[java] Undoing removal of Java 8 code

Will submit separate PR

Author: zodac-personal

java/src/org/openqa/selenium/PersistentCapabilities.java

@@ -17,9 +17,11 @@
 
 package org.openqa.selenium;
 
+import java.util.Collections;
 import java.util.Map;
 import java.util.Set;
 import java.util.function.Function;
+import java.util.stream.Collector;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 import org.openqa.selenium.internal.Require;
@@ -56,7 +58,7 @@ public PersistentCapabilities setCapability(String name, Object value) {
   @Override
   public Map<String, Object> asMap() {
     return getCapabilityNames().stream()
-        .collect(Collectors.toUnmodifiableMap(Function.identity(), this::getCapability));
+        .collect(toUnmodifiableMap(Function.identity(), this::getCapability));
   }
 
   @Override
@@ -79,7 +81,19 @@ public Capabilities merge(Capabilities other) {
   public Set<String> getCapabilityNames() {
     return Stream.concat(
             caps.getCapabilityNames().stream(), overrides.getCapabilityNames().stream())
-        .collect(Collectors.toUnmodifiableSet());
+        .collect(toUnmodifiableSet());
+  }
+
+  // Needed, since we're dependent on Java 8 as a minimum version
+  private <T, K, U> Collector<T, ?, Map<K, U>> toUnmodifiableMap(
+    Function<? super T, ? extends K> keyMapper, Function<? super T, ? extends U> valueMapper) {
+    return Collectors.collectingAndThen(
+      Collectors.toMap(keyMapper, valueMapper), Collections::unmodifiableMap);
+  }
+
+  // Needed, since we're dependent on Java 8 as a minimum version
+  private <T> Collector<T, ?, Set<T>> toUnmodifiableSet() {
+    return Collectors.collectingAndThen(Collectors.toSet(), Collections::unmodifiableSet);
   }
 
   @Override