Check to see if initialized

Author: ajermakovics

.gitignore

@@ -1,10 +1,9 @@
-.gradle/ 
+*.class
+*.jar
 .classpath 
 .project 
 .settings/
-*.class
-*.jar
+.gradle/
 bin/
 build/
 target/
-

LICENSE

@@ -18,5 +18,4 @@ FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-SOFTWARE.
-
+SOFTWARE.

src/main/java/lambda2sql/Lambda2Sql.java

@@ -1,6 +1,8 @@
 package lambda2sql;
 
+import static java.lang.System.getProperty;
 import static java.nio.file.Files.createTempDirectory;
+import static java.util.Objects.requireNonNull;
 
 import java.io.IOException;
 import java.util.function.Predicate;
@@ -36,6 +38,7 @@ public static void init() {
 	 * Supported operators: >,>=,<,<=,=,!=,&&,||,!
 	 */
 	public static <T> String toSql(Predicate<T> predicate) {
+		requireNonNull(getProperty(DUMP_CLASSES_PROP), "Call init() before creating the predicate.");
 		LambdaExpression<Predicate<T>> lambdaExpression = LambdaExpression.parse(predicate);
 		return lambdaExpression.accept(new ToSqlVisitor()).toString();
 	}