("/books.xml")) {
+            assertNotNull("books.xml must be on the class-path (src/test/resources)", in);
+
+            String xml;
+            try (Scanner scanner = new Scanner(in, StandardCharsets.UTF_8.name())) {
+                scanner.useDelimiter("\\A");
+                xml = scanner.hasNext() ? scanner.next() : "";
+            }
+
+            catalog = XML.toJSONObject(xml);
+        }
+    }
+
+    @Test
+    public void extractTitles() {
+        List<String> titles = catalog.toStream()
+                .filter(n -> n.getPath().endsWith("/title"))
+                .map(n -> n.getValue().toString())
+                .collect(Collectors.toList());
+
+        assertEquals(12, titles.size());
+        assertTrue(titles.contains("XML Developer's Guide"));
+        assertTrue(titles.contains("Visual Studio 7: A Comprehensive Guide"));
+    }
+
+    @Test
+    public void findExactPath() {
+        String wantedPath = "/catalog/book[0]/author";
+        Optional<JSONNode> node = catalog.toStream()
+                .filter(n -> n.getPath().equals(wantedPath))
+                .findFirst();
+
+        assertTrue(node.isPresent());
+        assertEquals("Gambardella, Matthew", node.get().getValue());
+    }
+
+    @Test
+    public void filterCheapBooks() {
+        List<JSONNode> cheapPrices = catalog.toStream()
+                .filter(n -> n.getPath().endsWith("/price"))
+                .filter(n -> Double.parseDouble(n.getValue().toString()) < 10.0)
+                .collect(Collectors.toList());
+
+        assertEquals(8, cheapPrices.size());
+        assertTrue(cheapPrices.stream()
+                .allMatch(n -> Double.parseDouble(n.getValue().toString()) < 10.0));
+    }
+}