Tighten the return types in Fix.

Remove the `default`ness of a method, given there's only one subclass.

PiperOrigin-RevId: 566989946

Author: graememorgan

check_api/src/main/java/com/google/errorprone/fixes/Fix.java

@@ -16,10 +16,9 @@
 
 package com.google.errorprone.fixes;
 
+import com.google.common.collect.ImmutableSet;
 import com.sun.tools.javac.tree.EndPosTable;
 import com.sun.tools.javac.tree.JCTree.JCCompilationUnit;
-import java.util.Collection;
-import java.util.Set;
 
 /**
  * Represents a source code transformation, usually used to fix a bug detected by error-prone.
@@ -36,17 +35,15 @@ public interface Fix {
    *
    * <p>Empty string generates the default description.
    */
-  default String getShortDescription() {
-    return "";
-  }
+  String getShortDescription();
 
   Replacements.CoalescePolicy getCoalescePolicy();
 
-  Set<Replacement> getReplacements(EndPosTable endPositions);
+  ImmutableSet<Replacement> getReplacements(EndPosTable endPositions);
 
-  Collection<String> getImportsToAdd();
+  ImmutableSet<String> getImportsToAdd();
 
-  Collection<String> getImportsToRemove();
+  ImmutableSet<String> getImportsToRemove();
 
   boolean isEmpty();
 }

check_api/src/main/java/com/google/errorprone/fixes/SuggestedFix.java

@@ -85,7 +85,7 @@ public String toString(JCCompilationUnit compilationUnit) {
   public abstract int hashCode();
 
   @Override
-  public Set<Replacement> getReplacements(EndPosTable endPositions) {
+  public ImmutableSet<Replacement> getReplacements(EndPosTable endPositions) {
     if (endPositions == null) {
       throw new IllegalArgumentException(
           "Cannot produce correct replacements without endPositions.");

check_api/src/test/java/com/google/errorprone/fixes/AppliedFixTest.java

@@ -24,14 +24,13 @@
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 
+import com.google.common.collect.ImmutableSet;
 import com.sun.source.tree.TreeVisitor;
 import com.sun.tools.javac.tree.EndPosTable;
 import com.sun.tools.javac.tree.JCTree;
 import com.sun.tools.javac.util.Position;
 import java.util.HashMap;
-import java.util.LinkedHashSet;
 import java.util.Map;
-import java.util.Set;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -156,9 +155,8 @@ public void shouldSuggestToRemoveLastLineIfAsked() {
   @Test
   public void shouldApplyFixesInReverseOrder() {
     // Have to use a mock Fix here in order to intentionally return Replacements in wrong order.
-    Set<Replacement> replacements = new LinkedHashSet<>();
-    replacements.add(Replacement.create(0, 1, ""));
-    replacements.add(Replacement.create(1, 1, ""));
+    ImmutableSet<Replacement> replacements =
+        ImmutableSet.of(Replacement.create(0, 1, ""), Replacement.create(1, 1, ""));
 
     Fix mockFix = mock(Fix.class);
     when(mockFix.getReplacements(any())).thenReturn(replacements);