如何使用lambda初始化字段 [英] How to initialize field using lambda

问题描述

我需要一个ConnectionFactory类型的实例字段。
供应商可以这样做：

I need an instance field of type ConnectionFactory. A supplier can do it:

private Supplier<ConnectionFactory> connectionFactorySupplier = () -> {
    // code omitted
    return null;
};

private ConnectionFactory connectionFactory = connectionFactorySupplier.get();

它可以缩短为一行，如下所示：

It can be shortened to one line like so:

private ConnectionFactory connectionFactory = new Supplier<ConnectionFactory>() {
    @Override
    public ConnectionFactory get() {
        // code omitted
        return null;
    }
}.get();

使用lambda有没有办法让它更简洁一些？我尝试过以下但不编译：

Is there a way to make this a bit less verbose, using lambda? I've tried the following but it doesn't compile:

private ConnectionFactory connectionFactory= () -> {
    // code omitted
    return null;
}.get(); 
// Compilation failure
// error: lambda expression not expected here

推荐答案

最后一个片段中的问题是编译器无法猜测

The problem in your last snippet is that there is no way for the compiler to guess that

() -> { 
// code omitted
    return null;
}

是实现的SAM的lambda表达式供应商界面（似乎你首先错过了表达式的括号，但无论如何）。

is a lambda expression that implements the SAM of the Supplier interface (and it seems you missed parentheses around the expression in the first place, but anyway).

你可以做的事情是转换lambda以告诉编译器这实际上是您实现的 Supplier 接口的抽象方法：

The thing you could do is to cast the lambda in order to tell the compiler that this is actually the abstract method of the Supplier interface that you implement:

private ConnectionFactory connectionFactory = 
     ((Supplier<ConnectionFactory>)() -> {
         // code omitted
         return null;
     }).get();

但是你得到的是什么，而不是有一个初始化器

But then what do you gain with that instead of having an initializer

private ConnectionFactory connectionFactory;
{
    //code omitted
    connectionFactory = null;
}

或初始化构造函数内的连接或使用最终方法：

or initialize the connection within the constructor or with a final method:

private ConnectionFactory connectionFactory = initConnection();

private final ConnectionFactory initConnection() {
    //code omitted
    return null;
}

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