Fix/suppress more warnings.

For the mutex change, compare cl/660005124.

Also, sneak in an implementation comment that would have been better in cl/704698587.

RELNOTES=n/a
PiperOrigin-RevId: 704748981

android/guava-tests/test/com/google/common/collect/SynchronizedNavigableMapTest.java

@@ -261,7 +261,7 @@ public static TestSuite suite() {
     suite.addTest(
         NavigableMapTestSuiteBuilder.using(
                 new TestStringSortedMapGenerator() {
-                  private final Object mutex = new Integer(1);
+                  private final Object mutex = new Object[0]; // something Serializable
 
                   @Override
                   protected SortedMap<String, String> create(Entry<String, String>[] entries) {

android/guava-tests/test/com/google/common/math/DoubleMathTest.java

@@ -447,6 +447,7 @@ public void testLog2NaNInfinity() {
   }
 
   @GwtIncompatible // StrictMath
+  @SuppressWarnings("strictfp") // Guava still supports Java 8
   private strictfp double trueLog2(double d) {
     double trueLog2 = StrictMath.log(d) / StrictMath.log(2);
     // increment until it's >= the true value

guava-tests/test/com/google/common/collect/SynchronizedNavigableMapTest.java

@@ -261,7 +261,7 @@ public static TestSuite suite() {
     suite.addTest(
         NavigableMapTestSuiteBuilder.using(
                 new TestStringSortedMapGenerator() {
-                  private final Object mutex = new Integer(1);
+                  private final Object mutex = new Object[0]; // something Serializable
 
                   @Override
                   protected SortedMap<String, String> create(Entry<String, String>[] entries) {

guava-tests/test/com/google/common/math/DoubleMathTest.java

@@ -447,6 +447,7 @@ public void testLog2NaNInfinity() {
   }
 
   @GwtIncompatible // StrictMath
+  @SuppressWarnings("strictfp") // Guava still supports Java 8
   private strictfp double trueLog2(double d) {
     double trueLog2 = StrictMath.log(d) / StrictMath.log(2);
     // increment until it's >= the true value

guava/src/com/google/common/base/Joiner.java

@@ -227,10 +227,18 @@ public String join(Iterable<? extends @Nullable Object> parts) {
       int i = 0;
       for (Object part : parts) {
         if (i == toJoin.length) {
+          /*
+           * We first initialized toJoin to the size of the input collection. However, that size can
+           * go out of date (for a collection like CopyOnWriteArrayList, which may have been safely
+           * modified concurrently), or it might have been only an estimate to begin with (for a
+           * collection like ConcurrentHashMap, which sums up several counters that may not be in
+           * sync with one another). We accommodate that by resizing as necessary.
+           */
           toJoin = Arrays.copyOf(toJoin, expandedCapacity(toJoin.length, toJoin.length + 1));
         }
         toJoin[i++] = toString(part);
       }
+      // We might not have seen the expected number of elements, as discussed above.
       if (i != toJoin.length) {
         toJoin = Arrays.copyOf(toJoin, i);
       }