@QElement("EXAMPLE") // must be unique - defines the name of your condition used in command and storage
public final class ExampleCondition extends Condition {

    /* Let's say this is condition that requires player's name to start or not start with certain string.*/

    private final String prefix;
    private final boolean inverted;

    public ExampleCondition(String prefix, boolean invert) {
        this.prefix - prefix;
        this.inverted = invert;
    }

    //This method is required when you want users to be able to have variables in custom comdition description
    @Override
    protected String parseDescription(String description) {
        //we are replacing every "%pref" in message with specified prefix
        return description.replaceAll("%pref", prefix);
    }

    //this method returns true if player meets the condition
    @Override
    public boolean isMet(Player player, Quester plugin) {
        return (player.getName().startsWith(prefix)) != inverted;
    }

    // this method returns string displayed in "/q show"
    @Override
    public String show() {
        String flag = inverted ? " not " : " ";
        return "Your name must" + flag + "start with " + prefix + ".";
    }

    // this method returns string that is appended to "EXAMPLE: " in "/q info"
    @Override
    public String info() {
        String flag = inverted ? " (-i)": "";
        return prefix + flag;
        // result is for example "EXAMPLE: prefix (-i)"
    }

    // element must contain this static fromCommand(QCommandContext context) method, that returns its supertype
    // it also hat to be annotated with @QCommand
    @QCommand(
            min = 1,
            max = 1,
            usage = "<prefix> (-i)")
    public static Condition fromCommand(QCommandContext context) throws QCommandException {
        String pre = context.getString(0);
        return new ExampleCondition(pre, context.hasFlag('i'));
    }

    // serialization to StorageKey
    @Override
    protected void save(StorageKey key) {
        key.setString("prefix", prefix);
        if(inverted) {
            key.setBoolean("inverted", inverted);
        }
    }

    // must contain static load(StorageKey key) method that returns its supertype, preferrably protected
    protected static Condition load(StorageKey key) {
        String pre = key.getString("amount", null);
        if(pre == null) { // invalid prefix
            return null;
        }

        return new ExampleCondition(pre, key.getBoolean("inverted", false));
    }
}