Java DiscordWebhook class to easily execute Discord Webhooks. Scroll down for Kotlin

ADiscordWebhook.kt

import java.awt.Color
import java.io.IOException
import java.lang.reflect.Array
import java.net.URL
import java.util.*
import javax.net.ssl.HttpsURLConnection
import kotlin.collections.HashMap
import kotlin.collections.set


/**
 * Class used to execute Discord Webhooks with low effort
 *
 * Kotlinified version of https://gist.github.com/k3kdude/fba6f6b37594eae3d6f9475330733bdb
 */
@Suppress("unused")
class DiscordWebhook(private val url: String) {

    private var content: String? = null
    private var username: String? = null
    private var avatarUrl: String? = null
    private var tts = false
    private val embeds: MutableList<EmbedObject> = ArrayList()

    fun setContent(content: String?) = apply { this.content = content }

    fun setUsername(username: String?) = apply { this.username = username }

    fun setAvatarUrl(avatarUrl: String?) = apply { this.avatarUrl = avatarUrl }

    fun setTts(tts: Boolean) = apply { this.tts = tts }

    fun addEmbed(embed: EmbedObject) = apply { embeds.add(embed) }

    @Throws(IOException::class)
    fun execute() {
        if (content == null && embeds.isEmpty()) {
            throw IllegalArgumentException("Set content or add at least one EmbedObject")
        }
        val json = JSONObject()
        json.put("content", content)
        json.put("username", username)
        json.put("avatar_url", avatarUrl)
        json.put("tts", tts)
        if (embeds.isNotEmpty()) {
            val embedObjects: MutableList<JSONObject> = ArrayList()
            for (embed: EmbedObject in embeds) {
                val jsonEmbed = JSONObject()
                jsonEmbed.put("title", embed.title)
                jsonEmbed.put("description", embed.description)
                jsonEmbed.put("url", embed.url)
                val color = embed.color
                if (color != null) {
                    var rgb = color.red
                    rgb = (rgb shl 8) + color.green
                    rgb = (rgb shl 8) + color.blue
                    jsonEmbed.put("color", rgb)
                }
                val footer: Footer? = embed.footer
                val image = embed.image
                val thumbnail: Thumbnail? = embed.thumbnail
                val author: Author? = embed.author
                val fields = embed.fields
                if (footer != null) {
                    val jsonFooter = JSONObject()
                    jsonFooter.put("text", footer.text)
                    jsonFooter.put("icon_url", footer.iconUrl)
                    jsonEmbed.put("footer", jsonFooter)
                }
                if (image != null) {
                    val jsonImage = JSONObject()
                    jsonImage.put("url", image.url)
                    jsonEmbed.put("image", jsonImage)
                }
                if (thumbnail != null) {
                    val jsonThumbnail = JSONObject()
                    jsonThumbnail.put("url", thumbnail.url)
                    jsonEmbed.put("thumbnail", jsonThumbnail)
                }
                if (author != null) {
                    val jsonAuthor = JSONObject()
                    jsonAuthor.put("name", author.name)
                    jsonAuthor.put("url", author.url)
                    jsonAuthor.put("icon_url", author.iconUrl)
                    jsonEmbed.put("author", jsonAuthor)
                }
                val jsonFields: MutableList<JSONObject> = ArrayList()
                for (field in fields) {
                    val jsonField = JSONObject()
                    jsonField.put("name", field.name)
                    jsonField.put("value", field.value)
                    jsonField.put("inline", field.isInline)
                    jsonFields.add(jsonField)
                }
                jsonEmbed.put("fields", jsonFields.toTypedArray())
                embedObjects.add(jsonEmbed)
            }
            json.put("embeds", embedObjects.toTypedArray())
        }
        val url = URL(url)
        val connection = url.openConnection() as HttpsURLConnection
        connection.addRequestProperty("Content-Type", "application/json")
        connection.addRequestProperty("User-Agent", "Java-DiscordWebhook-BY-Gelox_")
        connection.doOutput = true
        connection.requestMethod = "POST"
        val stream = connection.outputStream
        stream.write(json.toString().toByteArray())
        stream.flush()
        stream.close()
        connection.inputStream.close() //I'm not sure why but it doesn't work without getting the InputStream
        connection.disconnect()
    }
}

@Suppress("unused")
class EmbedObject {
    var title: String? = null
    var description: String? = null
    var url: String? = null
    var color: Color? = null
    var footer: Footer? = null
    var thumbnail: Thumbnail? = null
    var image: Image? = null
    var author: Author? = null
    val fields: MutableList<Field> = ArrayList()

    fun setTitle(title: String?): EmbedObject {
        this.title = title
        return this
    }

    fun setDescription(description: String?): EmbedObject {
        this.description = description
        return this
    }

    fun setUrl(url: String): EmbedObject {
        this.url = url
        return this
    }

    fun setColor(color: String): EmbedObject {
        fun String.toColor(): Color? {
            if (!this.matches("[#][0-9a-fA-F]{6}".toRegex())) return null
            val digits: String = this.substring(1, this.length.coerceAtMost(7))
            val hxstr = "0x$digits"
            return Color.decode(hxstr)
        }
        this.color = color.toColor()
        return this
    }

    fun setColor(color: Color?): EmbedObject {
        this.color = color
        return this
    }

    fun setFooter(text: String, icon: String): EmbedObject {
        footer = Footer(text, icon)
        return this
    }

    fun setThumbnail(url: String): EmbedObject {
        thumbnail = Thumbnail(url)
        return this
    }

    fun setImage(url: String): EmbedObject {
        image = Image(url)
        return this
    }

    fun setAuthor(name: String, url: String, icon: String): EmbedObject {
        author = Author(name, url, icon)
        return this
    }

    fun addField(name: String, value: String, inline: Boolean): EmbedObject {
        fields.add(Field(name, value, inline))
        return this
    }
}

data class Footer(val text: String, val iconUrl: String)

data class Thumbnail(val url: String)

data class Image(val url: String)

data class Author(
    val name: String,
    val url: String,
    val iconUrl: String
)

data class Field(
    val name: String,
    val value: String,
    val isInline: Boolean
)

class JSONObject {
    private val map = HashMap<String, Any>()
    fun put(key: String, value: Any?) {
        if (value != null) {
            map[key] = value
        }
    }

    override fun toString(): String {
        val builder = StringBuilder()
        val entrySet: Set<Map.Entry<String, Any>> = map.entries
        builder.append("{")
        var i = 0
        entrySet.forEach { (key, value) ->
            builder.append(quote(key)).append(":")
            when (value) {
                is String -> builder.append(quote(value))
                is Int -> builder.append(value)
                is Boolean -> builder.append(value)
                is JSONObject -> builder.append(value.toString())
                javaClass.isArray -> {
                    builder.append("[")
                    val len = Array.getLength(value)
                    for (j in 0 until len) {
                        builder.append(Array.get(value, j).toString())
                            .append(if (j != len - 1) "," else "")
                    }
                    builder.append("]")
                }
            }
            builder.append(if (++i == entrySet.size) "}" else ",")
        }
        return builder.toString()
    }

    private fun quote(string: String): String {
        return "\"$string\""
    }
}

DiscordWebhook.java

import javax.net.ssl.HttpsURLConnection;
import java.awt.Color;
import java.io.IOException;
import java.io.OutputStream;
import java.lang.reflect.Array;
import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;

/**
 * Class used to execute Discord Webhooks with low effort
 */
public class DiscordWebhook {

    private final String url;
    private String content;
    private String username;
    private String avatarUrl;
    private boolean tts;
    private List<EmbedObject> embeds = new ArrayList<>();

    /**
     * Constructs a new DiscordWebhook instance
     *
     * @param url The webhook URL obtained in Discord
     */
    public DiscordWebhook(String url) {
        this.url = url;
    }

    public void setContent(String content) {
        this.content = content;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public void setAvatarUrl(String avatarUrl) {
        this.avatarUrl = avatarUrl;
    }

    public void setTts(boolean tts) {
        this.tts = tts;
    }

    public void addEmbed(EmbedObject embed) {
        this.embeds.add(embed);
    }

    public void execute() throws IOException {
        if (this.content == null && this.embeds.isEmpty()) {
            throw new IllegalArgumentException("Set content or add at least one EmbedObject");
        }

        JSONObject json = new JSONObject();

        json.put("content", this.content);
        json.put("username", this.username);
        json.put("avatar_url", this.avatarUrl);
        json.put("tts", this.tts);

        if (!this.embeds.isEmpty()) {
            List<JSONObject> embedObjects = new ArrayList<>();

            for (EmbedObject embed : this.embeds) {
                JSONObject jsonEmbed = new JSONObject();

                jsonEmbed.put("title", embed.getTitle());
                jsonEmbed.put("description", embed.getDescription());
                jsonEmbed.put("url", embed.getUrl());

                if (embed.getColor() != null) {
                    Color color = embed.getColor();
                    int rgb = color.getRed();
                    rgb = (rgb << 8) + color.getGreen();
                    rgb = (rgb << 8) + color.getBlue();

                    jsonEmbed.put("color", rgb);
                }

                EmbedObject.Footer footer = embed.getFooter();
                EmbedObject.Image image = embed.getImage();
                EmbedObject.Thumbnail thumbnail = embed.getThumbnail();
                EmbedObject.Author author = embed.getAuthor();
                List<EmbedObject.Field> fields = embed.getFields();

                if (footer != null) {
                    JSONObject jsonFooter = new JSONObject();

                    jsonFooter.put("text", footer.getText());
                    jsonFooter.put("icon_url", footer.getIconUrl());
                    jsonEmbed.put("footer", jsonFooter);
                }

                if (image != null) {
                    JSONObject jsonImage = new JSONObject();

                    jsonImage.put("url", image.getUrl());
                    jsonEmbed.put("image", jsonImage);
                }

                if (thumbnail != null) {
                    JSONObject jsonThumbnail = new JSONObject();

                    jsonThumbnail.put("url", thumbnail.getUrl());
                    jsonEmbed.put("thumbnail", jsonThumbnail);
                }

                if (author != null) {
                    JSONObject jsonAuthor = new JSONObject();

                    jsonAuthor.put("name", author.getName());
                    jsonAuthor.put("url", author.getUrl());
                    jsonAuthor.put("icon_url", author.getIconUrl());
                    jsonEmbed.put("author", jsonAuthor);
                }

                List<JSONObject> jsonFields = new ArrayList<>();
                for (EmbedObject.Field field : fields) {
                    JSONObject jsonField = new JSONObject();

                    jsonField.put("name", field.getName());
                    jsonField.put("value", field.getValue());
                    jsonField.put("inline", field.isInline());

                    jsonFields.add(jsonField);
                }

                jsonEmbed.put("fields", jsonFields.toArray());
                embedObjects.add(jsonEmbed);
            }

            json.put("embeds", embedObjects.toArray());
        }

        URL url = new URL(this.url);
        HttpsURLConnection connection = (HttpsURLConnection) url.openConnection();
        connection.addRequestProperty("Content-Type", "application/json");
        connection.addRequestProperty("User-Agent", "Java-DiscordWebhook-BY-Gelox_");
        connection.setDoOutput(true);
        connection.setRequestMethod("POST");

        OutputStream stream = connection.getOutputStream();
        stream.write(json.toString().getBytes());
        stream.flush();
        stream.close();

        connection.getInputStream().close(); //I'm not sure why but it doesn't work without getting the InputStream
        connection.disconnect();
    }

    public static class EmbedObject {
        private String title;
        private String description;
        private String url;
        private Color color;

        private Footer footer;
        private Thumbnail thumbnail;
        private Image image;
        private Author author;
        private List<Field> fields = new ArrayList<>();

        public String getTitle() {
            return title;
        }

        public String getDescription() {
            return description;
        }

        public String getUrl() {
            return url;
        }

        public Color getColor() {
            return color;
        }

        public Footer getFooter() {
            return footer;
        }

        public Thumbnail getThumbnail() {
            return thumbnail;
        }

        public Image getImage() {
            return image;
        }

        public Author getAuthor() {
            return author;
        }

        public List<Field> getFields() {
            return fields;
        }

        public EmbedObject setTitle(String title) {
            this.title = title;
            return this;
        }

        public EmbedObject setDescription(String description) {
            this.description = description;
            return this;
        }

        public EmbedObject setUrl(String url) {
            this.url = url;
            return this;
        }

        public EmbedObject setColor(Color color) {
            this.color = color;
            return this;
        }

        public EmbedObject setFooter(String text, String icon) {
            this.footer = new Footer(text, icon);
            return this;
        }

        public EmbedObject setThumbnail(String url) {
            this.thumbnail = new Thumbnail(url);
            return this;
        }

        public EmbedObject setImage(String url) {
            this.image = new Image(url);
            return this;
        }

        public EmbedObject setAuthor(String name, String url, String icon) {
            this.author = new Author(name, url, icon);
            return this;
        }

        public EmbedObject addField(String name, String value, boolean inline) {
            this.fields.add(new Field(name, value, inline));
            return this;
        }

        private class Footer {
            private String text;
            private String iconUrl;

            private Footer(String text, String iconUrl) {
                this.text = text;
                this.iconUrl = iconUrl;
            }

            private String getText() {
                return text;
            }

            private String getIconUrl() {
                return iconUrl;
            }
        }

        private class Thumbnail {
            private String url;

            private Thumbnail(String url) {
                this.url = url;
            }

            private String getUrl() {
                return url;
            }
        }

        private class Image {
            private String url;

            private Image(String url) {
                this.url = url;
            }

            private String getUrl() {
                return url;
            }
        }

        private class Author {
            private String name;
            private String url;
            private String iconUrl;

            private Author(String name, String url, String iconUrl) {
                this.name = name;
                this.url = url;
                this.iconUrl = iconUrl;
            }

            private String getName() {
                return name;
            }

            private String getUrl() {
                return url;
            }

            private String getIconUrl() {
                return iconUrl;
            }
        }

        private class Field {
            private String name;
            private String value;
            private boolean inline;

            private Field(String name, String value, boolean inline) {
                this.name = name;
                this.value = value;
                this.inline = inline;
            }

            private String getName() {
                return name;
            }

            private String getValue() {
                return value;
            }

            private boolean isInline() {
                return inline;
            }
        }
    }

    private class JSONObject {

        private final HashMap<String, Object> map = new HashMap<>();

        void put(String key, Object value) {
            if (value != null) {
                map.put(key, value);
            }
        }

        @Override
        public String toString() {
            StringBuilder builder = new StringBuilder();
            Set<Map.Entry<String, Object>> entrySet = map.entrySet();
            builder.append("{");

            int i = 0;
            for (Map.Entry<String, Object> entry : entrySet) {
                Object val = entry.getValue();
                builder.append(quote(entry.getKey())).append(":");

                if (val instanceof String) {
                    builder.append(quote(String.valueOf(val)));
                } else if (val instanceof Integer) {
                    builder.append(Integer.valueOf(String.valueOf(val)));
                } else if (val instanceof Boolean) {
                    builder.append(val);
                } else if (val instanceof JSONObject) {
                    builder.append(val.toString());
                } else if (val.getClass().isArray()) {
                    builder.append("[");
                    int len = Array.getLength(val);
                    for (int j = 0; j < len; j++) {
                        builder.append(Array.get(val, j).toString()).append(j != len - 1 ? "," : "");
                    }
                    builder.append("]");
                }

                builder.append(++i == entrySet.size() ? "}" : ",");
            }

            return builder.toString();
        }

        private String quote(String string) {
            return "\"" + string + "\"";
        }
    }

}

Is there an example or and explanation somewhere? @Jaimss