如何使用lambda初始化地图？ [英] How to initialize a map using a lambda?

问题描述

我要在单个语句（可能包含多个嵌套语句）中声明完全填充的地图字段，如下所示：

I want to declare a fully populated map field in a single statement, (which may contain several nested statements,) like this:

private static final Map<Integer,Boolean> map = 
    something-returning-an-unmodifiable-fully-populated-HashMap;

匿名初始化器不会这样做，因为调用函数返回一个新的填充映射将不会：它们需要两个顶级语句：一个用于变量声明，一个用于方法或初始化器。

Anonymous initializers won't do, for the same reason that invoking a function which returns a new populated map won't do: they require two top-level statements: one for the variable declaration, and one for the method or initializer.

双花括号（ {{和}} ）idiom将工作，但它创建一个新的类，扩展 HashMap& > ，我不喜欢这个开销。

The double curly bracket ({{ and }}) idiom will work, but it creates a whole new class which extends HashMap<>, and I do not like the overhead represented by this.

Java 8的lambda可能提供一个更好的方法来完成这个？

Do the lambdas of Java 8 perhaps offer a better way of accomplishing this?

推荐答案

以下是如何使用lambda实现字段初始值设定。

Here is how to implement a field initializer using a lambda.

private static final Map<Integer,Boolean> map =
    ((Supplier<Map<Integer,Boolean>>)(() -> { 
        Map<Integer,Boolean> map = new HashMap();
        map.put( 1, false );
        map.put( 2, true );
        return Collections.unmodifiableMap( map );
    })).get();

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