如何在Java中初始化一个静态Map ?

方法一:静态初始化器 方法二:实例初始化器(匿名子类) 或 还有别的方法吗?

它们各自的优点和缺点是什么?

下面是一个例子来说明这两种方法:

import java.util.HashMap;
import java.util.Map;

public class Test {
    private static final Map<Integer, String> myMap = new HashMap<>();
    static {
        myMap.put(1, "one");
        myMap.put(2, "two");
    }

    private static final Map<Integer, String> myMap2 = new HashMap<>(){
        {
            put(1, "one");
            put(2, "two");
        }
    };
}

当前回答

如果你可以使用字符串表示你的数据,这也是一个选项在Java 8:

static Map<Integer, String> MAP = Stream.of(
        "1=one",
        "2=two"
).collect(Collectors.toMap(k -> Integer.parseInt(k.split("=")[0]), v -> v.split("=")[1]));

其他回答

现在Java 8已经发布了,这个问题值得重新审视。我尝试了一下——看起来也许你可以利用lambda表达式语法来获得一个非常漂亮和简洁(但类型安全)的映射文字语法,看起来像这样:

Map<String,Object> myMap = hashMap(
    bob -> 5,
    TheGimp -> 8,
    incredibleKoolAid -> "James Taylor",
    heyArnold -> new Date()
);

Map<String,Integer> typesafeMap = treeMap(
    a -> 5,
    bee -> 8,
    sea -> 13
    deep -> 21
);

未经测试的示例代码在https://gist.github.com/galdosd/10823529 会对其他人对此的看法感到好奇(这有点邪恶…)

下面是一个Java 8单行静态映射初始化器:

private static final Map<String, String> EXTENSION_TO_MIMETYPE =
    Arrays.stream(new String[][] {
        { "txt", "text/plain" }, 
        { "html", "text/html" }, 
        { "js", "application/javascript" },
        { "css", "text/css" },
        { "xml", "application/xml" },
        { "png", "image/png" }, 
        { "gif", "image/gif" }, 
        { "jpg", "image/jpeg" },
        { "jpeg", "image/jpeg" }, 
        { "svg", "image/svg+xml" },
    }).collect(Collectors.toMap(kv -> kv[0], kv -> kv[1]));

编辑:要初始化一个Map<Integer, String>,就像问题中那样,你需要这样的东西:

static final Map<Integer, String> MY_MAP = Arrays.stream(new Object[][]{
        {1, "one"},
        {2, "two"},
}).collect(Collectors.toMap(kv -> (Integer) kv[0], kv -> (String) kv[1]));

编辑(2):i_am_zero有一个更好的、支持混合类型的版本,它使用新的SimpleEntry<>(k, v)调用流。看看答案:https://stackoverflow.com/a/37384773/3950982

我已经读了答案,我决定写我自己的地图生成器。请随意复制粘贴并欣赏。

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

/**
 * A tool for easy creation of a map. Code example:<br/>
 * {@code MapBuilder.of("name", "Forrest").and("surname", "Gump").build()}
 * @param <K> key type (inferred by constructor)
 * @param <V> value type (inferred by constructor)
 * @author Vlasec (for http://stackoverflow.com/a/30345279/1977151)
 */
public class MapBuilder <K, V> {
    private Map<K, V> map = new HashMap<>();

    /** Constructor that also enters the first entry. */
    private MapBuilder(K key, V value) {
        and(key, value);
    }

    /** Factory method that creates the builder and enters the first entry. */
    public static <A, B> MapBuilder<A, B> mapOf(A key, B value) {
        return new MapBuilder<>(key, value);
    }

    /** Puts the key-value pair to the map and returns itself for method chaining */
    public MapBuilder<K, V> and(K key, V value) {
        map.put(key, value);
        return this;
    }

    /**
     * If no reference to builder is kept and both the key and value types are immutable,
     * the resulting map is immutable.
     * @return contents of MapBuilder as an unmodifiable map.
     */
    public Map<K, V> build() {
        return Collections.unmodifiableMap(map);
    }
}

编辑:最近,我经常发现公共静态方法,我有点喜欢它。我将它添加到代码中,并使构造函数私有,从而切换到静态工厂方法模式。

EDIT2:甚至最近,我不再喜欢被称为of的静态方法,因为它在使用静态导入时看起来非常糟糕。我将其重命名为mapOf,使其更适合静态导入。

如果你可以使用字符串表示你的数据,这也是一个选项在Java 8:

static Map<Integer, String> MAP = Stream.of(
        "1=one",
        "2=two"
).collect(Collectors.toMap(k -> Integer.parseInt(k.split("=")[0]), v -> v.split("=")[1]));

我做了一些不同的事情。不是最好的,但对我有用。也许它可以被“泛化”。

private static final Object[][] ENTRIES =
{
  {new Integer(1), "one"},
  {new Integer(2), "two"},
};
private static final Map myMap = newMap(ENTRIES);

private static Map newMap(Object[][] entries)
{
  Map map = new HashMap();

  for (int x = 0; x < entries.length; x++)
  {
    Object[] entry = entries[x];

    map.put(entry[0], entry[1]);
  }

  return map;
}