Make error-prone validate missing override annotations

build.gradle

@@ -1,3 +1,5 @@
+import net.ltgt.gradle.errorprone.CheckSeverity
+
 buildscript {
   ext.versions = [
       'airline': '0.8',
@@ -127,6 +129,11 @@ subprojects { project ->
     errorproneJavac 'com.google.errorprone:javac:9+181-r4173-1'
     errorprone 'com.google.errorprone:error_prone_core:2.3.3'
   }
+  tasks.withType(JavaCompile).configureEach {
+    options.errorprone {
+      check("MissingOverride", CheckSeverity.ERROR)
+    }
+  }
 
   apply plugin: 'org.jetbrains.dokka'
   dokka {

okhttp-logging-interceptor/src/test/java/okhttp3/logging/LoggingEventListenerTest.java

@@ -220,7 +220,7 @@ public final class LoggingEventListenerTest {
   }
 
   private static class LogRecorder extends HttpLoggingInterceptorTest.LogRecorder {
-    LogRecorder assertLogMatch(String pattern) {
+    @Override LogRecorder assertLogMatch(String pattern) {
       return (LogRecorder) super.assertLogMatch("\\[\\d+ ms] " + pattern);
     }
   }

okhttp/src/test/java/okhttp3/DelegatingSSLSocket.java

@@ -287,6 +287,7 @@ public abstract class DelegatingSSLSocket extends SSLSocket {
 
   // Java 9 methods.
 
+  @SuppressWarnings("MissingOverride") // Can only override with JDK 9+
   public SSLSession getHandshakeSession() {
     try {
       return (SSLSession) SSLSocket.class.getMethod("getHandshakeSession").invoke(delegate);
@@ -295,6 +296,7 @@ public abstract class DelegatingSSLSocket extends SSLSocket {
     }
   }
 
+  @SuppressWarnings("MissingOverride") // Can only override with JDK 9+
   public String getApplicationProtocol() {
     try {
       return (String) SSLSocket.class.getMethod("getApplicationProtocol").invoke(delegate);
@@ -303,6 +305,7 @@ public abstract class DelegatingSSLSocket extends SSLSocket {
     }
   }
 
+  @SuppressWarnings("MissingOverride") // Can only override with JDK 9+
   public String getHandshakeApplicationProtocol() {
     try {
       return (String) SSLSocket.class.getMethod("getHandshakeApplicationProtocol").invoke(delegate);
@@ -311,6 +314,7 @@ public abstract class DelegatingSSLSocket extends SSLSocket {
     }
   }
 
+  @SuppressWarnings("MissingOverride") // Can only override with JDK 9+
   public <T> Socket setOption(SocketOption<T> name, T value) throws IOException {
     try {
       SSLSocket.class.getMethod("setOption", SocketOption.class, Object.class).invoke(delegate, name, value);
@@ -320,7 +324,10 @@ public abstract class DelegatingSSLSocket extends SSLSocket {
     }
   }
 
-  @SuppressWarnings("unchecked") // Using reflection to delegate.
+  @SuppressWarnings({
+      "MissingOverride", // Can only override with JDK 9+
+      "unchecked" // Using reflection to delegate.
+  })
   public <T> T getOption(SocketOption<T> name) throws IOException {
     try {
       return (T) SSLSocket.class.getMethod("getOption", SocketOption.class).invoke(delegate, name);
@@ -329,7 +336,10 @@ public abstract class DelegatingSSLSocket extends SSLSocket {
     }
   }
 
-  @SuppressWarnings("unchecked") // Using reflection to delegate.
+  @SuppressWarnings({
+      "MissingOverride", // Can only override with JDK 9+
+      "unchecked" // Using reflection to delegate.
+  })
   public Set<SocketOption<?>> supportedOptions() {
     try {
       return (Set<SocketOption<?>>) SSLSocket.class.getMethod("supportedOptions").invoke(delegate);

okhttp/src/test/java/okhttp3/URLConnectionTest.java

@@ -746,24 +746,24 @@ public final class URLConnectionTest {
 
   private void testConnectViaSocketFactory(boolean useHttps) throws IOException {
     SocketFactory uselessSocketFactory = new SocketFactory() {
-      public Socket createSocket() {
+      @Override public Socket createSocket() {
         throw new IllegalArgumentException("useless");
       }
 
-      public Socket createSocket(InetAddress host, int port) {
+      @Override public Socket createSocket(InetAddress host, int port) {
         return null;
       }
 
-      public Socket createSocket(InetAddress address, int port, InetAddress localAddress,
+      @Override public Socket createSocket(InetAddress address, int port, InetAddress localAddress,
           int localPort) {
         return null;
       }
 
-      public Socket createSocket(String host, int port) {
+      @Override public Socket createSocket(String host, int port) {
         return null;
       }
 
-      public Socket createSocket(String host, int port, InetAddress localHost, int localPort) {
+      @Override public Socket createSocket(String host, int port, InetAddress localHost, int localPort) {
         return null;
       }
     };
@@ -3807,15 +3807,15 @@ public final class URLConnectionTest {
       this.delegate = delegate;
     }
 
-    public X509Certificate[] getAcceptedIssuers() {
+    @Override public X509Certificate[] getAcceptedIssuers() {
       return delegate.getAcceptedIssuers();
     }
 
-    public void checkClientTrusted(X509Certificate[] chain, String authType) {
+    @Override public void checkClientTrusted(X509Certificate[] chain, String authType) {
       calls.add("checkClientTrusted " + certificatesToString(chain));
     }
 
-    public void checkServerTrusted(X509Certificate[] chain, String authType) {
+    @Override public void checkServerTrusted(X509Certificate[] chain, String authType) {
       calls.add("checkServerTrusted " + certificatesToString(chain));
     }
 

okhttp/src/test/java/okhttp3/UrlComponentEncodingTester.java

@@ -306,13 +306,13 @@ class UrlComponentEncodingTester {
 
   public enum Encoding {
     IDENTITY {
-      public String encode(int codePoint) {
+      @Override public String encode(int codePoint) {
         return new String(new int[] {codePoint}, 0, 1);
       }
     },
 
     PERCENT {
-      public String encode(int codePoint) {
+      @Override public String encode(int codePoint) {
         ByteString utf8 = ByteString.encodeUtf8(IDENTITY.encode(codePoint));
         Buffer percentEncoded = new Buffer();
         for (int i = 0; i < utf8.size(); i++) {