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
第二种方法(双括号初始化)被认为是一种反模式，因此我将采用第一种方法。
另一个初始化静态Map的简单方法是使用这个实用函数:
public static <K, V> Map<K, V> mapOf(Object... keyValues) {
Map<K, V> map = new HashMap<>(keyValues.length / 2);
for (int index = 0; index < keyValues.length / 2; index++) {
map.put((K)keyValues[index * 2], (V)keyValues[index * 2 + 1]);
}
return map;
}
Map<Integer, String> map1 = mapOf(1, "value1", 2, "value2");
Map<String, String> map2 = mapOf("key1", "value1", "key2", "value2");
注意:在Java 9中你可以使用Map.of
其他回答
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。
我会用:
public class Test {
private static final Map<Integer, String> MY_MAP = createMap();
private static Map<Integer, String> createMap() {
Map<Integer, String> result = new HashMap<>();
result.put(1, "one");
result.put(2, "two");
return Collections.unmodifiableMap(result);
}
}
它避免了匿名类，我个人认为这是一种糟糕的风格，并避免 它使地图的创建更加明确 它使地图不可修改 因为MY_MAP是常量，所以我把它命名为常量
使用Eclipse Collections，以下所有功能都可以工作:
import java.util.Map;
import org.eclipse.collections.api.map.ImmutableMap;
import org.eclipse.collections.api.map.MutableMap;
import org.eclipse.collections.impl.factory.Maps;
public class StaticMapsTest
{
private static final Map<Integer, String> MAP =
Maps.mutable.with(1, "one", 2, "two");
private static final MutableMap<Integer, String> MUTABLE_MAP =
Maps.mutable.with(1, "one", 2, "two");
private static final MutableMap<Integer, String> UNMODIFIABLE_MAP =
Maps.mutable.with(1, "one", 2, "two").asUnmodifiable();
private static final MutableMap<Integer, String> SYNCHRONIZED_MAP =
Maps.mutable.with(1, "one", 2, "two").asSynchronized();
private static final ImmutableMap<Integer, String> IMMUTABLE_MAP =
Maps.mutable.with(1, "one", 2, "two").toImmutable();
private static final ImmutableMap<Integer, String> IMMUTABLE_MAP2 =
Maps.immutable.with(1, "one", 2, "two");
}
您还可以使用Eclipse Collections静态地初始化原始映射。
import org.eclipse.collections.api.map.primitive.ImmutableIntObjectMap;
import org.eclipse.collections.api.map.primitive.MutableIntObjectMap;
import org.eclipse.collections.impl.factory.primitive.IntObjectMaps;
public class StaticPrimitiveMapsTest
{
private static final MutableIntObjectMap<String> MUTABLE_INT_OBJ_MAP =
IntObjectMaps.mutable.<String>empty()
.withKeyValue(1, "one")
.withKeyValue(2, "two");
private static final MutableIntObjectMap<String> UNMODIFIABLE_INT_OBJ_MAP =
IntObjectMaps.mutable.<String>empty()
.withKeyValue(1, "one")
.withKeyValue(2, "two")
.asUnmodifiable();
private static final MutableIntObjectMap<String> SYNCHRONIZED_INT_OBJ_MAP =
IntObjectMaps.mutable.<String>empty()
.withKeyValue(1, "one")
.withKeyValue(2, "two")
.asSynchronized();
private static final ImmutableIntObjectMap<String> IMMUTABLE_INT_OBJ_MAP =
IntObjectMaps.mutable.<String>empty()
.withKeyValue(1, "one")
.withKeyValue(2, "two")
.toImmutable();
private static final ImmutableIntObjectMap<String> IMMUTABLE_INT_OBJ_MAP2 =
IntObjectMaps.immutable.<String>empty()
.newWithKeyValue(1, "one")
.newWithKeyValue(2, "two");
}
注意:我是Eclipse Collections的提交者
这里是算盘常用的代码
Map<Integer, String> map = N.asMap(1, "one", 2, "two");
// Or for Immutable map
ImmutableMap<Integer, String> = ImmutableMap.of(1, "one", 2, "two");
声明:我是普通算盘的开发者。
第二种方法(双括号初始化)被认为是一种反模式，因此我将采用第一种方法。
另一个初始化静态Map的简单方法是使用这个实用函数:
public static <K, V> Map<K, V> mapOf(Object... keyValues) {
Map<K, V> map = new HashMap<>(keyValues.length / 2);
for (int index = 0; index < keyValues.length / 2; index++) {
map.put((K)keyValues[index * 2], (V)keyValues[index * 2 + 1]);
}
return map;
}
Map<Integer, String> map1 = mapOf(1, "value1", 2, "value2");
Map<String, String> map2 = mapOf("key1", "value1", "key2", "value2");
注意:在Java 9中你可以使用Map.of
