Lambda expression's signature does not match the signature of the functional interface method apply

Question

I want to declare a function which accepts 3 parameters and returns an custom object in return like this

public static returnResult leadRatingFunction(LeadMaster lead,JSONObject json,String str)
{

}
// Where returnResult and LeadMaster are custom objects 

I have declared this function in functional interface as follows,

@SuppressWarnings("hiding")
@FunctionalInterface

interface Function<LeadMaster,JSONObject,String,returnResult>
{
    public returnResult apply(LeadMaster lead,JSONObject jsonObject,String str);
}

I want to use this function as hash map value like this,

 Map<String, Function<LeadMaster,JSONObject,String,returnResult>> commands = new HashMap<>();
         commands.put("leadRating",res -> leadRatingFunction(input1 ,input2 ,input3) ) ;

But it is giving error as "Lambda expression's signature does not match the signature of the functional interface method apply(LeadMaster, JSONObject, String)"

Thank you

Answer 1

A lambda expression that matches Function<LeadMaster,JSONObject,String,returnResult>, would require three arguments:

Map<String, Function<LeadMaster,JSONObject,String,returnResult>> commands = new HashMap<>();
commands.put("leadRating",(a,b,c) -> leadRatingFunction(a,b,c));

Alternately, as Lino commented, you can use a method reference:

commands.put("leadRating",YourClass::leadRatingFunction);

BTW, I'm not sure you want your Function<LeadMaster,JSONObject,String,returnResult> interface to be generic, since you placed names of actual classes as generic type parameters.

If you want generic parameters, use generic names:

interface Function<A,B,C,D>
{
    public D apply(A a ,B b , C c);
}

Otherwise, it doesn't have to be generic:

interface Function
{
    public returnResult apply(LeadMaster lead,JSONObject jsonObject,String str);
}