Return a char from formatReleaseVersion

Currently it always returns a `String` of length 1. This matches code in the JDK that is going to require updates for JDK 36, but I'm going to worry about that later.

The motivation is that this is the only place that `String#contains` is used in a context where string views could be used instead.

PiperOrigin-RevId: 690664119

java/com/google/turbine/binder/CtSymClassBinder.java

@@ -16,7 +16,6 @@
 
 package com.google.turbine.binder;
 
-import static com.google.common.base.Ascii.toUpperCase;
 import static com.google.common.base.StandardSystemProperty.JAVA_HOME;
 import static java.util.Objects.requireNonNull;
 
@@ -70,10 +69,7 @@ public final class CtSymClassBinder {
             return map.get(sym);
           }
         };
-    // ct.sym contains directories whose names are the concatentation of a list of target versions
-    // formatted as a single character 0-9 or A-Z (e.g. 789A) and which contain interface class
-    // files with a .sig extension.
-    String releaseString = formatReleaseVersion(version);
+    char releaseChar = formatReleaseVersion(version);
     for (Zip.Entry ze : new Zip.ZipIterable(ctSym)) {
       String name = ze.name();
       if (!name.endsWith(".sig")) {
@@ -84,7 +80,7 @@ public final class CtSymClassBinder {
         continue;
       }
       // check if the directory matches the desired release
-      if (!ze.name().substring(0, idx).contains(releaseString)) {
+      if (ze.name().substring(0, idx).indexOf(releaseChar) == -1) {
         continue;
       }
       // JDK >= 12 includes the module name as a prefix
@@ -138,12 +134,21 @@ public byte[] get() {
         });
   }
 
+  // ct.sym contains directories whose names are the concatenation of a list of target versions
+  // formatted as a single character 0-9 or A-Z (e.g. 789A) and which contain interface class
+  // files with a .sig extension.
+  // This was updated to use 36 as a radix in https://bugs.openjdk.org/browse/JDK-8245544,
+  // it's not clear what the plan is for JDK 36.
   @VisibleForTesting
-  static String formatReleaseVersion(int n) {
+  static char formatReleaseVersion(int n) {
     if (n <= 4 || n >= 36) {
       throw new IllegalArgumentException("invalid release version: " + n);
     }
-    return toUpperCase(Integer.toString(n, 36));
+    if (n < 10) {
+      return (char) ('0' + n);
+    } else {
+      return (char) ('A' + n - 10);
+    }
   }
 
   private CtSymClassBinder() {}

javatests/com/google/turbine/binder/CtSymClassBinderTest.java

@@ -31,12 +31,14 @@ public class CtSymClassBinderTest {
   public void formatReleaseVersion() {
     ImmutableList.of(5, 6, 7, 8, 9)
         .forEach(
-            x -> assertThat(CtSymClassBinder.formatReleaseVersion(x)).isEqualTo(String.valueOf(x)));
+            x ->
+                assertThat(Character.toString(CtSymClassBinder.formatReleaseVersion(x)))
+                    .isEqualTo(String.valueOf(x)));
     ImmutableMap.of(
-            10, "A",
-            11, "B",
-            12, "C",
-            35, "Z")
+            10, 'A',
+            11, 'B',
+            12, 'C',
+            35, 'Z')
         .forEach((k, v) -> assertThat(CtSymClassBinder.formatReleaseVersion(k)).isEqualTo(v));
     ImmutableList.of(4, 36)
         .forEach(