http://bufferings.hatenablog.com/entry/2017/10/23/000057 (assertjの3.8.0を使ってます)

DateAndTimeTest.java

package com.example.demo;

import org.junit.Test;

import java.time.LocalDate;
import java.time.chrono.JapaneseDate;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeParseException;
import java.time.format.ResolverStyle;
import java.util.Locale;

import static org.assertj.core.api.Assertions.assertThat;
import static org.assertj.core.api.Assertions.assertThatExceptionOfType;

public class DateAndTimeTest {

  @Test
  public void 普通にyyyyとuuuuを使ったら同じ結果になる() {
    LocalDate target = LocalDate.of(2017, 10, 22);
    assertThat(target.format(DateTimeFormatter.ofPattern("yyyy/MM/dd", Locale.US)))
        .isEqualTo("2017/10/22");
    assertThat(target.format(DateTimeFormatter.ofPattern("uuuu/MM/dd", Locale.US)))
        .isEqualTo("2017/10/22");
  }

  @Test
  public void Gをつけてみたらどうなる() {
    LocalDate target = LocalDate.of(2017, 10, 22);
    assertThat(target.format(DateTimeFormatter.ofPattern("G yyyy/MM/dd", Locale.US)))
        .isEqualTo("AD 2017/10/22");
    assertThat(target.format(DateTimeFormatter.ofPattern("G uuuu/MM/dd", Locale.US)))
        .isEqualTo("AD 2017/10/22");
  }

  @Test
  public void 紀元前を渡したらどうなる() {
    LocalDate target = LocalDate.of(-2017, 10, 22);
    assertThat(target.format(DateTimeFormatter.ofPattern("G yyyy/MM/dd", Locale.US)))
        .isEqualTo("BC 2018/10/22");
    assertThat(target.format(DateTimeFormatter.ofPattern("G uuuu/MM/dd", Locale.US)))
        .isEqualTo("BC -2017/10/22");
  }

  @Test
  public void こういう風に使うってことか() {
    LocalDate target = LocalDate.of(2017, 10, 22);
    assertThat(target.format(DateTimeFormatter.ofPattern("G yyyy/MM/dd", Locale.US)))
        .as("yyyy使うなら暦と一緒に")
        .isEqualTo("AD 2017/10/22");
    assertThat(target.format(DateTimeFormatter.ofPattern("uuuu/MM/dd", Locale.US)))
        .as("uuuu使うなら暦は使わない")
        .isEqualTo("2017/10/22");
  }

  @Test
  public void 和暦はこうなるってことか() {
    JapaneseDate japaneseDate = JapaneseDate.of(2017, 10, 22);
    assertThat(japaneseDate.format(DateTimeFormatter.ofPattern("G yyyy/MM/dd", Locale.JAPAN)))
        .isEqualTo("平成 0029/10/22");
    assertThat(japaneseDate.format(DateTimeFormatter.ofPattern("uuuu/MM/dd", Locale.JAPAN)))
        .isEqualTo("2017/10/22");
  }

  @Test
  public void STRICTモードで暦指定なしyyyyは例外発生() {
    assertThatExceptionOfType(DateTimeParseException.class)
        .isThrownBy(() -> LocalDate.parse("2017/10/22", DateTimeFormatter.ofPattern("yyyy/MM/dd")
            .withResolverStyle(ResolverStyle.STRICT)));
  }

  @Test
  public void yyyyを使うなら暦を指定する() {
    assertThat(LocalDate.parse("AD 2017/10/22",
        DateTimeFormatter.ofPattern("G yyyy/MM/dd").withResolverStyle(ResolverStyle.STRICT)))
        .isEqualTo(LocalDate.of(2017, 10, 22));
  }

  @Test
  public void でも西暦を扱うときには暦を指定したりしないだろうからuuuuを使うべきってことね() {
    assertThat(LocalDate.parse("2017/10/22",
        DateTimeFormatter.ofPattern("uuuu/MM/dd").withResolverStyle(ResolverStyle.STRICT)))
        .isEqualTo(LocalDate.of(2017, 10, 22));
  }
}