为什么Java集合有0个或1个元素,而不是更多的元素?

Collections Map

Guava Immutable* ImmutableMap

private static final ImmutableMap<String, String> MAP = ImmutableMap.of(
    "a", "b",
    "c", "d");

private static final ImmutableMap<String, String> MAP = 
    ImmutableMap.<String, String>builder()
      .put("a", "b")
      .put("b", "c")
      ...
      .put("y", "z")
      .build();

private static final Map<String, String> MAP;

static {
  Map<String, String> temp = new HashMap<String, String>();
  temp.put("a", "b");
  temp.put("b", "c");
  MAP = Collections.unmodifiableMap(temp);
}

MAP

static { 
   MAP.put("a", "b"); 
   MAP.put("c", "d");
}

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private static final Map<String, String> MAP;
static {
    Map<String, String> tempMap = new HashMap<String, String>();
    tempMap.put("a", "b");
    tempMap.put("c", "d");
    MAP = Collections.unmodifiableMap(tempMap);
}

private static final Map<String, String> Map = buildMap();

private static Map<String, String> buildMap() {
  Map<String, String> map = new HashMap<String, String>();
  map.put("a", "b");
  map.put("c", "d");
  return Collections.unmodifiableMap(map);
}

private static final Map<String, String> Map = buildMap();

private static Map<String, String> buildMap() {
  Map<String, String> map = new HashMap<String, String>();
  map.put("a", "b");
  map.put("c", "d");
  return map;
}