How to create a Java Function list?

Question

I want to create a list from Java Functions. But When I try to add a function to functions list, it says;

Non-static variable cannot be referenced from a static context

I don't get which method is static. Is there any idea?

My Main class:

public void createFunctionList() {
    List<Function> functions = new ArrayList<>();
    functions.add(Product::getInfo);
}

My product class:

public class Product 
{
    public Info getInfo() {
        return info;
    }
}

When you do Product::getInfo you make a reference to the function getInfo in the class Product. And thus without using an instance. So java is expecting getInfo to be static — vincrichaud
– vincrichaud, Commented Mar 20, 2018 at 10:32

Eran · Accepted Answer · 2018-03-20 10:39:14Z

3

Product::getInfo can be assigned to a Function<Product,Info> (since it's a non-static method, so it can be seen as a function that takes a Product instance and returns an Info instance). Change the declaration of functions from

List<Function> functions = new ArrayList<>();

to

List<Function<Product,Info>> functions = new ArrayList<>();

EDIT: I tested your code, and I get a different compilation error:

The type Product does not define getInfo(Object) that is applicable here.

The compilation error you got is misleading. Once I make the suggested change, the error goes away.

edited Mar 20, 2018 at 10:39

answered Mar 20, 2018 at 10:32

Eran

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6 Comments

Héctor Valls Over a year ago

I don't think this solves the problem. getInfo should be static to be referenced with Product::getinfo

Eran Over a year ago

@Héctor method references can refer to non static methods. A static method wouldn't fit the Function interface, since it would have a return type but no input argument.

Héctor Valls Over a year ago

Of course, but a this reference is needed, when it's not static.

Eran Over a year ago

@Héctor it's not needed. this is only needed if you want the method reference to refer to a specific instance of the class.

Eran Over a year ago

@hellzone do you want a List<Function<Object,Object>>? That's not type safe. Each function expects a specific type of input and produces a specific type of output. If you do use a List<Function<Object,Object>>, you can add functions to it such as functions.add(p -> ((Product)p).getInfo());, but that casting is not safe.

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