diff --git a/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java b/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java
index e22eed8384..3674d148ad 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java
@@ -41,6 +41,8 @@
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
+import java.util.function.Predicate;
+import java.util.function.Supplier;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
@@ -114,9 +116,7 @@
 import org.jivesoftware.smack.util.Objects;
 import org.jivesoftware.smack.util.PacketParserUtils;
 import org.jivesoftware.smack.util.ParserUtils;
-import org.jivesoftware.smack.util.Predicate;
 import org.jivesoftware.smack.util.StringUtils;
-import org.jivesoftware.smack.util.Supplier;
 import org.jivesoftware.smack.xml.XmlPullParser;
 import org.jivesoftware.smack.xml.XmlPullParserException;
 
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/XMPPConnection.java b/smack-core/src/main/java/org/jivesoftware/smack/XMPPConnection.java
index de5f46477f..efd0a46819 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/XMPPConnection.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/XMPPConnection.java
@@ -18,6 +18,7 @@
 
 import java.net.InetAddress;
 import java.util.concurrent.TimeUnit;
+import java.util.function.Predicate;
 
 import javax.xml.namespace.QName;
 
@@ -39,7 +40,6 @@
 import org.jivesoftware.smack.packet.StanzaFactory;
 import org.jivesoftware.smack.packet.XmlElement;
 import org.jivesoftware.smack.util.Consumer;
-import org.jivesoftware.smack.util.Predicate;
 import org.jivesoftware.smack.util.XmppElementUtil;
 
 import org.jxmpp.jid.DomainBareJid;
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/c2s/ModularXmppClientToServerConnection.java b/smack-core/src/main/java/org/jivesoftware/smack/c2s/ModularXmppClientToServerConnection.java
index 8cdac17b93..14ccb134ee 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/c2s/ModularXmppClientToServerConnection.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/c2s/ModularXmppClientToServerConnection.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2018-2022 Florian Schmaus
+ * Copyright 2018-2024 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -29,6 +29,7 @@
 import java.util.Map;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.TimeUnit;
+import java.util.function.Supplier;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
@@ -80,7 +81,6 @@
 import org.jivesoftware.smack.util.ExtendedAppendable;
 import org.jivesoftware.smack.util.PacketParserUtils;
 import org.jivesoftware.smack.util.StringUtils;
-import org.jivesoftware.smack.util.Supplier;
 import org.jivesoftware.smack.xml.XmlPullParser;
 import org.jivesoftware.smack.xml.XmlPullParserException;
 
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/c2s/internal/ModularXmppClientToServerConnectionInternal.java b/smack-core/src/main/java/org/jivesoftware/smack/c2s/internal/ModularXmppClientToServerConnectionInternal.java
index 42cf9c6508..876932316f 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/c2s/internal/ModularXmppClientToServerConnectionInternal.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/c2s/internal/ModularXmppClientToServerConnectionInternal.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2020-2021 Florian Schmaus
+ * Copyright 2020-2024 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -22,6 +22,7 @@
 import java.nio.channels.SelectionKey;
 import java.util.ListIterator;
 import java.util.Queue;
+import java.util.function.Supplier;
 
 import org.jivesoftware.smack.SmackException;
 import org.jivesoftware.smack.SmackException.NoResponseException;
@@ -42,7 +43,6 @@
 import org.jivesoftware.smack.packet.XmlEnvironment;
 import org.jivesoftware.smack.util.Consumer;
 import org.jivesoftware.smack.util.PacketParserUtils;
-import org.jivesoftware.smack.util.Supplier;
 import org.jivesoftware.smack.xml.XmlPullParser;
 import org.jivesoftware.smack.xml.XmlPullParserException;
 
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/filter/StanzaFilter.java b/smack-core/src/main/java/org/jivesoftware/smack/filter/StanzaFilter.java
index 147d0c9f1e..b06256f58e 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/filter/StanzaFilter.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/filter/StanzaFilter.java
@@ -17,8 +17,9 @@
 
 package org.jivesoftware.smack.filter;
 
+import java.util.function.Predicate;
+
 import org.jivesoftware.smack.packet.Stanza;
-import org.jivesoftware.smack.util.Predicate;
 
 /**
  * Defines a way to filter stanzas for particular attributes. Stanza filters are used when
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/packet/StanzaBuilder.java b/smack-core/src/main/java/org/jivesoftware/smack/packet/StanzaBuilder.java
index 2cc2a20edc..4fc57d7b1d 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/packet/StanzaBuilder.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/packet/StanzaBuilder.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2019-2021 Florian Schmaus
+ * Copyright 2019-2024 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,11 +18,11 @@
 
 import java.util.Collection;
 import java.util.List;
+import java.util.function.Function;
 
 import javax.xml.namespace.QName;
 
 import org.jivesoftware.smack.packet.id.StanzaIdSource;
-import org.jivesoftware.smack.util.Function;
 import org.jivesoftware.smack.util.MultiMap;
 import org.jivesoftware.smack.util.StringUtils;
 import org.jivesoftware.smack.util.ToStringUtil;
@@ -315,7 +315,7 @@ public static IqData buildIqData(String stanzaId) {
         return new IqData(stanzaId);
     }
 
-    public static <SB extends StanzaBuilder<?>> SB buildResponse(StanzaView request, Function<SB, String> builderFromStanzaId) {
+    public static <SB extends StanzaBuilder<?>> SB buildResponse(StanzaView request, Function<String, SB> builderFromStanzaId) {
         SB responseBuilder = builderFromStanzaId.apply(request.getStanzaId());
 
         responseBuilder.to(request.getFrom())
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/proxy/ProxySocketConnection.java b/smack-core/src/main/java/org/jivesoftware/smack/proxy/ProxySocketConnection.java
index beaf0ebade..41e6696d22 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/proxy/ProxySocketConnection.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/proxy/ProxySocketConnection.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2015-2019 Florian Schmaus.
+ * Copyright 2015-2024 Florian Schmaus.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,8 +18,7 @@
 
 import java.io.IOException;
 import java.net.Socket;
-
-import org.jivesoftware.smack.util.Function;
+import java.util.function.Function;
 
 public interface ProxySocketConnection {
 
@@ -36,7 +35,7 @@ public interface ProxySocketConnection {
     void connect(Socket socket, String host, int port, int timeout)
                     throws IOException;
 
-    static Function<ProxySocketConnection, ProxyInfo> forProxyType(ProxyInfo.ProxyType proxyType) {
+    static Function<ProxyInfo, ProxySocketConnection> forProxyType(ProxyInfo.ProxyType proxyType) {
         switch (proxyType) {
             case HTTP:
                 return HTTPProxySocketConnection::new;
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/util/Function.java b/smack-core/src/main/java/org/jivesoftware/smack/util/Function.java
deleted file mode 100644
index 2cc5552088..0000000000
--- a/smack-core/src/main/java/org/jivesoftware/smack/util/Function.java
+++ /dev/null
@@ -1,26 +0,0 @@
-/**
- *
- * Copyright 2019-2020 Florian Schmaus
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-package org.jivesoftware.smack.util;
-
-public interface Function<R, T> {
-
-    R apply(T t);
-
-    static <T> Function<T, T> identity() {
-        return t -> t;
-    }
-}
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/util/Predicate.java b/smack-core/src/main/java/org/jivesoftware/smack/util/Predicate.java
deleted file mode 100644
index ad4c11d0c6..0000000000
--- a/smack-core/src/main/java/org/jivesoftware/smack/util/Predicate.java
+++ /dev/null
@@ -1,24 +0,0 @@
-/**
- *
- * Copyright 2019 Florian Schmaus
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-package org.jivesoftware.smack.util;
-
-// TODO: Replace with java.util.function.Predicate once Smack's minimum Android SDK level is 24 or higher.
-public interface Predicate<T> {
-
-    boolean test(T t);
-
-}
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/util/Supplier.java b/smack-core/src/main/java/org/jivesoftware/smack/util/Supplier.java
deleted file mode 100644
index f0910adb7e..0000000000
--- a/smack-core/src/main/java/org/jivesoftware/smack/util/Supplier.java
+++ /dev/null
@@ -1,24 +0,0 @@
-/**
- *
- * Copyright 2020 Florian Schmaus
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-package org.jivesoftware.smack.util;
-
-// TODO: Replace with java.util.function.Supplier once Smack's minimum Android SDK level is 24 or higher.
-public interface Supplier<T> {
-
-    T get();
-
-}
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/util/ToStringUtil.java b/smack-core/src/main/java/org/jivesoftware/smack/util/ToStringUtil.java
index 447f1444f2..f7451cbbfa 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/util/ToStringUtil.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/util/ToStringUtil.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2019 Florian Schmaus
+ * Copyright 2019-2024 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,6 +19,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
+import java.util.function.Function;
 
 public class ToStringUtil {
 
@@ -46,7 +47,7 @@ public Builder addValue(String name, Object value) {
             return this;
         }
 
-        public <V> Builder add(String name, Collection<? extends V> values, Function<?, V> toStringFunction) {
+        public <V> Builder add(String name, Collection<? extends V> values, Function<V, ?> toStringFunction) {
             if (values.isEmpty()) {
                 return this;
             }
diff --git a/smack-integration-test/src/main/java/org/igniterealtime/smack/inttest/Configuration.java b/smack-integration-test/src/main/java/org/igniterealtime/smack/inttest/Configuration.java
index 3cad302cb7..dc29998f4a 100644
--- a/smack-integration-test/src/main/java/org/igniterealtime/smack/inttest/Configuration.java
+++ b/smack-integration-test/src/main/java/org/igniterealtime/smack/inttest/Configuration.java
@@ -32,6 +32,7 @@
 import java.util.Map;
 import java.util.Properties;
 import java.util.Set;
+import java.util.function.Function;
 import java.util.logging.Logger;
 import java.util.stream.Collectors;
 
@@ -41,7 +42,6 @@
 import org.jivesoftware.smack.debugger.ConsoleDebugger;
 import org.jivesoftware.smack.debugger.SmackDebuggerFactory;
 import org.jivesoftware.smack.util.CollectionUtil;
-import org.jivesoftware.smack.util.Function;
 import org.jivesoftware.smack.util.Objects;
 import org.jivesoftware.smack.util.ParserUtils;
 import org.jivesoftware.smack.util.SslContextFactory;