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...
}
}
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;
}
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
这一个使用Apache common -lang,它很可能已经在你的类路径上了:
Map<String, String> collect = Stream.of(
Pair.of("hello", "world"),
Pair.of("abc", "123"),
Pair.of("java", "eight")
).collect(Collectors.toMap(Pair::getKey, Pair::getValue));