When is a Method Reference Not Equivalent to a Lambda Expression?

Method Reference vs Lambda Expression: Exploring the Equivalency of System.out::println

When utilizing a method reference such as System.out::println, the function first evaluates System.out and captures the resulting value in a lambda expression. For instance, the lambda equivalent of this method reference would be:

<code class="java">Consumer<Integer> consumer = System.out::println;</code>

This lambda expression behaves similarly to the code below:

<code class="java">Consumer<Integer> consumer = new Consumer<Integer>() {
    @Override
    public void accept(Integer i) {
        System.out.println(i);
    }
};</code>

However, the method reference System.out::println offers an advantage over the lambda expression as it always evaluates System.out at compile time. In the case of the lambda expression, System.out is evaluated each time the method is invoked.

To ensure an exact equivalent to the method reference, the following code must be used:

<code class="java">PrintStream printStream = Objects.requireNonNull(System.out);
Consumer<Integer> consumer = o -> printStream.println(o);</code>

Here, printStream is a variable capturing the System.out value, guaranteeing that any subsequent changes to System.out will not affect this consumer.

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