diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 000000000000..0067d4488ef2
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,2 @@
+[*.kt]
+indent_size = 2
diff --git a/build.gradle b/build.gradle
index 6d9974354f02..9e8df402ea93 100644
--- a/build.gradle
+++ b/build.gradle
@@ -15,10 +15,10 @@ buildscript {
       'jnrUnixsocket': '0.28',
       'jsoup': '1.13.1',
       'junit': '4.13',
-      'kotlin': '1.3.72',
+      'kotlin': '1.4.0',
       'moshi': '1.9.2',
-      'okio': '2.7.0',
-      'ktlint': '0.36.0',
+      'okio': '2.8.0',
+      'ktlint': '0.38.0',
       'picocli': '4.2.0',
       'openjsse': '1.1.0'
   ]
@@ -47,11 +47,9 @@ buildscript {
   ]
 
   dependencies {
-    classpath 'net.ltgt.gradle:gradle-errorprone-plugin:1.1.1'
-    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:${versions.kotlin}"
+    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.4.0"
     classpath "org.jetbrains.dokka:dokka-gradle-plugin:0.10.1"
-    classpath 'com.diffplug.spotless:spotless-plugin-gradle:3.27.1'
-    classpath 'com.android.tools.build:gradle:3.6.2'
+    classpath "com.android.tools.build:gradle:4.0.1"
   }
 
   repositories {
@@ -62,9 +60,11 @@ buildscript {
 }
 
 plugins {
-  id 'ru.vyarus.animalsniffer' version '1.5.0'
-  id 'com.github.johnrengelman.shadow' version '5.2.0'
-  id 'me.champeau.gradle.japicmp' version '0.2.9'
+  id "ru.vyarus.animalsniffer" version "1.5.0"
+  id "com.github.johnrengelman.shadow" version "5.2.0"
+  id "me.champeau.gradle.japicmp" version "0.2.9"
+  id "com.diffplug.spotless" version "5.1.2"
+  id "net.ltgt.errorprone" version "1.2.1"
 }
 
 /** Returns the artifact ID for the project, or null if it is not published. */
@@ -112,14 +112,12 @@ subprojects { project ->
   if (project.name == 'android-test') return
   if (project.name == 'okhttp-bom') return
 
+  apply plugin: "org.jetbrains.kotlin.jvm"
   apply plugin: 'java'
   apply plugin: 'java-library'
-  apply plugin: 'org.jetbrains.kotlin.platform.jvm'
   apply plugin: 'checkstyle'
   apply plugin: 'ru.vyarus.animalsniffer'
-  apply plugin: 'net.ltgt.errorprone'
   apply plugin: 'org.jetbrains.dokka'
-  apply plugin: 'com.diffplug.gradle.spotless'
   sourceCompatibility = JavaVersion.VERSION_1_8
   targetCompatibility = JavaVersion.VERSION_1_8
 
@@ -152,13 +150,6 @@ subprojects { project ->
     signature 'org.codehaus.mojo.signature:java18:1.0@signature'
   }
 
-  spotless {
-    kotlin {
-      target "**/*.kt"
-      ktlint(versions.ktlint).userData(['indent_size': '2', 'continuation_indent_size': '2'])
-    }
-  }
-
   tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
     kotlinOptions {
       jvmTarget = "1.8"
@@ -205,20 +196,9 @@ subprojects { project ->
     }
   }
 
-  dependencies {
-    //noinspection GradleDynamicVersion
-    errorproneJavac 'com.google.errorprone:javac:9+181-r4173-1'
-    errorprone 'com.google.errorprone:error_prone_core:2.3.4'
-  }
   tasks.withType(JavaCompile).configureEach {
     sourceCompatibility = JavaVersion.VERSION_1_8
     targetCompatibility = JavaVersion.VERSION_1_8
-
-    options.errorprone {
-      check("MissingFail", CheckSeverity.ERROR)
-      check("MissingOverride", CheckSeverity.ERROR)
-      enabled = JavaVersion.current() < JavaVersion.VERSION_12
-    }
   }
 
   dokka {
diff --git a/okcurl/src/main/kotlin/okhttp3/curl/Main.kt b/okcurl/src/main/kotlin/okhttp3/curl/Main.kt
index 17dd983323ed..1c6d9d9df4f4 100644
--- a/okcurl/src/main/kotlin/okhttp3/curl/Main.kt
+++ b/okcurl/src/main/kotlin/okhttp3/curl/Main.kt
@@ -168,11 +168,7 @@ class Main : Runnable {
       builder.hostnameVerifier(createInsecureHostnameVerifier())
     }
     if (verbose) {
-      val logger = object : HttpLoggingInterceptor.Logger {
-        override fun log(message: String) {
-          println(message)
-        }
-      }
+      val logger = HttpLoggingInterceptor.Logger(::println)
       builder.eventListenerFactory(LoggingEventListener.Factory(logger))
     }
     return builder.build()
diff --git a/okhttp-logging-interceptor/src/main/kotlin/okhttp3/logging/HttpLoggingInterceptor.kt b/okhttp-logging-interceptor/src/main/kotlin/okhttp3/logging/HttpLoggingInterceptor.kt
index 0133309c669e..5b8537eb0dcb 100644
--- a/okhttp-logging-interceptor/src/main/kotlin/okhttp3/logging/HttpLoggingInterceptor.kt
+++ b/okhttp-logging-interceptor/src/main/kotlin/okhttp3/logging/HttpLoggingInterceptor.kt
@@ -104,7 +104,7 @@ class HttpLoggingInterceptor @JvmOverloads constructor(
     BODY
   }
 
-  interface Logger {
+  fun interface Logger {
     fun log(message: String)
 
     companion object {
diff --git a/okhttp-logging-interceptor/src/test/java/okhttp3/logging/HttpLoggingInterceptorTest.java b/okhttp-logging-interceptor/src/test/java/okhttp3/logging/HttpLoggingInterceptorTest.java
index 105dd5c4472f..2cb920a07675 100644
--- a/okhttp-logging-interceptor/src/test/java/okhttp3/logging/HttpLoggingInterceptorTest.java
+++ b/okhttp-logging-interceptor/src/test/java/okhttp3/logging/HttpLoggingInterceptorTest.java
@@ -107,7 +107,7 @@ private void setLevel(Level level) {
     try {
       applicationInterceptor.setLevel(null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
   }
 
diff --git a/okhttp-sse/src/main/kotlin/okhttp3/sse/EventSource.kt b/okhttp-sse/src/main/kotlin/okhttp3/sse/EventSource.kt
index 38174e1ab042..ec280d5b5df9 100644
--- a/okhttp-sse/src/main/kotlin/okhttp3/sse/EventSource.kt
+++ b/okhttp-sse/src/main/kotlin/okhttp3/sse/EventSource.kt
@@ -27,7 +27,7 @@ interface EventSource {
    */
   fun cancel()
 
-  interface Factory {
+  fun interface Factory {
     /**
      * Creates a new event source and immediately returns it. Creating an event source initiates an
      * asynchronous process to connect the socket. Once that succeeds or fails, `listener` will be
diff --git a/okhttp-sse/src/main/kotlin/okhttp3/sse/EventSources.kt b/okhttp-sse/src/main/kotlin/okhttp3/sse/EventSources.kt
index 8059ee709d7c..fcb6a555030e 100644
--- a/okhttp-sse/src/main/kotlin/okhttp3/sse/EventSources.kt
+++ b/okhttp-sse/src/main/kotlin/okhttp3/sse/EventSources.kt
@@ -16,25 +16,22 @@
 package okhttp3.sse
 
 import okhttp3.OkHttpClient
-import okhttp3.Request
 import okhttp3.Response
 import okhttp3.internal.sse.RealEventSource
 
 object EventSources {
   @JvmStatic
   fun createFactory(client: OkHttpClient): EventSource.Factory {
-    return object : EventSource.Factory {
-      override fun newEventSource(request: Request, listener: EventSourceListener): EventSource {
-        val actualRequest =
-          if (request.header("Accept") == null) {
-            request.newBuilder().addHeader("Accept", "text/event-stream").build()
-          } else {
-            request
-          }
-
-        return RealEventSource(actualRequest, listener).apply {
-          connect(client)
+    return EventSource.Factory { request, listener ->
+      val actualRequest =
+        if (request.header("Accept") == null) {
+          request.newBuilder().addHeader("Accept", "text/event-stream").build()
+        } else {
+          request
         }
+
+      RealEventSource(actualRequest, listener).apply {
+        connect(client)
       }
     }
   }
diff --git a/okhttp-testing-support/src/main/kotlin/okhttp3/OkHttpClientTestRule.kt b/okhttp-testing-support/src/main/kotlin/okhttp3/OkHttpClientTestRule.kt
index 8382129393b9..e0174acb4caa 100644
--- a/okhttp-testing-support/src/main/kotlin/okhttp3/OkHttpClientTestRule.kt
+++ b/okhttp-testing-support/src/main/kotlin/okhttp3/OkHttpClientTestRule.kt
@@ -92,14 +92,11 @@ class OkHttpClientTestRule : TestRule {
     Logger.getLogger("javax.net.ssl").fn()
   }
 
-  fun wrap(eventListener: EventListener) = object : EventListener.Factory {
-    override fun create(call: Call) = ClientRuleEventListener(eventListener) { addEvent(it) }
-  }
+  fun wrap(eventListener: EventListener) =
+    EventListener.Factory { ClientRuleEventListener(eventListener, ::addEvent) }
 
-  fun wrap(eventListenerFactory: EventListener.Factory) = object : EventListener.Factory {
-    override fun create(call: Call) =
-      ClientRuleEventListener(eventListenerFactory.create(call)) { addEvent(it) }
-  }
+  fun wrap(eventListenerFactory: EventListener.Factory) =
+    EventListener.Factory { call -> ClientRuleEventListener(eventListenerFactory.create(call), ::addEvent) }
 
   /**
    * Returns an OkHttpClient for tests to use as a starting point.
@@ -115,9 +112,8 @@ class OkHttpClientTestRule : TestRule {
     if (client == null) {
       client = OkHttpClient.Builder()
           .dns(SINGLE_INET_ADDRESS_DNS) // Prevent unexpected fallback addresses.
-          .eventListenerFactory(object : EventListener.Factory {
-            override fun create(call: Call) = ClientRuleEventListener { addEvent(it) }
-          })
+          .eventListenerFactory(
+              EventListener.Factory { ClientRuleEventListener(logger = ::addEvent) })
           .build()
       testClient = client
     }
diff --git a/okhttp/build.gradle b/okhttp/build.gradle
index 6804ce950794..df42f42810a2 100644
--- a/okhttp/build.gradle
+++ b/okhttp/build.gradle
@@ -108,6 +108,8 @@ task japicmp(type: me.champeau.gradle.japicmp.JapicmpTask, dependsOn: 'jar') {
       'okhttp3.OkHttpClient#socketFactory()',
       'okhttp3.OkHttpClient#sslSocketFactory()',
       'okhttp3.OkHttpClient#writeTimeoutMillis()',
+      'okhttp3.OkHttpClient#writeTimeoutMillis()',
+      'okhttp3.Request$Builder#delete()',
   ]
 }
 check.dependsOn(japicmp)
diff --git a/okhttp/src/main/kotlin/okhttp3/Authenticator.kt b/okhttp/src/main/kotlin/okhttp3/Authenticator.kt
index 4cb89fb0c9b8..0d6872edc6b6 100644
--- a/okhttp/src/main/kotlin/okhttp3/Authenticator.kt
+++ b/okhttp/src/main/kotlin/okhttp3/Authenticator.kt
@@ -112,7 +112,7 @@ import okhttp3.internal.authenticator.JavaNetAuthenticator
  *
  * [1]: https://tools.ietf.org/html/rfc2817
  */
-interface Authenticator {
+fun interface Authenticator {
   /**
    * Returns a request that includes a credential to satisfy an authentication challenge in
    * [response]. Returns null if the challenge cannot be satisfied.
diff --git a/okhttp/src/main/kotlin/okhttp3/Cache.kt b/okhttp/src/main/kotlin/okhttp3/Cache.kt
index 491c8dac9911..82baf682d63d 100644
--- a/okhttp/src/main/kotlin/okhttp3/Cache.kt
+++ b/okhttp/src/main/kotlin/okhttp3/Cache.kt
@@ -386,12 +386,12 @@ class Cache internal constructor(
 
   @Synchronized fun requestCount(): Int = requestCount
 
-  private inner class RealCacheRequest internal constructor(
+  private inner class RealCacheRequest(
     private val editor: DiskLruCache.Editor
   ) : CacheRequest {
     private val cacheOut: Sink = editor.newSink(ENTRY_BODY)
     private val body: Sink
-    internal var done = false
+    var done = false
 
     init {
       this.body = object : ForwardingSink(cacheOut) {
@@ -489,8 +489,7 @@ class Cache internal constructor(
      * each on their own line. A length of -1 is used to encode a null array. The last line is
      * optional. If present, it contains the TLS version.
      */
-    @Throws(IOException::class)
-    internal constructor(rawSource: Source) {
+    @Throws(IOException::class) constructor(rawSource: Source) {
       try {
         val source = rawSource.buffer()
         url = source.readUtf8LineStrict()
@@ -542,7 +541,7 @@ class Cache internal constructor(
       }
     }
 
-    internal constructor(response: Response) {
+    constructor(response: Response) {
       this.url = response.request.url.toString()
       this.varyHeaders = response.varyHeaders()
       this.requestMethod = response.request.method
@@ -665,8 +664,8 @@ class Cache internal constructor(
     }
   }
 
-  private class CacheResponseBody internal constructor(
-    internal val snapshot: DiskLruCache.Snapshot,
+  private class CacheResponseBody(
+    val snapshot: DiskLruCache.Snapshot,
     private val contentType: String?,
     private val contentLength: String?
   ) : ResponseBody() {
diff --git a/okhttp/src/main/kotlin/okhttp3/Call.kt b/okhttp/src/main/kotlin/okhttp3/Call.kt
index 6df8da0e9cbd..66677f62726c 100644
--- a/okhttp/src/main/kotlin/okhttp3/Call.kt
+++ b/okhttp/src/main/kotlin/okhttp3/Call.kt
@@ -93,7 +93,7 @@ interface Call : Cloneable {
    */
   public override fun clone(): Call
 
-  interface Factory {
+  fun interface Factory {
     fun newCall(request: Request): Call
   }
 }
diff --git a/okhttp/src/main/kotlin/okhttp3/EventListener.kt b/okhttp/src/main/kotlin/okhttp3/EventListener.kt
index ea0b6eb45071..b742d1d9b650 100644
--- a/okhttp/src/main/kotlin/okhttp3/EventListener.kt
+++ b/okhttp/src/main/kotlin/okhttp3/EventListener.kt
@@ -457,7 +457,7 @@ abstract class EventListener {
   open fun cacheConditionalHit(call: Call, cachedResponse: Response) {
   }
 
-  interface Factory {
+  fun interface Factory {
     /**
      * Creates an instance of the [EventListener] for a particular [Call]. The returned
      * [EventListener] instance will be used during the lifecycle of [call].
diff --git a/okhttp/src/main/kotlin/okhttp3/Handshake.kt b/okhttp/src/main/kotlin/okhttp3/Handshake.kt
index 4bc98acc1b69..e030b1be4b8f 100644
--- a/okhttp/src/main/kotlin/okhttp3/Handshake.kt
+++ b/okhttp/src/main/kotlin/okhttp3/Handshake.kt
@@ -52,7 +52,7 @@ class Handshake internal constructor(
     try {
       peerCertificatesFn()
     } catch (spue: SSLPeerUnverifiedException) {
-      listOf<Certificate>()
+      listOf()
     }
   }
 
@@ -160,7 +160,7 @@ class Handshake internal constructor(
       val peerCertificatesCopy = try {
         peerCertificates.toImmutableList()
       } catch (_: SSLPeerUnverifiedException) {
-        listOf<Certificate>()
+        listOf()
       }
 
       return Handshake(tlsVersion, cipherSuite,
diff --git a/okhttp/src/main/kotlin/okhttp3/Interceptor.kt b/okhttp/src/main/kotlin/okhttp3/Interceptor.kt
index 2452cff367f7..8da5b803d468 100644
--- a/okhttp/src/main/kotlin/okhttp3/Interceptor.kt
+++ b/okhttp/src/main/kotlin/okhttp3/Interceptor.kt
@@ -23,7 +23,7 @@ import java.util.concurrent.TimeUnit
  * responses coming back in. Typically interceptors add, remove, or transform headers on the request
  * or response.
  */
-interface Interceptor {
+fun interface Interceptor {
   @Throws(IOException::class)
   fun intercept(chain: Chain): Response
 
@@ -39,9 +39,7 @@ interface Interceptor {
      * ```
      */
     inline operator fun invoke(crossinline block: (chain: Chain) -> Response): Interceptor =
-        object : Interceptor {
-          override fun intercept(chain: Chain) = block(chain)
-        }
+      Interceptor { block(it) }
   }
 
   interface Chain {
diff --git a/okhttp/src/main/kotlin/okhttp3/WebSocket.kt b/okhttp/src/main/kotlin/okhttp3/WebSocket.kt
index 01d2faa245d0..ac731e62e1b1 100644
--- a/okhttp/src/main/kotlin/okhttp3/WebSocket.kt
+++ b/okhttp/src/main/kotlin/okhttp3/WebSocket.kt
@@ -109,7 +109,7 @@ interface WebSocket {
    */
   fun cancel()
 
-  interface Factory {
+  fun interface Factory {
     /**
      * Creates a new web socket and immediately returns it. Creating a web socket initiates an
      * asynchronous process to connect the socket. Once that succeeds or fails, `listener` will be
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/Util.kt b/okhttp/src/main/kotlin/okhttp3/internal/Util.kt
index bb5a569ae749..55a8b7b2c76c 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/Util.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/Util.kt
@@ -29,7 +29,6 @@ import java.nio.charset.Charset
 import java.nio.charset.StandardCharsets.UTF_16BE
 import java.nio.charset.StandardCharsets.UTF_16LE
 import java.nio.charset.StandardCharsets.UTF_8
-import java.util.Arrays
 import java.util.Collections
 import java.util.Comparator
 import java.util.LinkedHashMap
@@ -300,9 +299,7 @@ fun HttpUrl.canReuseConnectionFor(other: HttpUrl): Boolean = host == other.host
     port == other.port &&
     scheme == other.scheme
 
-fun EventListener.asFactory() = object : EventListener.Factory {
-  override fun create(call: Call): EventListener = this@asFactory
-}
+fun EventListener.asFactory() = EventListener.Factory { this }
 
 infix fun Byte.and(mask: Int): Int = toInt() and mask
 infix fun Short.and(mask: Int): Int = toInt() and mask
@@ -378,18 +375,18 @@ fun Socket.peerName(): String {
  * @param source the source used to read bytes from the socket.
  */
 fun Socket.isHealthy(source: BufferedSource): Boolean {
-  try {
+  return try {
     val readTimeout = soTimeout
     try {
       soTimeout = 1
-      return !source.exhausted()
+      !source.exhausted()
     } finally {
       soTimeout = readTimeout
     }
   } catch (_: SocketTimeoutException) {
-    return true // Read timed out; socket is good.
+    true // Read timed out; socket is good.
   } catch (_: IOException) {
-    return false // Couldn't read; socket is closed.
+    false // Couldn't read; socket is closed.
   }
 }
 
@@ -473,7 +470,7 @@ fun <T> List<T>.toImmutableList(): List<T> {
 /** Returns an immutable list containing [elements]. */
 @SafeVarargs
 fun <T> immutableListOf(vararg elements: T): List<T> {
-  return Collections.unmodifiableList(Arrays.asList(*elements.clone()))
+  return Collections.unmodifiableList(listOf(*elements.clone()))
 }
 
 /** Returns an immutable copy of this. */
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/connection/Exchange.kt b/okhttp/src/main/kotlin/okhttp3/internal/connection/Exchange.kt
index fc44f72be339..ca93f27c4b4c 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/connection/Exchange.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/connection/Exchange.kt
@@ -198,7 +198,7 @@ class Exchange(
   }
 
   /** A request body that fires events when it completes. */
-  private inner class RequestBodySink internal constructor(
+  private inner class RequestBodySink(
     delegate: Sink,
     /** The exact number of bytes to be written, or -1L if that is unknown. */
     private val contentLength: Long
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/http1/Http1ExchangeCodec.kt b/okhttp/src/main/kotlin/okhttp3/internal/http1/Http1ExchangeCodec.kt
index 76f03597dac7..156085e14329 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/http1/Http1ExchangeCodec.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/http1/Http1ExchangeCodec.kt
@@ -340,7 +340,7 @@ class Http1ExchangeCodec(
      * Closes the cache entry and makes the socket available for reuse. This should be invoked when
      * the end of the body has been reached.
      */
-    internal fun responseBodyComplete() {
+    fun responseBodyComplete() {
       if (state == STATE_CLOSED) return
       if (state != STATE_READING_RESPONSE_BODY) throw IllegalStateException("state: $state")
 
@@ -351,7 +351,7 @@ class Http1ExchangeCodec(
   }
 
   /** An HTTP body with a fixed length specified in advance. */
-  private inner class FixedLengthSource internal constructor(private var bytesRemaining: Long) :
+  private inner class FixedLengthSource(private var bytesRemaining: Long) :
       AbstractSource() {
 
     init {
@@ -394,7 +394,7 @@ class Http1ExchangeCodec(
   }
 
   /** An HTTP body with alternating chunk sizes and chunk bodies. */
-  private inner class ChunkedSource internal constructor(private val url: HttpUrl) :
+  private inner class ChunkedSource(private val url: HttpUrl) :
       AbstractSource() {
     private var bytesRemainingInChunk = NO_CHUNK_YET
     private var hasMoreChunks = true
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/http2/Http2Connection.kt b/okhttp/src/main/kotlin/okhttp3/internal/http2/Http2Connection.kt
index 80cd5875dc3a..3f5c0714174a 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/http2/Http2Connection.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/http2/Http2Connection.kt
@@ -737,10 +737,10 @@ class Http2Connection internal constructor(builder: Builder) : Closeable {
       synchronized(writer) {
         synchronized(this@Http2Connection) {
           val previousPeerSettings = peerSettings
-          if (clearPrevious) {
-            newPeerSettings = settings
+          newPeerSettings = if (clearPrevious) {
+            settings
           } else {
-            newPeerSettings = Settings().apply {
+            Settings().apply {
               merge(previousPeerSettings)
               merge(settings)
             }
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/http2/Http2Reader.kt b/okhttp/src/main/kotlin/okhttp3/internal/http2/Http2Reader.kt
index 8ad9428f0a8b..477ec3bd1260 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/http2/Http2Reader.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/http2/Http2Reader.kt
@@ -75,7 +75,7 @@ class Http2Reader(
       }
     } else {
       // The server reads the CONNECTION_PREFACE byte string.
-      val connectionPreface = source.readByteString(Http2.CONNECTION_PREFACE.size.toLong())
+      val connectionPreface = source.readByteString(CONNECTION_PREFACE.size.toLong())
       if (logger.isLoggable(FINE)) logger.fine(format("<< CONNECTION ${connectionPreface.hex()}"))
       if (CONNECTION_PREFACE != connectionPreface) {
         throw IOException("Expected a connection header but was ${connectionPreface.utf8()}")
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/platform/BouncyCastlePlatform.kt b/okhttp/src/main/kotlin/okhttp3/internal/platform/BouncyCastlePlatform.kt
index d53ebe46cf16..e0bcb5530450 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/platform/BouncyCastlePlatform.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/platform/BouncyCastlePlatform.kt
@@ -48,7 +48,7 @@ class BouncyCastlePlatform private constructor() : Platform() {
     return trustManagers[0] as X509TrustManager
   }
 
-  public override fun trustManager(sslSocketFactory: SSLSocketFactory): X509TrustManager? =
+  override fun trustManager(sslSocketFactory: SSLSocketFactory): X509TrustManager? =
       throw UnsupportedOperationException(
           "clientBuilder.sslSocketFactory(SSLSocketFactory) not supported with BouncyCastle")
 
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/platform/Jdk8WithJettyBootPlatform.kt b/okhttp/src/main/kotlin/okhttp3/internal/platform/Jdk8WithJettyBootPlatform.kt
index 8d59af4b0b11..e7ffc9437971 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/platform/Jdk8WithJettyBootPlatform.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/platform/Jdk8WithJettyBootPlatform.kt
@@ -77,14 +77,14 @@ class Jdk8WithJettyBootPlatform(
    * Handle the methods of ALPN's ClientProvider and ServerProvider without a compile-time
    * dependency on those interfaces.
    */
-  private class AlpnProvider internal constructor(
+  private class AlpnProvider(
     /** This peer's supported protocols. */
     private val protocols: List<String>
   ) : InvocationHandler {
     /** Set when remote peer notifies ALPN is unsupported. */
-    internal var unsupported: Boolean = false
+    var unsupported: Boolean = false
     /** The protocol the server selected. */
-    internal var selected: String? = null
+    var selected: String? = null
 
     @Throws(Throwable::class)
     override fun invoke(proxy: Any, method: Method, args: Array<Any>?): Any? {
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/platform/Jdk9Platform.kt b/okhttp/src/main/kotlin/okhttp3/internal/platform/Jdk9Platform.kt
index fd4f0b56de1a..fd29f340a44e 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/platform/Jdk9Platform.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/platform/Jdk9Platform.kt
@@ -53,7 +53,7 @@ open class Jdk9Platform : Platform() {
     }
   }
 
-  public override fun trustManager(sslSocketFactory: SSLSocketFactory): X509TrustManager? {
+  override fun trustManager(sslSocketFactory: SSLSocketFactory): X509TrustManager? {
     // Not supported due to access checks on JDK 9+:
     // java.lang.reflect.InaccessibleObjectException: Unable to make member of class
     // sun.security.ssl.SSLSocketFactoryImpl accessible:  module java.base does not export
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/platform/OpenJSSEPlatform.kt b/okhttp/src/main/kotlin/okhttp3/internal/platform/OpenJSSEPlatform.kt
index 15818d289e6a..040585cbe153 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/platform/OpenJSSEPlatform.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/platform/OpenJSSEPlatform.kt
@@ -50,7 +50,7 @@ class OpenJSSEPlatform private constructor() : Platform() {
     return trustManagers[0] as X509TrustManager
   }
 
-  public override fun trustManager(sslSocketFactory: SSLSocketFactory): X509TrustManager? =
+  override fun trustManager(sslSocketFactory: SSLSocketFactory): X509TrustManager? =
       throw UnsupportedOperationException(
           "clientBuilder.sslSocketFactory(SSLSocketFactory) not supported with OpenJSSE")
 
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/publicsuffix/PublicSuffixDatabase.kt b/okhttp/src/main/kotlin/okhttp3/internal/publicsuffix/PublicSuffixDatabase.kt
index 614fd818eb71..451f67e60ae7 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/publicsuffix/PublicSuffixDatabase.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/publicsuffix/PublicSuffixDatabase.kt
@@ -203,8 +203,8 @@ class PublicSuffixDatabase {
 
   @Throws(IOException::class)
   private fun readTheList() {
-    var publicSuffixListBytes: ByteArray? = null
-    var publicSuffixExceptionListBytes: ByteArray? = null
+    var publicSuffixListBytes: ByteArray?
+    var publicSuffixExceptionListBytes: ByteArray?
 
     val resource =
         PublicSuffixDatabase::class.java.getResourceAsStream(PUBLIC_SUFFIX_RESOURCE) ?: return
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/tls/BasicCertificateChainCleaner.kt b/okhttp/src/main/kotlin/okhttp3/internal/tls/BasicCertificateChainCleaner.kt
index 35d7f721fdf6..ee8a78414120 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/tls/BasicCertificateChainCleaner.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/tls/BasicCertificateChainCleaner.kt
@@ -46,7 +46,7 @@ class BasicCertificateChainCleaner(
    */
   @Throws(SSLPeerUnverifiedException::class)
   override fun clean(chain: List<Certificate>, hostname: String): List<Certificate> {
-    val queue: Deque<Certificate> = ArrayDeque<Certificate>(chain)
+    val queue: Deque<Certificate> = ArrayDeque(chain)
     val result = mutableListOf<Certificate>()
     result.add(queue.removeFirst())
     var foundTrustedCertificate = false
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/ws/WebSocketExtensions.kt b/okhttp/src/main/kotlin/okhttp3/internal/ws/WebSocketExtensions.kt
index 850e7abc7341..4e82c290f8dc 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/ws/WebSocketExtensions.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/ws/WebSocketExtensions.kt
@@ -80,10 +80,10 @@ data class WebSocketExtensions(
 ) {
 
   fun noContextTakeover(clientOriginated: Boolean): Boolean {
-    if (clientOriginated) {
-      return clientNoContextTakeover // Client is deflating.
+    return if (clientOriginated) {
+      clientNoContextTakeover // Client is deflating.
     } else {
-      return serverNoContextTakeover // Server is deflating.
+      serverNoContextTakeover // Server is deflating.
     }
   }
 
diff --git a/okhttp/src/main/kotlin/okhttp3/internal/ws/WebSocketReader.kt b/okhttp/src/main/kotlin/okhttp3/internal/ws/WebSocketReader.kt
index 92b75fc63ca6..6d306cbcd9b4 100644
--- a/okhttp/src/main/kotlin/okhttp3/internal/ws/WebSocketReader.kt
+++ b/okhttp/src/main/kotlin/okhttp3/internal/ws/WebSocketReader.kt
@@ -133,11 +133,11 @@ class WebSocketReader(
     val reservedFlag1 = b0 and B0_FLAG_RSV1 != 0
     when (opcode) {
       OPCODE_TEXT, OPCODE_BINARY -> {
-        if (reservedFlag1) {
+        readingCompressedMessage = if (reservedFlag1) {
           if (!perMessageDeflate) throw ProtocolException("Unexpected rsv1 flag")
-          readingCompressedMessage = true
+          true
         } else {
-          readingCompressedMessage = false
+          false
         }
       }
       else -> {
diff --git a/okhttp/src/test/java/okhttp3/CipherSuiteTest.java b/okhttp/src/test/java/okhttp3/CipherSuiteTest.java
index d70d7d4b17f4..cf3dd0c179cd 100644
--- a/okhttp/src/test/java/okhttp3/CipherSuiteTest.java
+++ b/okhttp/src/test/java/okhttp3/CipherSuiteTest.java
@@ -31,7 +31,7 @@ public class CipherSuiteTest {
     try {
       forJavaName(null);
       fail("Should have thrown");
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
   }
 
diff --git a/okhttp/src/test/java/okhttp3/CookieTest.java b/okhttp/src/test/java/okhttp3/CookieTest.java
index bf2add65e3a9..e90343610746 100644
--- a/okhttp/src/test/java/okhttp3/CookieTest.java
+++ b/okhttp/src/test/java/okhttp3/CookieTest.java
@@ -427,7 +427,7 @@ public final class CookieTest {
     try {
       new Cookie.Builder().name(null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
     try {
       new Cookie.Builder().name(" a ");
@@ -440,7 +440,7 @@ public final class CookieTest {
     try {
       new Cookie.Builder().value(null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
     try {
       new Cookie.Builder().value(" b ");
@@ -485,7 +485,7 @@ public final class CookieTest {
     try {
       new Cookie.Builder().hostOnlyDomain(null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
     try {
       new Cookie.Builder().hostOnlyDomain("a/b");
@@ -518,7 +518,7 @@ public final class CookieTest {
     try {
       new Cookie.Builder().path(null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
     try {
       new Cookie.Builder().path("foo");
diff --git a/okhttp/src/test/java/okhttp3/HeadersTest.java b/okhttp/src/test/java/okhttp3/HeadersTest.java
index 6ada63c49147..cbb86ae8c570 100644
--- a/okhttp/src/test/java/okhttp3/HeadersTest.java
+++ b/okhttp/src/test/java/okhttp3/HeadersTest.java
@@ -15,14 +15,12 @@
  */
 package okhttp3;
 
-import java.io.IOException;
 import java.time.Instant;
 import java.util.Collections;
 import java.util.Date;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
-import kotlin.TypeCastException;
 import okhttp3.internal.Util;
 import okhttp3.internal.http.HttpHeaders;
 import okhttp3.internal.http2.Header;
@@ -201,7 +199,7 @@ public final class HeadersTest {
     try {
       Headers.of(Collections.singletonMap("User-Agent", null));
       fail();
-    } catch (TypeCastException expected) {
+    } catch (NullPointerException expected) {
     }
   }
 
@@ -831,7 +829,7 @@ public final class HeadersTest {
           .add("testDate", (Date) null)
           .build();
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
   }
 
@@ -850,7 +848,7 @@ public final class HeadersTest {
           .add("Test-Instant", (Instant) null)
           .build();
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
   }
 
@@ -870,7 +868,7 @@ public final class HeadersTest {
           .set("testDate", (Date) null)
           .build();
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
   }
 
@@ -890,7 +888,7 @@ public final class HeadersTest {
           .set("Test-Instant", (Instant) null)
           .build();
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
   }
 }
diff --git a/okhttp/src/test/java/okhttp3/OkHttpClientTest.java b/okhttp/src/test/java/okhttp3/OkHttpClientTest.java
index 1724af3ba31c..9dd1e818f780 100644
--- a/okhttp/src/test/java/okhttp3/OkHttpClientTest.java
+++ b/okhttp/src/test/java/okhttp3/OkHttpClientTest.java
@@ -169,7 +169,7 @@ public final class OkHttpClientTest {
     try {
       builder.addInterceptor(null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
   }
 
@@ -178,7 +178,7 @@ public final class OkHttpClientTest {
     try {
       builder.addNetworkInterceptor(null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
   }
 
diff --git a/okhttp/src/test/java/okhttp3/RequestTest.java b/okhttp/src/test/java/okhttp3/RequestTest.java
index 9638b3ba4043..4257f03bdd34 100644
--- a/okhttp/src/test/java/okhttp3/RequestTest.java
+++ b/okhttp/src/test/java/okhttp3/RequestTest.java
@@ -195,22 +195,22 @@ public final class RequestTest {
     try {
       builder.header(null, "Value");
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
     try {
       builder.addHeader(null, "Value");
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
     try {
       builder.header("Name", null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
     try {
       builder.addHeader("Name", null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
   }
 
diff --git a/okhttp/src/test/java/okhttp3/URLConnectionTest.java b/okhttp/src/test/java/okhttp3/URLConnectionTest.java
index 30e3d0213a59..95172f010316 100644
--- a/okhttp/src/test/java/okhttp3/URLConnectionTest.java
+++ b/okhttp/src/test/java/okhttp3/URLConnectionTest.java
@@ -156,25 +156,25 @@ public final class URLConnectionTest {
       new Request.Builder()
           .header(null, "j");
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
     try {
       new Request.Builder()
           .addHeader(null, "k");
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
     try {
       new Request.Builder()
           .addHeader("NullValue", null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
     try {
       new Request.Builder()
           .addHeader("AnotherNullValue", null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
 
     Response response = getResponse(request);
@@ -211,7 +211,7 @@ public final class URLConnectionTest {
     try {
       response.header(null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
     Headers responseHeaders = response.headers();
     assertThat(new LinkedHashSet<>(responseHeaders.values("A"))).isEqualTo(
@@ -3578,6 +3578,8 @@ private void zeroLengthPayload(String method) throws Exception {
     try {
       client.newBuilder().sslSocketFactory(null);
       fail();
+    } catch (NullPointerException expected) {
+      // Kotlin 1.4
     } catch (IllegalArgumentException expected) {
     }
   }
diff --git a/okhttp/src/test/java/okhttp3/internal/publicsuffix/PublicSuffixDatabaseTest.java b/okhttp/src/test/java/okhttp3/internal/publicsuffix/PublicSuffixDatabaseTest.java
index 46a107e46f33..3bc2c1857c27 100644
--- a/okhttp/src/test/java/okhttp3/internal/publicsuffix/PublicSuffixDatabaseTest.java
+++ b/okhttp/src/test/java/okhttp3/internal/publicsuffix/PublicSuffixDatabaseTest.java
@@ -269,7 +269,7 @@ private void checkPublicSuffix(String domain, String registrablePart) {
       try {
         publicSuffixDatabase.getEffectiveTldPlusOne(null);
         fail();
-      } catch (IllegalArgumentException expected) {
+      } catch (NullPointerException expected) {
       }
       return;
     }
diff --git a/okhttp/src/test/java/okhttp3/internal/ws/WebSocketHttpTest.java b/okhttp/src/test/java/okhttp3/internal/ws/WebSocketHttpTest.java
index 166a1263fbfa..82a2bfb9ec58 100644
--- a/okhttp/src/test/java/okhttp3/internal/ws/WebSocketHttpTest.java
+++ b/okhttp/src/test/java/okhttp3/internal/ws/WebSocketHttpTest.java
@@ -139,7 +139,7 @@ private OkHttpClientTestRule configureClientTestRule() {
     try {
       webSocket.send((String) null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
 
     closeWebSockets(webSocket, server);
@@ -156,7 +156,7 @@ private OkHttpClientTestRule configureClientTestRule() {
     try {
       webSocket.send((ByteString) null);
       fail();
-    } catch (IllegalArgumentException expected) {
+    } catch (NullPointerException expected) {
     }
 
     closeWebSockets(webSocket, server);