Extracted superclass for Terminals implementations, and fixed thread-safety.

src/main/java/net/rubygrapefruit/platform/internal/AbstractTerminals.java

@@ -0,0 +1,28 @@
+package net.rubygrapefruit.platform.internal;
+
+import net.rubygrapefruit.platform.Terminal;
+import net.rubygrapefruit.platform.Terminals;
+
+public abstract class AbstractTerminals implements Terminals {
+    private static Output currentlyOpen;
+    private static AbstractTerminal current;
+
+    public Terminal getTerminal(Output output) {
+        synchronized (AbstractTerminals.class) {
+            if (currentlyOpen != null && currentlyOpen != output) {
+                throw new UnsupportedOperationException("Currently only one output can be used as a terminal.");
+            }
+
+            if (current == null) {
+                AbstractTerminal terminal = createTerminal(output);
+                terminal.init();
+                currentlyOpen = output;
+                current = terminal;
+            }
+
+            return current;
+        }
+    }
+
+    protected abstract AbstractTerminal createTerminal(Output output);
+}