Implementing fold and reduce in Java

fold-reduce.java

import java.util.function.BiFunction;
import java.util.List;
import java.util.Arrays;

public class FunctionsExample {

  public static void main(String[] args) {
    String[] strs = {"Example", "Strings", "To", "Join"};
    List<String> strsList = Arrays.asList(strs);
    
    BiFunction<String, String, String> concat = (str1, str2) -> str1 + str2;
    final String EMPTY_STRING = "";
    
    System.out.println(reduce(concat, strsList));
    System.out.println(fold(concat, EMPTY_STRING, strsList));
  }
  
  static <T> T reduce(BiFunction<T, T, T> f, List<T> list) {
    if (list == null || list.isEmpty()) {
      throw new IllegalArgumentException("The list argument cannot be null or empty.");
    }
      
    T head = list.get(0);
    List<T> rest = list.subList(1, list.size());
      
    return fold(f, head, rest);
  }
  
  static <State, T> State fold(BiFunction<State, T, State> f, State state, List<T> list) {
    if (list == null || list.isEmpty()) {
      return state;
    }
      
    T head = list.get(0);
    List<T> rest = list.subList(1, list.size());
      
    return fold(f, f.apply(state, head), rest);
  }
}