Here you can find the source of first(Collection extends T> collection)
public static <T> T first(Collection<? extends T> collection)
//package com.java2s; //License from project: Open Source License import java.util.ArrayList; import java.util.Collection; public class Main { public static <T> T first(Collection<? extends T> collection) { ArrayList<T> list = new ArrayList<>(collection); return list.isEmpty() ? null : list.get(0); }/*from w ww .ja va2s .c om*/ }