Question

I’d like to write a method that can accept a type param (or whatever the method can figure out the type from) and return a value of this type so I don’t have to cast the return type.

Here is a method:

public Object doIt(Object param){
    if(param instanceof String){
        return "string";
    }else if(param instanceof Integer){
        return 1;
    }else{
        return null;
    }
}

When I call this method, and pass in it a String, even if I know the return type will be a String I have to cast the return Object. This is similar to the int param.

How shall I write this method to accept a type param, and return this type?

Answer 1

if you don’t want to have a specific interface to handle that stuff you can use a generic method in this way:

public <T> T mymethod(T type)
{
  return type;
}

Mind that in this way the compiler doesn’t know anything about the type that you plan to use inside that method, so you should use a bound type, for example:

public <T extends YourType> T mymethod(T type)
{
  // now you can use YourType methods
  return type;
}

But you should be sure that you need a generic method, that means that the implementation of doIt will be the same for all the types you are planning to use it with. Otherwise if every implementation is different just overload the methods, it will work fine since return type is not used for dynamic binding:

public String my(String s)
{
  return s;
}

public int my(int s)
{
  return s;
}

int i = my(23);
String s = my("lol");