Add Utils.isLinux() and Utils.isMac() helpers.

core/src/main/java/org/bitcoinj/core/Utils.java

@@ -544,7 +544,15 @@ public class Utils {
         return isAndroid == 1;
     }
 
+    public static boolean isLinux() {
+        return System.getProperty("os.name").toLowerCase().contains("linux");
+    }
+
     public static boolean isWindows() {
         return System.getProperty("os.name").toLowerCase().contains("win");
     }
+
+    public static boolean isMac() {
+        return System.getProperty("os.name").toLowerCase().contains("mac");
+    }
 }

core/src/main/java/org/bitcoinj/net/discovery/DnsDiscovery.java

@@ -67,7 +67,7 @@ public class DnsDiscovery extends MultiplexingDiscovery {
     protected ExecutorService createExecutor() {
         // Attempted workaround for reported bugs on Linux in which gethostbyname does not appear to be properly
         // thread safe and can cause segfaults on some libc versions.
-        if (System.getProperty("os.name").toLowerCase().contains("linux"))
+        if (Utils.isLinux())
             return Executors.newSingleThreadExecutor(new ContextPropagatingThreadFactory("DNS seed lookups"));
         else
             return Executors.newFixedThreadPool(seeds.size(), new DaemonThreadFactory("DNS seed lookups"));

core/src/main/java/org/bitcoinj/utils/BlockFileLoader.java

@@ -54,15 +54,16 @@ public class BlockFileLoader implements Iterable<Block>, Iterator<Block> {
      */
     public static List<File> getReferenceClientBlockFileList() {
         String defaultDataDir;
-        String OS = System.getProperty("os.name").toLowerCase();
         if (Utils.isWindows()) {
             defaultDataDir = System.getenv("APPDATA") + "\\.bitcoin\\blocks\\";
-        } else if (OS.indexOf("mac") >= 0 || (OS.indexOf("darwin") >= 0)) {
+        } else if (Utils.isMac()) {
             defaultDataDir = System.getProperty("user.home") + "/Library/Application Support/Bitcoin/blocks/";
-        } else {
+        } else if (Utils.isLinux()) {
             defaultDataDir = System.getProperty("user.home") + "/.bitcoin/blocks/";
+        } else {
+            throw new RuntimeException("Unsupported system");
         }
-        
+
         List<File> list = new LinkedList<>();
         for (int i = 0; true; i++) {
             File file = new File(defaultDataDir + String.format(Locale.US, "blk%05d.dat", i));

wallettemplate/src/main/java/wallettemplate/Main.java

@@ -19,6 +19,7 @@ package wallettemplate;
 import com.google.common.util.concurrent.*;
 import javafx.scene.input.*;
 import org.bitcoinj.core.NetworkParameters;
+import org.bitcoinj.core.Utils;
 import org.bitcoinj.kits.WalletAppKit;
 import org.bitcoinj.params.*;
 import org.bitcoinj.utils.BriefLogFormatter;
@@ -74,7 +75,7 @@ public class Main extends Application {
         // Show the crash dialog for any exceptions that we don't handle and that hit the main loop.
         GuiUtils.handleCrashesOnThisThread();
 
-        if (System.getProperty("os.name").toLowerCase().contains("mac")) {
+        if (Utils.isMac()) {
             // We could match the Mac Aqua style here, except that (a) Modena doesn't look that bad, and (b)
             // the date picker widget is kinda broken in AquaFx and I can't be bothered fixing it.
             // AquaFx.style();