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Cast an object to its specific type at runtime Java [duplicate]

I'm experimenting with this code:
interface Callee {
public void foo(Object o);
public void foo(String s);
public void foo(Integer i);
}
class CalleeImpl implements Callee
public void foo(Object o) {
logger.debug("foo(Object o)");
}
public void foo(String s) {
logger.debug("foo(\"" + s + "\")");
}
public void foo(Integer i) {
logger.debug("foo(" + i + ")");
}
}
Callee callee = new CalleeImpl();
Object i = new Integer(12);
Object s = "foobar";
Object o = new Object();
callee.foo(i);
callee.foo(s);
callee.foo(o);
This prints foo(Object o) three times. I expect the method selection to take in consideration the real (not the declared) parameter type. Am I missing something? Is there a way to modify this code so that it'll print foo(12), foo("foobar") and foo(Object o)?

I expect the method selection to take
in consideration the real (not the
declared) parameter type. Am I missing
something?
Yes. Your expectation is wrong. In Java, dynamic method dispatch happens only for the object the method is called on, not for the parameter types of overloaded methods.
Citing the Java Language Specification:
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). If
the method that is to be invoked is an
instance method, the actual method to
be invoked will be determined at run
time, using dynamic method lookup
(§15.12.4).

As mentioned before overloading resolution is performed at compile time.
Java Puzzlers has a nice example for that:
Puzzle 46: The Case of the Confusing Constructor
This puzzle presents you with two Confusing constructors. The main method invokes a constructor,
but which one? The program's output depends on the answer. What does the program print, or is it
even legal?
public class Confusing {
private Confusing(Object o) {
System.out.println("Object");
}
private Confusing(double[] dArray) {
System.out.println("double array");
}
public static void main(String[] args) {
new Confusing(null);
}
}
Solution 46: Case of the Confusing Constructor
...
Java's overload resolution process operates in two phases. The first phase selects all the methods or constructors that are accessible and applicable. The second phase selects the most specific of the methods or constructors selected in the first phase. One method or constructor is less specific than another if it can accept any parameters passed to the other [JLS 15.12.2.5].
In our program, both constructors are accessible and applicable. The constructor
Confusing(Object) accepts any parameter passed to Confusing(double[]), so
Confusing(Object) is less specific. (Every double array is an Object, but not every Object is a double array.) The most specific constructor is therefore Confusing(double[]), which explains the program's output.
This behavior makes sense if you pass a value of type double[]; it is counterintuitive if you pass null. The key to understanding this puzzle is that the test for which method or constructor is most specific does not use the actual parameters: the parameters appearing in the invocation.
They are used only to determine which overloadings are applicable. Once the compiler determines which overloadings are applicable and accessible, it selects the most specific overloading, using only the formal parameters: the parameters appearing in the declaration.
To invoke the Confusing(Object) constructor with a null parameter, write new
Confusing((Object)null). This ensures that only Confusing(Object) is applicable. More
generally, to force the compiler to select a specific overloading, cast actual parameters to the declared types of the formal parameters.

Ability to dispatch a call to a method based on types of arguments is called multiple dispatch. In Java this is done with Visitor pattern.
However, since you're dealing with Integers and Strings, you cannot easily incorporate this pattern (you just cannot modify these classes). Thus, a giant switch on object run-time will be your weapon of choice.

In Java the method to call (as in which method signature to use) is determined at compile time, so it goes with the compile time type.
The typical pattern for working around this is to check the object type in the method with the Object signature and delegate to the method with a cast.
public void foo(Object o) {
if (o instanceof String) foo((String) o);
if (o instanceof Integer) foo((Integer) o);
logger.debug("foo(Object o)");
}
If you have many types and this is unmanageable, then method overloading is probably not the right approach, rather the public method should just take Object and implement some kind of strategy pattern to delegate the appropriate handling per object type.

I had a similar issue with calling the right constructor of a class called "Parameter" that could take several basic Java types such as String, Integer, Boolean, Long, etc. Given an array of Objects, I want to convert them into an array of my Parameter objects by calling the most-specific constructor for each Object in the input array. I also wanted to define the constructor Parameter(Object o) that would throw an IllegalArgumentException. I of course found this method being invoked for every Object in my array.
The solution I used was to look up the constructor via reflection...
public Parameter[] convertObjectsToParameters(Object[] objArray) {
Parameter[] paramArray = new Parameter[objArray.length];
int i = 0;
for (Object obj : objArray) {
try {
Constructor<Parameter> cons = Parameter.class.getConstructor(obj.getClass());
paramArray[i++] = cons.newInstance(obj);
} catch (Exception e) {
throw new IllegalArgumentException("This method can't handle objects of type: " + obj.getClass(), e);
}
}
return paramArray;
}
No ugly instanceof, switch statements, or visitor pattern required! :)

Java looks at the reference type when trying to determine which method to call. If you want to force your code you choose the 'right' method, you can declare your fields as instances of the specific type:
Integeri = new Integer(12);
String s = "foobar";
Object o = new Object();
You could also cast your params as the type of the param:
callee.foo(i);
callee.foo((String)s);
callee.foo(((Integer)o);

If there is an exact match between the number and types of arguments specified in the method call and the method signature of an overloaded method then that is the method that will be invoked. You are using Object references, so java decides at compile time that for Object param, there is a method which accepts directly Object. So it called that method 3 times.

Cannot find matching method from Groovy to Java

Groovyc: [Static type checking] - Cannot find matching method io.swagger.client.util.EmUtil#addLobList(java.lang.Object, java.lang.Object). Please check if the declared type is correct and if the method exists.
I am calling this function in a static block in groovy, as shown below:
static {
Arrays.asList(LOBEnum.values()).forEach {lob -> EmUtil.getInstance().addLobList(lob.name(), lob.getLob())}
EmUtil.getInstance().setPrefix("CCB_Reference_Data_")
EmUtil.getInstance().init()
}
This calls a java function, as shown below:
public void addLobList(String lob, String licenseLob) {
lobList.add(lob);
lobLicenseList.add(licenseLob);
}

It looks like lob.name() and lob.getLob() each have a return type of Object. If you want to use the return values of those methods as parameters to your addLobList method, the static type checker needs to make sure those are String instance. You can cast those with (String). Alternatively, you could change the return value of those methods to String.

Overloading method with parent type and child type argument and passing null while calling. Why method with childtype argument is getting called [duplicate]

I have added three methods with parameters:
public static void doSomething(Object obj) {
System.out.println("Object called");
}
public static void doSomething(char[] obj) {
System.out.println("Array called");
}
public static void doSomething(Integer obj) {
System.out.println("Integer called");
}
When I am calling doSomething(null) , then compiler throws error as ambiguous methods. So is the issue because Integer and char[] methods or Integer and Object methods?

Java will always try to use the most specific applicable version of a method that's available (see JLS §15.12.2).
Object, char[] and Integer can all take null as a valid value. Therefore all 3 version are applicable, so Java will have to find the most specific one.
Since Object is the super-type of char[], the array version is more specific than the Object-version. So if only those two methods exist, the char[] version will be chosen.
When both the char[] and Integer versions are available, then both of them are more specific than Object but none is more specific than the other, so Java can't decide which one to call. In this case you'll have to explicitly mention which one you want to call by casting the argument to the appropriate type.
Note that in practice this problem occurs far more seldom than one might think. The reason for this is that it only happens when you're explicitly calling a method with null or with a variable of a rather un-specific type (such as Object).
On the contrary, the following invocation would be perfectly unambiguous:
char[] x = null;
doSomething(x);
Although you're still passing the value null, Java knows exactly which method to call, since it will take the type of the variable into account.

Each pair of these three methods is ambiguous by itself when called with a null argument. Because each parameter type is a reference type.
The following are the three ways to call one specific method of yours with null.
doSomething( (Object) null);
doSomething( (Integer) null);
doSomething( (char[]) null);
May I suggest to remove this ambiguity if you actually plan to call these methods with null arguments. Such a design invites errors in the future.

null is a valid value for any of the three types; so the compiler cannot decide which function to use. Use something like doSomething((Object)null) or doSomething((Integer)null) instead.

Every class in Java extends Object class.Even Integer class also extends Object. Hence both Object and Integer are considered as Object instance. So when you pass null as a parameter than compiler gets confused that which object method to call i.e. With parameter Object or parameter Integer since they both are object and their reference can be null. But the primitives in java does not extends Object.

I Have tried this and when there is exactly one pair of overloaded method and one of them has a parameter type Object then the compiler will always select the method with more specific type. But when there is more than one specific type, then the compiler throws an ambiguous method error.
Since this is a compile time event, this can only happen when one intentionally passes null to this method. If this is done intentionally then it is better to overload this method again with no parameter or create another method altogether.

class Sample{
public static void main (String[] args) {
Sample s = new Sample();
s.printVal(null);
}
public static void printVal(Object i){
System.out.println("obj called "+i);
}
public static void printVal(Integer i){
System.out.println("Int called "+i);
}
}
The output is Int called null and so ambiguity is with char[] and Integer

there is an ambiguity because of doSomething(char[] obj) and doSomething(Integer obj).
char[] and Integer both are the same superior for null that's why they are ambiguous.

Create generic Java method wrapper for pre and post processing

I'm basically trying to create a static method that will serve as a wrapper for any method I pass and will execute something before and after the actual execution of the method itself. I'd prefer to do it using Java 8 new coding style. So far I have a class that has a static method, but I'm not sure what the parameter type should be so it can take any method with any type of parameter and then execute it. Like i mentioned I want to do some stuff before and after the method executes.
For example: executeAndProcess(anyMethod(anyParam));

Your method can accept a Supplier instance and return its result:
static <T> T executeAndProcess(Supplier<T> s) {
preExecute();
T result = s.get();
postExecute();
return result;
}
Call it like this:
AnyClass result = executeAndProcess(() -> anyMethod(anyParam));

Call method through method in parameter

How can I call a method that is passed on through a parameter in a method in Java 8?
Small example I have a method like that:
void output(String text) {
System.out.println(text)
}
Now I want to pass that method to another class which should call output and set something for text. Is that possible?

The class you want to pass the function to must take a parameter of type Consumer<String>. This class represents a function that takes a parameter of some type (String in this case), and has return type void. A Consumer has a method accept that takes the parameter and calls the function.
You can create you class like this:
class Test {
Test(Consumer<String> consumer) {
consumer.accept("This is a string!");
}
}
Now, when you want to instantiate this class, you need to pass your function to it like this:
Test t = new Test(this::output);
The :: notation is called a method reference. The this (before the colons) means that the method is located in the object you're in. It can be changed to, for example, MyClass::output if it is a static method on MyClass, or myObject::output if it is a method on the object myObject.