MergeTip: Extract null-safe factory method

Change-Id: I3cc331050732f28b5341384f729f48cfb0311131

gerrit-server/src/main/java/com/google/gerrit/server/git/MergeTip.java

@@ -15,6 +15,7 @@
 package com.google.gerrit.server.git;
 
 import com.google.common.collect.Maps;
+import com.google.gerrit.common.Nullable;
 
 import java.util.Collection;
 import java.util.Map;
@@ -23,6 +24,14 @@ import java.util.Map;
  * Class describing a merge tip during merge operation.
  */
 public class MergeTip {
+  public static MergeTip from(@Nullable CodeReviewCommit initial,
+      Collection<CodeReviewCommit> toMerge) {
+    if (initial == null) {
+      return null;
+    }
+    return new MergeTip(initial, toMerge);
+  }
+
   private CodeReviewCommit branchTip;
   private Map<String,String> mergeResults;
 
@@ -30,7 +39,8 @@ public class MergeTip {
    * @param initial Tip before the merge operation.
    * @param toMerge List of CodeReview commits to be merged in merge operation.
    */
-  public MergeTip(CodeReviewCommit initial, Collection<CodeReviewCommit> toMerge) {
+  private MergeTip(CodeReviewCommit initial,
+      Collection<CodeReviewCommit> toMerge) {
     this.mergeResults = Maps.newHashMap();
     this.branchTip = initial;
     // Assume fast-forward merge until opposite is proven.

gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/CherryPick.java

@@ -66,8 +66,7 @@ public class CherryPick extends SubmitStrategy {
   @Override
   protected MergeTip _run(CodeReviewCommit branchTip,
       Collection<CodeReviewCommit> toMerge) throws MergeException {
-    MergeTip mergeTip = branchTip != null
+    MergeTip mergeTip = MergeTip.from(branchTip, toMerge);
-        ? new MergeTip(branchTip, toMerge) : null;
     List<CodeReviewCommit> sorted = CodeReviewCommit.ORDER.sortedCopy(toMerge);
     while (!sorted.isEmpty()) {
       CodeReviewCommit n = sorted.remove(0);
@@ -90,7 +89,7 @@ public class CherryPick extends SubmitStrategy {
 
   private MergeTip cherryPickUnbornRoot(CodeReviewCommit n) {
     // The branch is unborn. Take fast-forward resolution to create the branch.
-    MergeTip mergeTip = new MergeTip(n, Lists.newArrayList(n));
+    MergeTip mergeTip = MergeTip.from(n, Lists.newArrayList(n));
     n.setStatusCode(CommitMergeStatus.CLEAN_MERGE);
     return mergeTip;
   }

gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/FastForwardOnly.java

@@ -31,7 +31,7 @@ public class FastForwardOnly extends SubmitStrategy {
   @Override
   protected MergeTip _run(final CodeReviewCommit branchTip,
       final Collection<CodeReviewCommit> toMerge) throws MergeException {
-    MergeTip mergeTip = new MergeTip(branchTip, toMerge);
+    MergeTip mergeTip = MergeTip.from(branchTip, toMerge);
     List<CodeReviewCommit> sorted = args.mergeUtil.reduceToMinimalMerge(
         args.mergeSorter, toMerge);
     final CodeReviewCommit newMergeTipCommit =

gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/MergeAlways.java

@@ -35,10 +35,10 @@ public class MergeAlways extends SubmitStrategy {
     if (branchTip == null) {
       // The branch is unborn. Take a fast-forward resolution to
       // create the branch.
-      mergeTip = new MergeTip(sorted.get(0), toMerge);
+      mergeTip = MergeTip.from(sorted.get(0), toMerge);
       sorted.remove(0);
     } else {
-      mergeTip = new MergeTip(branchTip, toMerge);
+      mergeTip = MergeTip.from(branchTip, toMerge);
     }
     while (!sorted.isEmpty()) {
       CodeReviewCommit mergedFrom = sorted.remove(0);

gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/MergeIfNecessary.java

@@ -36,10 +36,10 @@ public class MergeIfNecessary extends SubmitStrategy {
     if (branchTip == null) {
       // The branch is unborn. Take a fast-forward resolution to
       // create the branch.
-      mergeTip = new MergeTip(sorted.get(0), toMerge);
+      mergeTip = MergeTip.from(sorted.get(0), toMerge);
       branchTip = sorted.remove(0);
     } else {
-      mergeTip = new MergeTip(branchTip, toMerge);
+      mergeTip = MergeTip.from(branchTip, toMerge);
       branchTip =
           args.mergeUtil.getFirstFastForward(branchTip, args.rw, sorted);
     }

gerrit-server/src/main/java/com/google/gerrit/server/git/strategy/RebaseIfNecessary.java

@@ -58,7 +58,7 @@ public class RebaseIfNecessary extends SubmitStrategy {
   @Override
   protected MergeTip _run(final CodeReviewCommit branchTip,
       final Collection<CodeReviewCommit> toMerge) throws MergeException {
-    MergeTip mergeTip = new MergeTip(branchTip, toMerge);
+    MergeTip mergeTip = MergeTip.from(branchTip, toMerge);
     List<CodeReviewCommit> sorted = sort(toMerge);
     while (!sorted.isEmpty()) {
       CodeReviewCommit n = sorted.remove(0);