[split] Add ExceptionalFunction0 for easier use from Java

util-core/src/main/scala/com/twitter/util/Function.scala

@@ -2,6 +2,16 @@ package com.twitter.util
 
 abstract class Function0[R] extends (() => R)
 
+abstract class ExceptionalFunction0[R] extends Function0[R] {
+   /**
+    * Implements apply in terms of abstract applyE, to allow Java code to throw checked exceptions.
+    */
+   final override def apply(): R = applyE()
+
+   @throws(classOf[Throwable])
+   def applyE(): R
+}
+
 /**
  * This class is for Java friendliness. Any Scala method that takes Function1[A, B]
  * can now take a Function[A, B]. Because Function1 is a trait, it is very difficult

util-core/src/test/java/com/twitter/util/FunctionTest.java

@@ -22,4 +22,21 @@ public String applyE(Integer in) throws Exception {
       // pass: expected
     }
   }
+
+  /** Confirm that we can extend ExceptionalFunction0 with applyE(). */
+  public void testExceptionalFunction0() {
+    ExceptionalFunction0<Integer> fun = new ExceptionalFunction0<Integer>() {
+      @Override
+      public Integer applyE() throws Exception {
+        throw new Exception("Expected");
+      }
+    };
+    try {
+      fun.apply();
+      assert false : "Should have thrown";
+    } catch (Exception e) {
+      // pass: expected
+    }
+  }
+
 }