How to write overloading functions in java

Question

I wonder what is missing to get the following overloading functions to work?

e.g.

List<?> list = ...;
for (Object element : list)
  Fun(element);

public void Fun(Integer element) { ... }
public void Fun(String element) { ... }
public void Fun(MyClass element) { ... }

`element` is not declared as any of those types. Overloading is a compile time concept. — Sotirios Delimanolis, Feb 04 '14 at 04:10
There is no such thing. Go through [the tutorial](http://docs.oracle.com/javase/tutorial/java/javaOO/methods.html) again. — Sotirios Delimanolis, Feb 04 '14 at 04:11
@Kon I want the above code compile and based on different lists, the corresponding Fun will be run using that generic for loop, thank you. — user1589188, Feb 04 '14 at 04:13
Then you need to add an overloaded Fun method which accepts an Object as a parameter. — Kon, Feb 04 '14 at 04:14
@Kon sorry that was not my intend. I dont process Object, instead I process Integer/String/MyClass. So instead of writing a for loop for Integer/String/MyClass, I want to write just one for loop using Object. — user1589188, Feb 04 '14 at 04:17

score 2 · Accepted Answer · edited May 23 '17 at 10:31

Java uses double-dispatch -- the compiler chooses among overloaded methods based on parameter types, and later, at runtime, the JVM chooses among implementations (overridings) of a method based on the runtime type of this.

You're supplying multiple overloadings, but Java will not automatically switch between them. It looks for an overloading that can accept (Object element) and finds none so the compiler rejects your program.

One way to fix this is to have a runtime dispatch method:

public void Fun(Object element) {
  if (element instanceof Integer) { Fun((Integer) element); }
  ...
}

This works because its static type is very general, and it calls Fun with a parameter type whose type is more specific so is dispatched to a different overloading.

Frequent use of instanceof is often considered an anti-pattern.

Yes thank you, I see where this is going. I was just hoping those instanceof can be done implicitly by jvm at runtime based on the actual instance of the Object element, instead of doing it explicitly using that dispatch method. — user1589188, Feb 04 '14 at 04:23

score 2 · Answer 2 · answered Feb 04 '14 at 04:17

2

Fun(x) where x is of type Object will try to find a function with the signature Fun(Object), and you've never declared one. I think you want something like this instead.

public void fun(Object obj) {
    if (obj instanceof Integer)
        ((Integer) obj).someMethod(); // line 3
    else if (obj instanceof String)
        ...
}

Note cast in line 3. I would also recommend you to follow Java naming conventions: someMethodName for functions, and SomeClassName for classes.

answered Feb 04 '14 at 04:17

Christian Tapia

33,620
7
56
73

Yes, thank you, I fully understand this can be done. Actually thats my original code. I was only thinking if those instanceof can be removed because obviously jvm knows the actual instance of Object obj and would be nice to call appropriate overloading functions for me automatically. – user1589188 Feb 04 '14 at 04:26

score 2 · Answer 3 · answered Feb 04 '14 at 04:17

2

Add the logic in for identifying the type of object and typecast it. Then the code will work as you expected.

List<?> list = ...;
for (Object element : list)
// Type cast element to one of the type (Integer, String or MyClass) using Instance of  
  Fun(element);

public void Fun(Integer element) { ... }
public void Fun(String element) { ... }
public void Fun(MyClass element) { ... }

answered Feb 04 '14 at 04:17

Sureshkumar Panneerselvan

749
6
22

Yes sure. I came from that code and was hoping to simplify it further. – user1589188 Feb 04 '14 at 04:27

score 2 · Answer 4 · answered Feb 04 '14 at 04:18

2

you can do something like that

public void SuperFun(Object o){
     if(o instanceof Integer)
          Fun((Integer)o);
     if(o instanceof String)
          Fun((String)o);
     if(o instanceof MyClass)
          Fun((MyClass)o);

}

answered Feb 04 '14 at 04:18

stinepike

54,068
14
92
112

score 1 · Answer 5 · answered Feb 04 '14 at 04:18

The Java Language Specification says the following about overloading

When a method is invoked (§15.12), the number of actual arguments (and any explicit type arguments) and the compile-time types of the arguments are used, at compile time, to determine the signature of the method that will be invoked (§15.12.2).

The reference you are trying to use as a method argument is declared as type Object, but you don't have an overloaded method that accepts an Object argument.

You will have to rethink your design.

How to write overloading functions in java

5 Answers5