Improved the readability and fixed few java warnings (#9163)

* Improved the readability and fixed few java warnings

* Fix the checkstyle

Co-authored-by: Gian Merlino <gianmerlino@gmail.com>

sql/src/main/java/org/apache/druid/sql/calcite/expression/Expressions.java

@@ -264,14 +264,9 @@ public class Expressions
           rexNode,
           postAggregatorVisitor
       );
-
-      if (expression == null) {
-        return null;
-      } else {
       return expression;
     }
   }
-  }
 
   private static DruidExpression literalToDruidExpression(
       final PlannerContext plannerContext,

sql/src/main/java/org/apache/druid/sql/calcite/expression/builtin/TimeArithmeticOperatorConversion.java

@@ -48,7 +48,7 @@ public abstract class TimeArithmeticOperatorConversion implements SqlOperatorCon
   {
     this.operator = operator;
     this.direction = direction;
-    Preconditions.checkArgument(direction > 0 || direction < 0);
+    Preconditions.checkArgument(direction != 0);
   }
 
   @Override

sql/src/main/java/org/apache/druid/sql/calcite/filtration/BoundRefKey.java

@@ -24,6 +24,8 @@ import org.apache.druid.query.filter.BoundDimFilter;
 import org.apache.druid.query.filter.SelectorDimFilter;
 import org.apache.druid.query.ordering.StringComparator;
 
+import java.util.Objects;
+
 public class BoundRefKey
 {
   private final String dimension;
@@ -82,13 +84,13 @@ public class BoundRefKey
 
     BoundRefKey boundRefKey = (BoundRefKey) o;
 
-    if (dimension != null ? !dimension.equals(boundRefKey.dimension) : boundRefKey.dimension != null) {
+    if (!Objects.equals(dimension, boundRefKey.dimension)) {
       return false;
     }
-    if (extractionFn != null ? !extractionFn.equals(boundRefKey.extractionFn) : boundRefKey.extractionFn != null) {
+    if (!Objects.equals(extractionFn, boundRefKey.extractionFn)) {
       return false;
     }
-    return comparator != null ? comparator.equals(boundRefKey.comparator) : boundRefKey.comparator == null;
+    return Objects.equals(comparator, boundRefKey.comparator);
   }
 
   @Override

sql/src/main/java/org/apache/druid/sql/calcite/filtration/BoundValue.java

@@ -22,6 +22,8 @@ package org.apache.druid.sql.calcite.filtration;
 import org.apache.druid.java.util.common.ISE;
 import org.apache.druid.query.ordering.StringComparator;
 
+import java.util.Objects;
+
 public class BoundValue implements Comparable<BoundValue>
 {
   private final String value;
@@ -55,10 +57,10 @@ public class BoundValue implements Comparable<BoundValue>
 
     BoundValue that = (BoundValue) o;
 
-    if (value != null ? !value.equals(that.value) : that.value != null) {
+    if (!Objects.equals(value, that.value)) {
       return false;
     }
-    return comparator != null ? comparator.equals(that.comparator) : that.comparator == null;
+    return Objects.equals(comparator, that.comparator);
 
   }
 

sql/src/main/java/org/apache/druid/sql/calcite/filtration/ConvertSelectorsToIns.java

@@ -66,11 +66,7 @@ public class ConvertSelectorsToIns extends BottomUpTransform
                   SimpleExtraction.of(selector.getDimension(), selector.getExtractionFn())
               )
           );
-          List<SelectorDimFilter> filterList = selectors.get(boundRefKey);
+          List<SelectorDimFilter> filterList = selectors.computeIfAbsent(boundRefKey, k -> new ArrayList<>());
-          if (filterList == null) {
-            filterList = new ArrayList<>();
-            selectors.put(boundRefKey, filterList);
-          }
           filterList.add(selector);
         }
       }

sql/src/main/java/org/apache/druid/sql/calcite/filtration/Filtration.java

@@ -32,6 +32,7 @@ import org.apache.druid.sql.calcite.table.RowSignature;
 import org.joda.time.Interval;
 
 import java.util.List;
+import java.util.Objects;
 
 public class Filtration
 {
@@ -162,10 +163,10 @@ public class Filtration
 
     Filtration that = (Filtration) o;
 
-    if (intervals != null ? !intervals.equals(that.intervals) : that.intervals != null) {
+    if (!Objects.equals(intervals, that.intervals)) {
       return false;
     }
-    return dimFilter != null ? dimFilter.equals(that.dimFilter) : that.dimFilter == null;
+    return Objects.equals(dimFilter, that.dimFilter);
 
   }
 

sql/src/main/java/org/apache/druid/sql/calcite/rel/DruidConvention.java

@@ -73,7 +73,7 @@ public class DruidConvention implements Convention
   @Override
   public boolean satisfies(RelTrait trait)
   {
-    return trait == this;
+    return trait.equals(this);
   }
 
   @Override

sql/src/main/java/org/apache/druid/sql/calcite/table/DruidTable.java

@@ -34,6 +34,8 @@ import org.apache.calcite.sql.SqlCall;
 import org.apache.calcite.sql.SqlNode;
 import org.apache.druid.query.DataSource;
 
+import java.util.Objects;
+
 public class DruidTable implements TranslatableTable
 {
   private final DataSource dataSource;
@@ -111,10 +113,10 @@ public class DruidTable implements TranslatableTable
 
     DruidTable that = (DruidTable) o;
 
-    if (dataSource != null ? !dataSource.equals(that.dataSource) : that.dataSource != null) {
+    if (!Objects.equals(dataSource, that.dataSource)) {
       return false;
     }
-    return rowSignature != null ? rowSignature.equals(that.rowSignature) : that.rowSignature == null;
+    return Objects.equals(rowSignature, that.rowSignature);
   }
 
   @Override

sql/src/main/java/org/apache/druid/sql/calcite/table/RowSignature.java

@@ -42,6 +42,7 @@ import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 
 /**
  * Type signature for a row in a Druid dataSource ("DruidTable") or query result. Rows have an ordering and every
@@ -196,10 +197,10 @@ public class RowSignature
 
     RowSignature that = (RowSignature) o;
 
-    if (columnTypes != null ? !columnTypes.equals(that.columnTypes) : that.columnTypes != null) {
+    if (!Objects.equals(columnTypes, that.columnTypes)) {
       return false;
     }
-    return columnNames != null ? columnNames.equals(that.columnNames) : that.columnNames == null;
+    return Objects.equals(columnNames, that.columnNames);
   }
 
   @Override