Enable and fix more Eclipse warnings

Enable and fix warnings:

- Missing `hashCode` method on classes that define `equals`
- Redundant null check

Also enable "possible accidental boolean assignment" warning.  There
are currently no instances of this but it might be useful to enable.

Change-Id: I7659ae66bedb6a17e088c90d2165282b7fe33695

.settings/org.eclipse.jdt.core.prefs

@@ -33,7 +33,7 @@ org.eclipse.jdt.core.compiler.problem.methodWithConstructorName=warning
 org.eclipse.jdt.core.compiler.problem.missingDefaultCase=ignore
 org.eclipse.jdt.core.compiler.problem.missingDeprecatedAnnotation=ignore
 org.eclipse.jdt.core.compiler.problem.missingEnumCaseDespiteDefault=disabled
-org.eclipse.jdt.core.compiler.problem.missingHashCodeMethod=ignore
+org.eclipse.jdt.core.compiler.problem.missingHashCodeMethod=warning
 org.eclipse.jdt.core.compiler.problem.missingOverrideAnnotation=warning
 org.eclipse.jdt.core.compiler.problem.missingOverrideAnnotationForInterfaceMethodImplementation=enabled
 org.eclipse.jdt.core.compiler.problem.missingSerialVersion=warning
@@ -48,12 +48,12 @@ org.eclipse.jdt.core.compiler.problem.nullSpecViolation=error
 org.eclipse.jdt.core.compiler.problem.nullUncheckedConversion=warning
 org.eclipse.jdt.core.compiler.problem.overridingPackageDefaultMethod=warning
 org.eclipse.jdt.core.compiler.problem.parameterAssignment=ignore
-org.eclipse.jdt.core.compiler.problem.possibleAccidentalBooleanAssignment=ignore
+org.eclipse.jdt.core.compiler.problem.possibleAccidentalBooleanAssignment=warning
 org.eclipse.jdt.core.compiler.problem.potentialNullReference=ignore
 org.eclipse.jdt.core.compiler.problem.potentiallyUnclosedCloseable=ignore
 org.eclipse.jdt.core.compiler.problem.rawTypeReference=warning
 org.eclipse.jdt.core.compiler.problem.redundantNullAnnotation=warning
-org.eclipse.jdt.core.compiler.problem.redundantNullCheck=ignore
+org.eclipse.jdt.core.compiler.problem.redundantNullCheck=warning
 org.eclipse.jdt.core.compiler.problem.redundantSpecificationOfTypeArguments=warning
 org.eclipse.jdt.core.compiler.problem.redundantSuperinterface=ignore
 org.eclipse.jdt.core.compiler.problem.reportMethodCanBePotentiallyStatic=ignore

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/LineMapper.java

@@ -17,6 +17,7 @@ package com.google.gerrit.client.diff;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Objects;
 
 /** Helper class to handle calculations involving line gaps. */
 class LineMapper {
@@ -192,6 +193,11 @@ class LineMapper {
       return false;
     }
 
+    @Override
+    public int hashCode() {
+      return Objects.hash(this);
+    }
+
     @Override
     public String toString() {
       return line + " " + aligned;

gerrit-server/src/main/java/com/google/gerrit/server/mail/CommentSender.java

@@ -121,7 +121,7 @@ public class CommentSender extends ReplyToChangeSender {
         try {
           patchList = getPatchList();
         } catch (PatchListNotAvailableException e) {
-          patchList = null;
+          log.error("Failed to get patch list", e);
         }
       }