What is the lambda expression equivalent to the System.out::println method reference?

Equivalence of System.out::println Method Reference in Lambda Expression

In Java, the method reference System.out::println allows for a concise way to refer to the System.out.println() method. It evaluates System.out and assigns it to a variable.

Question:

What is the corresponding lambda expression for System.out::println that provides the same behavior?

Answer:

An exact equivalent lambda expression would be:

num -> {
    PrintStream p = Objects.requireNonNull(System.out);
    p.println(num);
}

This lambda expression captures the reference to System.out in the variable p before executing the println method. This prevents any changes to System.out from affecting the behavior of the lambda expression.

Difference from Simple Lambda Expression:

A simpler lambda expression like num -> System.out.println(num) would evaluate System.out each time it's called, making it susceptible to changes in System.out while the lambda is executed.

Usage:

The equivalent lambda expression can be used in the same way as System.out::println, such as:

List numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);
numbers.forEach(num -> {
    PrintStream p = Objects.requireNonNull(System.out);
    p.println(num);
});

This code will print the numbers in the list to the console.

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