型クラス.java

TypeClass.java

public class TypeClass {
  
  interface Addable<T> {
    T add(T a, T b);
  }

  public static class IntAdd implements Addable<Integer> {
    @Override
    public Integer add(Integer a, Integer b) {
      return a + b;
    }
  }

  public static class StringAdd implements Addable<String> {
    @Override
    public String add(String a, String b) {
      return a + b;
    }
  }

  public static void main(String[] args) {

    List<Integer> integerList = Arrays.asList(1 ,2, 3);
    List<String> stringList = Arrays.asList("a", "b", "c");

    Integer result1 = addAll(integerList, 0, new IntAdd());
    String result2 = addAll(stringList, "", new StringAdd());
  }

  public static <T> T addAll(List<T> list, T init, Addable<T> addable) {

    T result = init;
    for(T x : list){
      result = addable.add(result, x);
    }

    return result;
  }
}