Makes some improvements

Saves the downloaded BuildTools version to the server version container
Adds a check to see whether the BuildTools file exists
Adds a helper function for reading local files

src/main/java/net/knarcraft/minecraftserverlauncher/server/ServerVersionContainer.java

@@ -21,6 +21,7 @@ public class ServerVersionContainer {
     private Map<String, String> travertineVersions;
     private Map<String, String> spongeVanillaVersions;
     private Map<String, String> spongeForgeVersions;
+    private String downloadedBuildToolsVersion;
 
     /**
      * Initializes a new server version container
@@ -56,6 +57,7 @@ public class ServerVersionContainer {
         this.travertineVersions = new HashMap<>();
         this.spongeVanillaVersions = new HashMap<>();
         this.spongeForgeVersions = new HashMap<>();
+        this.downloadedBuildToolsVersion = null;
     }
 
     @Override
@@ -66,7 +68,8 @@ public class ServerVersionContainer {
                 "waterfallVersions;" + mapToString(waterfallVersions) + "\n" +
                 "travertineVersions;" + mapToString(travertineVersions) + "\n" +
                 "spongeVanillaVersions;" + mapToString(spongeVanillaVersions) + "\n" +
-                "spongeForgeVersions;" + mapToString(spongeForgeVersions);
+                "spongeForgeVersions;" + mapToString(spongeForgeVersions) + "\n" +
+                "downloadedBuildToolsVersion;" + downloadedBuildToolsVersion;
     }
 
     /**
@@ -105,7 +108,7 @@ public class ServerVersionContainer {
                 }
             }
             file = new PrintWriter(versionFile);
-            file.println(this.toString());
+            file.println(this);
             file.close();
         } catch (IOException e) {
             e.printStackTrace();
@@ -163,6 +166,9 @@ public class ServerVersionContainer {
             case "spongeForgeVersions":
                 parseVersionsToMap(spongeForgeVersions, variableValue);
                 break;
+            case "downloadedBuildToolsVersion":
+                downloadedBuildToolsVersion = variableValue;
+                break;
             default:
                 throw new IllegalArgumentException("Invalid key encountered in the server version file.");
         }
@@ -323,5 +329,25 @@ public class ServerVersionContainer {
         spongeForgeVersions.put(mapKey, newValue);
         saveState();
     }
+
+    /**
+     * Gets the version of the downloaded BuildTools file
+     *
+     * @return <p>The version of the downloaded BuildTools file</p>
+     */
+    public String getDownloadedBuildToolsVersion() {
+        return this.downloadedBuildToolsVersion;
+    }
+
+    /**
+     * Sets the version of the downloaded BuildTools file
+     *
+     * @param newValue <p>The new version</p></p>
+     */
+    public void setDownloadedBuildToolsVersion(String newValue) {
+        this.downloadedBuildToolsVersion = newValue;
+        saveState();
+    }
+
 }