package rocks.matchmaker;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Util {

    public static <T> List<T> append(List<T> list, T element) {
        ArrayList<T> newList = new ArrayList<>(list);
        newList.add(element);
        return Collections.unmodifiableList(newList);
    }

    public static <T> T checkNotNull(T value) {
        if (value == null) {
            throw new NullPointerException("Value cannot be null");
        }
        return value;
    }
}

@@ -20,16 +20,62 @@
 maker;%0A%0A
+import com.google.common.base.Preconditions;%0A%0A
 import j
@@ -560,11 +560,151 @@
 e;%0A    %7D
+%0A%0A    public static void checkArgument(boolean expression, String message) %7B%0A        Preconditions.checkArgument(expression, message);%0A    %7D
 %0A%7D%0A

Add `Util.checkArgument` method