keyboardr · December 8, 2016 01:54 · Dec 8, 2016
diff --git a/HeadlessFragmentFactory.java b/HeadlessFragmentFactory.java
@@ -0,0 +1,30 @@
+/**
+ * What you'd think an abstract factory would look like. Unfortunately, this class won't compile.
+ */
+public abstract class HeadlessFragmentFactory<F extends Fragment & ParentedFragment<F, ? extends P>, P> {
+
+  protected abstract F newInstance();
+
+  /* There will be a compiler warning on P. When joining types, only the first type is allowed to
+     be a class. All others MUST be interfaces. In this situation, there is no way for the compiler
+     to know that P will be an interface. If P is removed, there is no longer a compile-time check
+     that the parent implements the necessary callback interface */
+  public <A extends FragmentActivity & P> F attach(A parent, String tag) {
+    return attach(parent.getSupportFragmentManager(), tag);
+  }
+
+  // Same as above
+  public <A extends Fragment & P> F attach(A parent, String tag) {
+    return attach(parent.getChildFragmentManager(), tag);
+  }
+
+  private F attach(FragmentManager fragmentManager, String tag) {
+    F frag = ((F) fragmentManager.findFragmentByTag(tag));
+    if (frag == null) {
+      frag = newInstance();
+      fragmentManager.beginTransaction().add(frag, tag).commitNow();
+    }
+    return frag;
+  }
+
+}
diff --git a/ParentedFragment.java b/ParentedFragment.java
@@ -0,0 +1,23 @@
+/** 
+ *  Fragment that requires a parent of type P. T is a self-referential generic that also forces
+ *  the implementation to extend Fragment
+ */
+public interface ParentedFragment<T extends Fragment & ParentedFragment<T, P>, P> {
+
+  //Example parent interface
+  interface FragmentParent {}
+
+  //Example fragment implementation
+  class ParentedFragmentImpl extends Fragment
+      implements ParentedFragment<ParentedFragmentImpl, FragmentParent> {
+  }
+
+  //Example factory
+  HeadlessFragmentFactory<ParentedFragmentImpl, FragmentParent> IMPL_FACTORY
+      = new HeadlessFragmentFactory<ParentedFragmentImpl, FragmentParent>() {
+    @Override
+    protected ParentedFragmentImpl newInstance() {
+      return new ParentedFragmentImpl();
+    }
+  };
+}
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