Method declaration with a function to extract fields of an object and a binary operator which will b-CodePudding

Given the following sample class Foo

@Getter
@AllArgsConstructor
@ToString
static class Foo {
    int field1;
    int field2;
    String field3;
}

I want to declare a method, let's call it applyBinOperator, which accepts two Foo objects a key-extractor and a binary operator and apply that operator to the extracted fields of the Foo objects and returns the result. So basically I want to call the method in the following way or something similar:

Foo foo1      = new Foo(10, 20, "first");
Foo foo2      = new Foo(100, 200, "second");

int sum       = applyBinOperator(foo1, foo2, Foo::getField1, (a,b) -> a b);

int prod      = applyBinOperator(foo1, foo2, Foo::getField2, (a,b) -> a*b);

String concat = applyBinOperator(foo1, foo2, Foo::getField3, (a,b) -> String.join(a,b));

//and get sum = 110, prod = 4000 and concat = firstsecond

But I am struggling to find the correct syntax to define the method since I am not very familiar with generics. I have tried the following but it doesn't compile yet:

public  static <T> T applyBinOperator(Foo foo1, Foo foo2, 
       Function<? super T, ?> keyExtractor, BinaryOperator<T> binaryOperator) {

    return binaryOperator.apply(keyExtractor.apply(foo1), keyExtractor.apply(foo2));
}

Can someone help me with the correct syntax?

CodePudding user response：

You seem to have just mixed up the order of the type parameters of Function. To write "a function that takes a Foo and returns a T", it is Function<Foo, T>:

public  static <T> T applyBinOperator(Foo foo1, Foo foo2,
                                      Function<Foo, T> keyExtractor, BinaryOperator<T> binaryOperator) {
    return binaryOperator.apply(keyExtractor.apply(foo1), keyExtractor.apply(foo2));
}

We can make the Foo part generic too, and also apply PECS:

public  static <T, U> T applyBinOperator(U foo1, U foo2,
                                      Function<? super U, ? extends T> keyExtractor, BinaryOperator<T> binaryOperator) {
    return binaryOperator.apply(keyExtractor.apply(foo1), keyExtractor.apply(foo2));
}

Also, String.join does not join "first" and "second" together. You probably meant String.concat, or just use , which works with strings too.