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");
}
};
}
我喜欢使用静态初始化“技术”,当我有一个抽象类的具体实现,它定义了一个初始化构造函数,但没有默认构造函数,但我希望我的子类有一个默认构造函数。
例如:
public abstract class Shape {
public static final String COLOR_KEY = "color_key";
public static final String OPAQUE_KEY = "opaque_key";
private final String color;
private final Boolean opaque;
/**
* Initializing constructor - note no default constructor.
*
* @param properties a collection of Shape properties
*/
public Shape(Map<String, Object> properties) {
color = ((String) properties.getOrDefault(COLOR_KEY, "black"));
opaque = (Boolean) properties.getOrDefault(OPAQUE_KEY, false);
}
/**
* Color property accessor method.
*
* @return the color of this Shape
*/
public String getColor() {
return color;
}
/**
* Opaque property accessor method.
*
* @return true if this Shape is opaque, false otherwise
*/
public Boolean isOpaque() {
return opaque;
}
}
以及这个类的具体实现——但它想要/需要一个默认构造函数:
public class SquareShapeImpl extends Shape {
private static final Map<String, Object> DEFAULT_PROPS = new HashMap<>();
static {
DEFAULT_PROPS.put(Shape.COLOR_KEY, "yellow");
DEFAULT_PROPS.put(Shape.OPAQUE_KEY, false);
}
/**
* Default constructor -- intializes this square to be a translucent yellow
*/
public SquareShapeImpl() {
// the static initializer was useful here because the call to
// this(...) must be the first statement in this constructor
// i.e., we can't be mucking around and creating a map here
this(DEFAULT_PROPS);
}
/**
* Initializing constructor -- create a Square with the given
* collection of properties.
*
* @param props a collection of properties for this SquareShapeImpl
*/
public SquareShapeImpl(Map<String, Object> props) {
super(props);
}
}
然后要使用这个默认构造函数,只需执行以下操作:
public class StaticInitDemo {
public static void main(String[] args) {
// create a translucent, yellow square...
Shape defaultSquare = new SquareShapeImpl();
// etc...
}
}
这是我最喜欢的
不想(或不能)使用Guava的ImmutableMap.of()
或者我需要一个可变Map
或者我需要从JDK9+的Map.of()中超过10个条目限制
public static <A> Map<String, A> asMap(Object... keysAndValues) {
return new LinkedHashMap<String, A>() {{
for (int i = 0; i < keysAndValues.length - 1; i++) {
put(keysAndValues[i].toString(), (A) keysAndValues[++i]);
}
}};
}
它非常紧凑,并且忽略了杂散值(即没有值的最终键)。
用法:
Map<String, String> one = asMap("1stKey", "1stVal", "2ndKey", "2ndVal");
Map<String, Object> two = asMap("1stKey", Boolean.TRUE, "2ndKey", new Integer(2));
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…)
第二个方法的一个优点是,你可以用Collections.unmodifiableMap()来包装它,以确保以后不会更新集合:
private static final Map<Integer, String> CONSTANT_MAP =
Collections.unmodifiableMap(new HashMap<Integer, String>() {{
put(1, "one");
put(2, "two");
}});
// later on...
CONSTANT_MAP.put(3, "three"); // going to throw an exception!
和往常一样,apache-commons有合适的方法MapUtils。putAll(地图、对象[]):
例如,要创建一个彩色地图:
Map<String, String> colorMap = MapUtils.putAll(new HashMap<String, String>(), new String[][] {
{"RED", "#FF0000"},
{"GREEN", "#00FF00"},
{"BLUE", "#0000FF"}
});