This question already has answers here:
Overloaded method selection based on the parameter's real type
(7 answers)
Closed 9 years ago.
I have two methods in a class...
public void method(String var1, String var2, Object var3) {
//do stuff
}
public void method(String var1, String var2, Object[] var3) {
//do stuff
}
When I make the following call... obj.method("string", "string", obj) the correct method is called, however, when I try to call obj.method("string", "string", obj[]) the first incarnation of that method is called.
Is there any annotation or "hint" I can give to make sure the method I anticipate being called will be called? I know an Object[] is anObject, but I would hope that at runtime the method with Object[] would be called before Object.
Nope, for me this calls the second method as expected!
In the case where you want one to be called over the other, then the hint you'd give is usually a cast to the exact parameter type expected by the method (this cast will work even on null.) So using (Object)arr would force the first method to be called, but by default it will definitely call the second method (on all correct versions of the JVM anyway - contrary to popular belief, this scenario is covered explicitly in the JLS and thus is not ambiguous.)
In Java the method overload happens during compile time. That means which method is called was decided during program compilation. Note this is different from method overriding.
Related
This question already has answers here:
java why should equals method input parameter be Object
(6 answers)
Overloading in Java and multiple dispatch
(6 answers)
Closed 4 years ago.
I was making some tests with the java equals method and I figured out that if my parameter is not of the generic type Object the test won’t pass, even if the two objects I create are of that type. Let’s say I want to check if an object is an animal, what I tried to do is writing down:
public boolean equals(Animal other) {
*some code*
}
And then I create a test for that method to compare the animals. But if I do that the test will fail, on the other side, if I write down:
public boolean equals(Object other) {
*some code*
}
and then test it, the test will pass. I understand that’s useless declaring the object of the desired type and try to test it but I don’t get why it doesn’t work in a good weather test case.
It is simple, Object class equals method signature is this
public boolean equals(Object obj)
But if you write equals method with Animal parameter then it will not be the Overridden equals method from object class. and when you try to compare objects by using .equals() Object class equals will be invoked
For this reason and to make it clear it is always recommended to use #Override annotation
The equals method is part of the base Object class in Java and the only way to make benefit of it is to override it. To override it you need to stick to the same signature which will tell any libraries using equals to invoke your method instead of the base one.
Your above code is doing an overloading which is a totally different method to the Java compiler.
This question already has answers here:
In Java, why does ((A)b).disp() call derived class method disp() instead of base class method disp() ?
(4 answers)
Closed 3 years ago.
I am learning more about polymorphism and casting in Java. I was running some code and I am quite confused with the output. I would really appreciate it if someone can explain what the compiler is doing here.
When I wrote this code, I thought that since newCar() is being cast to Vehicle, I assumed it would use the default method that was written in the Vehicle interface. But it looks like it used the overridden method in Car.
Thanks in advance!!
public interface Vehicle {
default boolean canFly() {return true;}
}
public class Car implements Vehicle {
boolean canFly() {return false;}
public static void main(String[] args) {
Vehicle vehicle = (Vehicle) new Car();
System.out.println(vehicle.canFly());
}
}
I saw possible duplicates; however, the question was targeted to only classes. My question is specifically with interfaces.
The whole point of inheritance and polymorphing is that it doesn't matter what type the variable has, the actual type of the created object determines which method will be called.
There is no way for you to call the Vehicle version of the canFly method, since it has been overridden by the Car class, and the real object is a Car.
The Lambda faq lists conflict resolution rules for default methods. First rule is:
Classes always win. A declaration in the class or a superclass takes priority over any default method declaration.
Because the class provides an implementation of the method, that always takes precedence over the default method on the interface. ("Default" means a fallback for cases where no implementation is provided otherwise.)
Casting only keeps the compiler from reporting an error due to some variable's type not matching the type of the thing being assigned to it. It's there for cases where you know it's ok to treat something as another type and you need to overrule the compiler. Casting has no effect at runtime, except to cause a ClassCastException to be thrown if the cast is erroneous. See here for more details.
TLDR: The idea that you could use casts to change what method gets called is not accurate.
This question already has answers here:
java why should equals method input parameter be Object
(6 answers)
Overloading in Java and multiple dispatch
(6 answers)
Closed 4 years ago.
I was making some tests with the java equals method and I figured out that if my parameter is not of the generic type Object the test won’t pass, even if the two objects I create are of that type. Let’s say I want to check if an object is an animal, what I tried to do is writing down:
public boolean equals(Animal other) {
*some code*
}
And then I create a test for that method to compare the animals. But if I do that the test will fail, on the other side, if I write down:
public boolean equals(Object other) {
*some code*
}
and then test it, the test will pass. I understand that’s useless declaring the object of the desired type and try to test it but I don’t get why it doesn’t work in a good weather test case.
It is simple, Object class equals method signature is this
public boolean equals(Object obj)
But if you write equals method with Animal parameter then it will not be the Overridden equals method from object class. and when you try to compare objects by using .equals() Object class equals will be invoked
For this reason and to make it clear it is always recommended to use #Override annotation
The equals method is part of the base Object class in Java and the only way to make benefit of it is to override it. To override it you need to stick to the same signature which will tell any libraries using equals to invoke your method instead of the base one.
Your above code is doing an overloading which is a totally different method to the Java compiler.
This question already has answers here:
Which overload will get selected for null in Java?
(3 answers)
Closed 8 years ago.
I want to know whether this is a valid overloading :
public class OverLoadingTest{
private void callFunction(Object object){
System.out.println("Printing Object");
}
private void callFunction(String string){
System.out.println("Printing String");
}
}
Further more, since someone asked me this question.
If I do like this,
OverLoadingTest test = new OverLoadingTest();
test.callFunction(null);
what will be printed ?
Of course my opinion is that it isn't valid overloading at all.
So no question of the second part.
Please tell me about this with some explanation.
The method with the least generic argument is called. So, in your case, it will be method accepting String
Note : If 2 classes are at the same level, then you will get an ambiguous call exception. For example if one method took String and another took Exception.
If more than one member method is both accessible and applicable to a method
invocation, it is necessary to choose one to provide the descriptor for
the run-time method dispatch.
The Java programming language uses the rule that the most specific method is chosen.
See more details in JSL 15.12.2.5
In your case, String method will be invoked, if argument is String or null and for other argument's types Object method will be invoked.
In your example, if you define one more method with argument type that is not String (e.g Integer), can't compile the source as it is ambiguous to be invoked between the methods with String and Integer as they are same level.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Method Overloading for NULL parameter
In the code below the output is
String
and if I remove the method with the parameter of type String then the output is
Object
I know how overloading of methods acts when the parameter types don't match exactly but I can not understand how null can be treated as an Object and/or a String parameter.
What is the explanation for this?
class C {
static void m1(Object x) {
System.out.print("Object");
}
static void m1(String x) {
System.out.print("String");
}
public static void main(String[] args) {
m1(null);
}
}
It always uses the most specific method according to the Java specs, section 15.12.2.5.
The intro is reasonably specific about it:
If more than one member method is both accessible and applicable to a method invocation, it is necessary to choose one to provide the descriptor for the run-time method dispatch. The Java programming language uses the rule that the most specific method is chosen.
The informal intuition is that one method is more specific than another if any invocation handled by the first method could be passed on to the other one without a compile-time type error.
Generally speaking, and at least for code readability, it's always best to try to be as explicit as possible. You could cast your null into the type that matches the signature you want to call. But that's definitely a questionable practice. It assumes everyone knows this rule and makes the code more difficult to read.
But it's a good academic question, so I +1 your question.
When multiple overloads match a signature, Java picks the most specific method from among them.
The value of null matches both Object and String, but String is a subclass of Object, so String is picked. If you add another overload with a sibling of String in the class hierarchy, you'd get a compile error.\
// DOES NOT COMPILE
class C {
static void m1(Object x) {
System.out.print("Object");
}
static void m1(String x) {
System.out.print("String");
}
static void m1(Integer x) {
System.out.print("Integer");
}
public static void main(String[] args) {
m1(null);
}
}
Here is a link to a post that discusses your code example at some length.
If you need to force the call of a aprticular overloaded method when passing null as parameter, you have to cast it, like this:
m1((String)null);
By doing this, you make sure you're calling the correct overloaded version of the method.
You have to set the type of null to tell Java what function you want to call.
So you do: m1((Object) null); to call the implementation with the Object parameter and you do m1((String) null); to call the other one.
1. As String is also an object the JVM got confused to call which method at runtime.
2. If you want to specify which method to call at runtime, you can do this by explicit casting
eg:
m1((String)null);