Add nullability annotations to linq4j

Author: vlsi

linq4j/build.gradle.kts

@@ -19,4 +19,5 @@ dependencies {
 
     implementation("com.google.guava:guava")
     implementation("org.apache.calcite.avatica:avatica-core")
+    implementation("org.checkerframework:checker-qual")
 }

linq4j/src/main/java/org/apache/calcite/linq4j/BaseQueryable.java

@@ -18,6 +18,8 @@
 
 import org.apache.calcite.linq4j.tree.Expression;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.Iterator;
 
@@ -34,10 +36,10 @@ public abstract class BaseQueryable<TSource>
     extends AbstractQueryable<TSource> {
   protected final QueryProvider provider;
   protected final Type elementType;
-  protected final Expression expression;
+  protected final @Nullable Expression expression;
 
   public BaseQueryable(QueryProvider provider, Type elementType,
-      Expression expression) {
+      @Nullable Expression expression) {
     this.provider = provider;
     this.elementType = elementType;
     this.expression = expression;
@@ -51,7 +53,7 @@ public Type getElementType() {
     return elementType;
   }
 
-  public Expression getExpression() {
+  public @Nullable Expression getExpression() {
     return expression;
   }
 

linq4j/src/main/java/org/apache/calcite/linq4j/DefaultEnumerable.java

@@ -33,6 +33,9 @@
 import org.apache.calcite.linq4j.function.Predicate1;
 import org.apache.calcite.linq4j.function.Predicate2;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.PolyNull;
+
 import java.math.BigDecimal;
 import java.util.Collection;
 import java.util.Comparator;
@@ -69,7 +72,7 @@ protected OrderedEnumerable<T> getThisOrdered() {
     return this;
   }
 
-  public <R> R foreach(Function1<T, R> func) {
+  public <R> @Nullable R foreach(Function1<T, R> func) {
     R result = null;
     try (Enumerator<T> enumerator = enumerator()) {
       while (enumerator.moveNext()) {
@@ -89,12 +92,12 @@ protected OrderedQueryable<T> asOrderedQueryable() {
     return EnumerableDefaults.asOrderedQueryable(this);
   }
 
-  public T aggregate(Function2<T, T, T> func) {
+  public @Nullable T aggregate(Function2<@Nullable T, T, T> func) {
     return EnumerableDefaults.aggregate(getThis(), func);
   }
 
-  public <TAccumulate> TAccumulate aggregate(TAccumulate seed,
-      Function2<TAccumulate, T, TAccumulate> func) {
+  public <TAccumulate> @PolyNull TAccumulate aggregate(@PolyNull TAccumulate seed,
+      Function2<@PolyNull TAccumulate, T, @PolyNull TAccumulate> func) {
     return EnumerableDefaults.aggregate(getThis(), seed, func);
   }
 
@@ -191,11 +194,11 @@ public <TKey> OrderedEnumerable<T> createOrderedEnumerable(
         keySelector, comparator, descending);
   }
 
-  public Enumerable<T> defaultIfEmpty() {
+  public Enumerable<@Nullable T> defaultIfEmpty() {
     return EnumerableDefaults.defaultIfEmpty(getThis());
   }
 
-  public Enumerable<T>  defaultIfEmpty(T value) {
+  public Enumerable<@PolyNull T> defaultIfEmpty(@PolyNull T value) {
     return EnumerableDefaults.defaultIfEmpty(getThis(), value);
   }
 
@@ -211,7 +214,7 @@ public T elementAt(int index) {
     return EnumerableDefaults.elementAt(getThis(), index);
   }
 
-  public T elementAtOrDefault(int index) {
+  public @Nullable T elementAtOrDefault(int index) {
     return EnumerableDefaults.elementAtOrDefault(getThis(), index);
   }
 
@@ -241,11 +244,11 @@ public T first(Predicate1<T> predicate) {
     return EnumerableDefaults.first(getThis(), predicate);
   }
 
-  public T firstOrDefault() {
+  public @Nullable T firstOrDefault() {
     return EnumerableDefaults.firstOrDefault(getThis());
   }
 
-  public T firstOrDefault(Predicate1<T> predicate) {
+  public @Nullable T firstOrDefault(Predicate1<T> predicate) {
     return EnumerableDefaults.firstOrDefault(getThis(), predicate);
   }
 
@@ -428,11 +431,11 @@ public T last(Predicate1<T> predicate) {
     return EnumerableDefaults.last(getThis(), predicate);
   }
 
-  public T lastOrDefault() {
+  public @Nullable T lastOrDefault() {
     return EnumerableDefaults.lastOrDefault(getThis());
   }
 
-  public T lastOrDefault(Predicate1<T> predicate) {
+  public @Nullable T lastOrDefault(Predicate1<T> predicate) {
     return EnumerableDefaults.lastOrDefault(getThis(), predicate);
   }
 
@@ -621,11 +624,11 @@ public T single(Predicate1<T> predicate) {
     return EnumerableDefaults.single(getThis(), predicate);
   }
 
-  public T singleOrDefault() {
+  public @Nullable T singleOrDefault() {
     return EnumerableDefaults.singleOrDefault(getThis());
   }
 
-  public T singleOrDefault(Predicate1<T> predicate) {
+  public @Nullable T singleOrDefault(Predicate1<T> predicate) {
     return EnumerableDefaults.singleOrDefault(getThis(), predicate);
   }
 

linq4j/src/main/java/org/apache/calcite/linq4j/DefaultQueryable.java

@@ -33,6 +33,8 @@
 import org.apache.calcite.linq4j.function.Predicate2;
 import org.apache.calcite.linq4j.tree.FunctionExpression;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.math.BigDecimal;
 import java.util.Comparator;
 
@@ -148,7 +150,7 @@ public Enumerator<T> enumerator() {
     return factory.ofType(getThis(), clazz);
   }
 
-  @Override public Queryable<T> defaultIfEmpty() {
+  @Override public Queryable<@Nullable T> defaultIfEmpty() {
     return factory.defaultIfEmpty(getThis());
   }
 
@@ -162,7 +164,7 @@ public Enumerator<T> enumerator() {
 
   // End disambiguate
 
-  public T aggregate(FunctionExpression<Function2<T, T, T>> selector) {
+  public @Nullable T aggregate(FunctionExpression<Function2<@Nullable T, T, T>> selector) {
     return factory.aggregate(getThis(), selector);
   }
 
@@ -243,7 +245,7 @@ public T first(FunctionExpression<Predicate1<T>> predicate) {
     return factory.first(getThis(), predicate);
   }
 
-  public T firstOrDefault(FunctionExpression<Predicate1<T>> predicate) {
+  public @Nullable T firstOrDefault(FunctionExpression<Predicate1<T>> predicate) {
     return factory.firstOrDefault(getThis(), predicate);
   }
 
@@ -421,7 +423,7 @@ public T single(FunctionExpression<Predicate1<T>> predicate) {
     return factory.single(getThis(), predicate);
   }
 
-  public T singleOrDefault(FunctionExpression<Predicate1<T>> predicate) {
+  public @Nullable T singleOrDefault(FunctionExpression<Predicate1<T>> predicate) {
     return factory.singleOrDefault(getThis(), predicate);
   }
 

linq4j/src/main/java/org/apache/calcite/linq4j/Enumerable.java

@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j;
 
+import org.checkerframework.framework.qual.Covariant;
+
 /**
  * Exposes the enumerator, which supports a simple iteration over a collection.
  *
@@ -26,6 +28,7 @@
  *
  * @param <T> Element type
  */
+@Covariant(0)
 public interface Enumerable<T>
     extends RawEnumerable<T>, Iterable<T>, ExtendedEnumerable<T> {
   /**