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
如果你想要一些简洁和相对安全的东西，你可以将编译时类型检查转移到运行时:
static final Map<String, Integer> map = MapUtils.unmodifiableMap(
String.class, Integer.class,
"cat", 4,
"dog", 2,
"frog", 17
);
这个实现应该捕获任何错误:
import java.util.HashMap;
public abstract class MapUtils
{
private MapUtils() { }
public static <K, V> HashMap<K, V> unmodifiableMap(
Class<? extends K> keyClazz,
Class<? extends V> valClazz,
Object...keyValues)
{
return Collections.<K, V>unmodifiableMap(makeMap(
keyClazz,
valClazz,
keyValues));
}
public static <K, V> HashMap<K, V> makeMap(
Class<? extends K> keyClazz,
Class<? extends V> valClazz,
Object...keyValues)
{
if (keyValues.length % 2 != 0)
{
throw new IllegalArgumentException(
"'keyValues' was formatted incorrectly! "
+ "(Expected an even length, but found '" + keyValues.length + "')");
}
HashMap<K, V> result = new HashMap<K, V>(keyValues.length / 2);
for (int i = 0; i < keyValues.length;)
{
K key = cast(keyClazz, keyValues[i], i);
++i;
V val = cast(valClazz, keyValues[i], i);
++i;
result.put(key, val);
}
return result;
}
private static <T> T cast(Class<? extends T> clazz, Object object, int i)
{
try
{
return clazz.cast(object);
}
catch (ClassCastException e)
{
String objectName = (i % 2 == 0) ? "Key" : "Value";
String format = "%s at index %d ('%s') wasn't assignable to type '%s'";
throw new IllegalArgumentException(String.format(format, objectName, i, object.toString(), clazz.getSimpleName()), e);
}
}
}
其他回答
我喜欢匿名类语法;只是代码更少。然而，我发现的一个主要缺点是，您将无法通过远程序列化该对象。您将得到一个关于无法在远程端找到匿名类的异常。
因为Java不支持地图文字，所以必须始终显式地实例化和填充地图实例。
幸运的是，在Java中可以使用工厂方法来近似映射字面量的行为。
例如:
public class LiteralMapFactory {
// Creates a map from a list of entries
@SafeVarargs
public static <K, V> Map<K, V> mapOf(Map.Entry<K, V>... entries) {
LinkedHashMap<K, V> map = new LinkedHashMap<>();
for (Map.Entry<K, V> entry : entries) {
map.put(entry.getKey(), entry.getValue());
}
return map;
}
// Creates a map entry
public static <K, V> Map.Entry<K, V> entry(K key, V value) {
return new AbstractMap.SimpleEntry<>(key, value);
}
public static void main(String[] args) {
System.out.println(mapOf(entry("a", 1), entry("b", 2), entry("c", 3)));
}
}
输出:
{a=1, b=2, c=3}
这比一次创建和填充一个元素要方便得多。
如果你想要不可修改的映射，最后java 9增加了一个很酷的工厂方法的映射接口。在Set, List中也添加了类似的方法。
Map<String, String> unmodifiableMap = Map。Of ("key1"， "value1"， "key2"， "value2");
如果你想要一些简洁和相对安全的东西，你可以将编译时类型检查转移到运行时:
static final Map<String, Integer> map = MapUtils.unmodifiableMap(
String.class, Integer.class,
"cat", 4,
"dog", 2,
"frog", 17
);
这个实现应该捕获任何错误:
import java.util.HashMap;
public abstract class MapUtils
{
private MapUtils() { }
public static <K, V> HashMap<K, V> unmodifiableMap(
Class<? extends K> keyClazz,
Class<? extends V> valClazz,
Object...keyValues)
{
return Collections.<K, V>unmodifiableMap(makeMap(
keyClazz,
valClazz,
keyValues));
}
public static <K, V> HashMap<K, V> makeMap(
Class<? extends K> keyClazz,
Class<? extends V> valClazz,
Object...keyValues)
{
if (keyValues.length % 2 != 0)
{
throw new IllegalArgumentException(
"'keyValues' was formatted incorrectly! "
+ "(Expected an even length, but found '" + keyValues.length + "')");
}
HashMap<K, V> result = new HashMap<K, V>(keyValues.length / 2);
for (int i = 0; i < keyValues.length;)
{
K key = cast(keyClazz, keyValues[i], i);
++i;
V val = cast(valClazz, keyValues[i], i);
++i;
result.put(key, val);
}
return result;
}
private static <T> T cast(Class<? extends T> clazz, Object object, int i)
{
try
{
return clazz.cast(object);
}
catch (ClassCastException e)
{
String objectName = (i % 2 == 0) ? "Key" : "Value";
String format = "%s at index %d ('%s') wasn't assignable to type '%s'";
throw new IllegalArgumentException(String.format(format, objectName, i, object.toString(), clazz.getSimpleName()), e);
}
}
}
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。
