initial implementation of JSONPointer

JSONPointer.java

@@ -0,0 +1,61 @@
+package org.json;
+
+import static java.lang.String.format;
+import static java.util.Collections.emptyList;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class JSONPointer {
+
+    private List<String> refTokens;
+
+    public JSONPointer(String pointer) {
+        if (pointer == null) {
+            throw new NullPointerException("pointer cannot be null");
+        }
+        if (pointer.isEmpty()) {
+            refTokens = emptyList();
+            return;
+        }
+        if (pointer.startsWith("#/")) {
+            pointer = pointer.substring(2);
+        } else if (pointer.startsWith("/")) {
+            pointer = pointer.substring(1);
+        } else {
+            throw new IllegalArgumentException("a JSON pointer should start with '/' or '#/'");
+        }
+        refTokens = new ArrayList<String>();
+        for (String token : pointer.split("/")) {
+            refTokens.add(unescape(token));
+        }
+    }
+
+    private String unescape(String token) {
+        return token.replace("~1", "/").replace("~0", "~");
+    }
+
+    public Object queryFrom(JSONObject document) {
+        if (refTokens.isEmpty()) {
+            return document;
+        }
+        Object current = document;
+        for (String token : refTokens) {
+            if (current instanceof JSONObject) {
+                current = ((JSONObject) current).opt(unescape(token));
+            } else if (current instanceof JSONArray) {
+                current = readByIndexToken(current, unescape(token));
+            }
+        }
+        return current;
+    }
+
+    private Object readByIndexToken(Object current, String indexToken) {
+        try {
+            return ((JSONArray) current).opt(Integer.parseInt(unescape(indexToken)));
+        } catch (NumberFormatException e) {
+            throw new JSONPointerException(format("%s is not an array index", unescape(indexToken)), e);
+        }
+    }
+
+}