Avoid calling checkNotNull on nullable values except during actual precondition checks.

It's not that we're not going to make such calls illegal, I promise :) I mean, we certainly aren't going to _in general_, but I am tempted for `com.google.common`, as discussed on cl/372346107 :) (It would have caught the problem of cl/612591080!) I'm testing what would happen if we did do it for `com.google.common` in case it shakes out any more bugs.

It does reveal that I didn't complete the cleanup of cl/612591080. And it reveals a few places where we'd normally use `requireNonNull`, since the checks aren't "preconditions" in the sense of "the caller did something wrong" (from cl/15376243 and cl/526930990). I've made those changes. (I would have made some more changes if I had tried to address more of `com.google.common`. But I stuck to the "main" packages, and I didn't even fix enough errors to see full results.)

Honestly, the more interesting thing that this exercise revealed was that there are more cases in which I'm especially sympathetic to calling `checkNotNull` on nullable values:
- `DummyProxy` is making an `InvocationHandler` perform automatic precondition tests based on annotations on the interface it's implementing.
- `EqualsTester` and Truth have permissive signatures because they're test utilities, as documented in cl/578260904 and discussed during the Truth CLs.

And the yet more interesting thing that it revealed is that we may want to use `@NonNull` here in the future, similar to what we've discussed in #6824.

RELNOTES=n/a
PiperOrigin-RevId: 612937549

Author: cpovirk

android/guava-testlib/src/com/google/common/testing/FreshValueGenerator.java

@@ -18,6 +18,7 @@
 
 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.base.Throwables.throwIfUnchecked;
+import static java.util.Objects.requireNonNull;
 
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
@@ -211,7 +212,7 @@ final <T> T newFreshProxy(final Class<T> interfaceType) {
       return pickInstance(rawType.getEnumConstants(), null);
     }
     if (type.isArray()) {
-      TypeToken<?> componentType = checkNotNull(type.getComponentType());
+      TypeToken<?> componentType = requireNonNull(type.getComponentType());
       Object array = Array.newInstance(componentType.getRawType(), 1);
       Array.set(array, 0, generate(componentType));
       return array;

android/guava/src/com/google/common/collect/TableCollectors.java

@@ -199,14 +199,12 @@ void merge(V value, BinaryOperator<V> mergeFunction) {
     }
   }
 
-  private static <
-          R extends @Nullable Object, C extends @Nullable Object, V extends @Nullable Object>
-      void mergeTables(
-          Table<R, C, V> table,
-          @ParametricNullness R row,
-          @ParametricNullness C column,
-          @ParametricNullness V value,
-          BinaryOperator<V> mergeFunction) {
+  private static <R extends @Nullable Object, C extends @Nullable Object, V> void mergeTables(
+      Table<R, C, V> table,
+      @ParametricNullness R row,
+      @ParametricNullness C column,
+      V value,
+      BinaryOperator<V> mergeFunction) {
     checkNotNull(value);
     V oldValue = table.get(row, column);
     if (oldValue == null) {

android/guava/src/com/google/common/collect/TreeMultimap.java

@@ -17,6 +17,7 @@
 package com.google.common.collect;
 
 import static com.google.common.base.Preconditions.checkNotNull;
+import static java.util.Objects.requireNonNull;
 
 import com.google.common.annotations.GwtCompatible;
 import com.google.common.annotations.GwtIncompatible;
@@ -217,8 +218,8 @@ private void writeObject(ObjectOutputStream stream) throws IOException {
   @SuppressWarnings("unchecked") // reading data stored by writeObject
   private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
     stream.defaultReadObject();
-    keyComparator = checkNotNull((Comparator<? super K>) stream.readObject());
-    valueComparator = checkNotNull((Comparator<? super V>) stream.readObject());
+    keyComparator = requireNonNull((Comparator<? super K>) stream.readObject());
+    valueComparator = requireNonNull((Comparator<? super V>) stream.readObject());
     setMap(new TreeMap<K, Collection<V>>(keyComparator));
     Serialization.populateMultimap(this, stream);
   }

guava-testlib/src/com/google/common/testing/FreshValueGenerator.java

@@ -18,6 +18,7 @@
 
 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.base.Throwables.throwIfUnchecked;
+import static java.util.Objects.requireNonNull;
 
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
@@ -215,7 +216,7 @@ final <T> T newFreshProxy(final Class<T> interfaceType) {
       return pickInstance(rawType.getEnumConstants(), null);
     }
     if (type.isArray()) {
-      TypeToken<?> componentType = checkNotNull(type.getComponentType());
+      TypeToken<?> componentType = requireNonNull(type.getComponentType());
       Object array = Array.newInstance(componentType.getRawType(), 1);
       Array.set(array, 0, generate(componentType));
       return array;

guava/src/com/google/common/collect/TableCollectors.java

@@ -195,14 +195,12 @@ void merge(V value, BinaryOperator<V> mergeFunction) {
     }
   }
 
-  private static <
-          R extends @Nullable Object, C extends @Nullable Object, V extends @Nullable Object>
-      void mergeTables(
-          Table<R, C, V> table,
-          @ParametricNullness R row,
-          @ParametricNullness C column,
-          @ParametricNullness V value,
-          BinaryOperator<V> mergeFunction) {
+  private static <R extends @Nullable Object, C extends @Nullable Object, V> void mergeTables(
+      Table<R, C, V> table,
+      @ParametricNullness R row,
+      @ParametricNullness C column,
+      V value,
+      BinaryOperator<V> mergeFunction) {
     checkNotNull(value);
     V oldValue = table.get(row, column);
     if (oldValue == null) {

guava/src/com/google/common/collect/TreeMultimap.java

@@ -17,6 +17,7 @@
 package com.google.common.collect;
 
 import static com.google.common.base.Preconditions.checkNotNull;
+import static java.util.Objects.requireNonNull;
 
 import com.google.common.annotations.GwtCompatible;
 import com.google.common.annotations.GwtIncompatible;
@@ -217,8 +218,8 @@ private void writeObject(ObjectOutputStream stream) throws IOException {
   @SuppressWarnings("unchecked") // reading data stored by writeObject
   private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
     stream.defaultReadObject();
-    keyComparator = checkNotNull((Comparator<? super K>) stream.readObject());
-    valueComparator = checkNotNull((Comparator<? super V>) stream.readObject());
+    keyComparator = requireNonNull((Comparator<? super K>) stream.readObject());
+    valueComparator = requireNonNull((Comparator<? super V>) stream.readObject());
     setMap(new TreeMap<K, Collection<V>>(keyComparator));
     Serialization.populateMultimap(this, stream);
   }