Java 8, Static methods vs Functions

Question

In Java 8 I want to create something that returns an argument or creates an instance if the argument is null.

I could do this by creating a static method or a UnaryOperator. Are the following approaches technically the same or are there technical differences that I should be aware of with either approach:

Static Method

static Cat initOrReturn(Cat c) {
    if (c==null) {
        return new Cat();
    }
    return c;
}

Function

UnaryOperator<Cat> initOrReturn = c -> {
    if (c==null) {
        return new Cat();
    }
    return c;
}

Answer 1

You can think about function as a "value" - something that can be stored to variable, and passed around.

This "value" can be used as e.g. method parameter to (during runtime) dynamically change part of method implementation.

Take a look at this basic example. Hope that can illustrate idea:

static Number functionsUsageExample(Integer someValue, UnaryOperator<Number> unaryOperator) {
    if (someValue == 1) {
        //do something
    }
    Number result = unaryOperator.apply(someValue); // dynamically apply supplied implementation
    // do something else
    return result;
}

public static void main(String[] args) {
    UnaryOperator<Number> add = i -> i.doubleValue() + 20;
    UnaryOperator<Number> multiply = i -> i.intValue() * 3;

    var additionResult = functionsUsageExample(1, add);
    var multiplicationResult = functionsUsageExample(1, multiply);
    
    //additionResult value is:           21.0
    //multiplicationResult value is:     3
}

Function can be also used as a 'helper methods' stored inside method block. This way you will not corrupt class scope with method that is used only in one place.