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

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"}
 });

在这种情况下，我绝不会创建匿名子类。静态初始化器同样有效，如果你想让映射不可修改，例如:

private static final Map<Integer, String> MY_MAP;
static
{
    Map<Integer, String>tempMap = new HashMap<Integer, String>();
    tempMap.put(1, "one");
    tempMap.put(2, "two");
    MY_MAP = Collections.unmodifiableMap(tempMap);
}

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。