Warn when not using Java 8 or greater.

Note: this warning is not in the shared WorldEdit class because it
doesn't get loaded as early as the individual plugin/mod
implementations for some reason.

worldedit-core/src/main/java/com/sk89q/worldedit/util/Java8Detector.java

@@ -0,0 +1,46 @@
+/*
+ * WorldEdit, a Minecraft world manipulation toolkit
+ * Copyright (C) sk89q <http://www.sk89q.com>
+ * Copyright (C) WorldEdit team and contributors
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License
+ * for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+package com.sk89q.worldedit.util;
+
+import com.google.common.base.Joiner;
+import com.sk89q.worldedit.WorldEdit;
+
+public final class Java8Detector {
+
+    public static void notifyIfNot8() {
+        String[] ver = System.getProperty("java.version").split("\\.");
+        int major = Integer.parseInt(ver[1]);
+        if (major <= 7) {
+            // Implicitly java 7 because we compile against 7, so this won't
+            // even launch on 6.
+            WorldEdit.logger
+                    .warning("WorldEdit has detected you are using Java 7"
+                            + " (based on detected version "
+                            + Joiner.on('.').join(ver) + ").");
+            WorldEdit.logger.warning(
+                    "WorldEdit will stop supporting Java less than version 8 in the future,"
+                            + " due to Java 7 being EOL since April 2015."
+                            + " Please update your server to Java 8.");
+        }
+    }
+
+    private Java8Detector() {
+    }
+
+}