This question already has answers here:
Why doesn't Java allow overriding of static methods?
(22 answers)
Closed 5 years ago.
This is the code that I'm dealing with:
class Animal {
public void hide() {
System.out.println("The hide method in Animal.");
}
public void override() {
System.out.println("The override method in Animal.");
}
public static void clank()
{
System.out.println("Clank in Animal");
}
}
and
public class Cat extends Animal {
public void hide() {
System.out.println("The hide method in Cat.");
}
public void override() {
System.out.println("The override method in Cat.");
}
public static void clank()
{
System.out.println("Clank in Cat");
}
public static void main(String[] args) {
Cat myCat = new Cat();
Animal myAnimal = new Cat();
myAnimal.hide();
myAnimal.override();
myAnimal.clank();
}
}
The output of this code is:
The hide method in Cat.
The override method in Cat.
Clank in Animal
why does declaring both clank methods as static doesn't allow the clank in Animal to be overridden? should the output be "Clank in Cat"? Thanks!
Static methods are class methods, they don't need any instance to be called. Access to them is always resolved during compile time. So in this case, since the compile time type of myAnimal is Animal, calling myAnimal.clank() is equal to Animal.clank(). But if you write Cat myAnimal = new Cat(); and then call myAnimal.clank(), you will get Clank in Cat.
Java documentation says that
Static methods, which have the static modifier in their declarations,
should be invoked with the class name, without the need for creating
an instance of the class
Note: You can also refer to static methods with an object reference
like instanceName.methodName(args) but this is discouraged because it
does not make it clear that they are class methods.
This question already has answers here:
What does a "Cannot find symbol" or "Cannot resolve symbol" error mean?
(18 answers)
Closed 5 years ago.
I am only learning about polymorphism so be easy on me (literally copying from the book). I try to pass a class as an argument to a method. When I do that I can call the superclass methods, but not the actual subclass. Using the start() method, I try to make the wolf howl:
public class experiment {
public static void main(String[] args) {
PetOwner own = new PetOwner();
own.start();
}
}
//Trying polymorphic arguments
class Vet {
public void giveShot(Animal a) {
a.howl();
}
}
class PetOwner {
public void start() {
Vet v = new Vet();
Wolf w = new Wolf();
v.giveShot(w);
}
}
//Inheritance//
//Kingdom - Animal
class Animal {
public void move() {
System.out.println("*motions softly*");
}
}
//Family - canine
class Canine extends Animal {
public void bark() {
System.out.println("Woof!");
}
}
//Species - wolf
class Wolf extends Canine {
public void howl() {
System.out.println("Howl! Howl!");
}
}
If I pass the howl method to the superclass (Animal) it works fine. If I call it directly from the Wolf class - it works fine. The only instance where it doesn't work is if I try to pass the wolf class as an argument and call it from there.
Here is why I try it that way, quoted from Head First Java pg 187:
The Vet's giveShot() method can take any Animal you give it. As long
as the object you in as the argument is a subclass of Animal, it
will work
I am getting a "cannot find symbol symbol: method howl(), Location variable of type animal" error.
You're calling a.howl() while a is an instance of Animal class. Animal does not know how to howl. The only Wolf does.
You can defin a method say react() and then override it for any particular subclass of Animal;
You are calling the howl-Method inside the giveShot-Method. Animal doesn't have a howl method.
EDIT: If you cast animal to wolf inside the giveShot-Method it should work.
Yes, you could do something like:
class Vet {
public void giveShot(Animal a) {
a.makeNoise();
}
}
class Animal {
public void move() {
System.out.println("*motions softly*");
}
public void makeNoise() {
}
}
//Family - canine
class Canine extends Animal {
#Override
public void makeNoise() {
System.out.println("Woof!");
}
}
//Species - wolf
class Wolf extends Canine {
#Override
public void makeNoise() {
System.out.println("Howl! Howl!");
}
}
This question already has answers here:
Why bark method can not be called
(5 answers)
Closed 6 years ago.
The Superclass reference variable can hold the subclass object, but using that variable you can access only the members of the superclass, so to access the members of both classes it is recommended to always create reference variable to the subclass.
class Animal {
public void move() {
System.out.println("Animals can move");
}
}
class Dog extends Animal {
public void move() {
System.out.println("Dogs can walk and run");
}
public void bark() {
System.out.println("Dogs can bark");
}
}
public class TestDog {
public static void main(String args[]) {
Animal a = new Animal(); // Animal reference and object
Animal b = new Dog(); // Animal reference but Dog object
a.move(); // runs the method in Animal class
b.move(); // runs the method in Dog class
b.bark();
}
}
output:
TestDog.java:26: error: cannot find symbol
b.bark();
^
symbol: method bark()
location: variable b of type Animal
1 error
What I do not understand here is why is the object 'b' able to access the Dog.move() and not Dog.bark() because the statement mentioned above says it can access only the members of the superclass and not the subclass.Following this logic the output of b.move() should be "Animals can move" and not "Dogs can walk and run".But that is not case.Can anyone help me with this?Thanks in advance!
Congratulations - you just discovered polymorphism.
In Java the classes are bound dynamically. That is if you are invoking a method the implementation of the object is invoked (in your case the Dog) and not the method of the reference type (in your case the Animal).
This allows overwriting methods and replace or fulfill their implementation.
On the other hand, you can only access methods that are available in the type you are referencing, not the implementing type (in your case the Animal). To invoke the methods of the instance, you would have to use it as the reference type (in your case the Dog).
In your question Animal is a parent class which doesn't have bark() method so that method isn't overridden. If you were able to access bark() from parent class without declaring either abstract method or defining it, then that would be violation of the Polymorphism principle.
If you really want to access it that way, then you can either define a abstract public void bark(); in your parent or access that method by typecasting like this
((Dog) b).bark();
This will not compile since Animal does not have a method called bark.
Think of it this way, all dogs are animals, but not all animals are dogs. All dogs bark, but not all animals bark.
This code is wrong, as the line b.bark(); will give you a compiler error, because b is only defined as an Animal, which cannot bark().
If you change Animal b = new Dog(); to Dog d = new Dog(); it will work properly.
You've got inheritance mixed up. Dog can do what Animal can do, not vice versa.
class Animal {
public void move() {
System.out.println("Animals can move");
}
}
class Dog extends Animal {
#Override public void move() {
System.out.println("Dogs can walk and run");
}
public void bark() {
System.out.println("Dogs can bark");
}
public void moveSuper() {
super.move();
}
}
public class TestDog {
public static void main(final String args[]) {
final Animal a = new Animal(); // Animal reference and object
a.move(); // runs the method in Animal class
final Dog d = new Dog(); // Animal reference but Dog object
d.move(); // runs the method in Dog class
d.bark();
d.moveSuper();
}
}
I'm confused on how overriding differs from hiding in Java. Can anyone provide more details on how these differ? I read the Java Tutorial but the sample code still left me confused.
To be more clear, I understand overriding well. My issue is that I don't see how hiding is any different, except for the fact that one is at the instance level while the other is at the class level.
Looking at the Java tutorial code:
public class Animal {
public static void testClassMethod() {
System.out.println("Class" + " method in Animal.");
}
public void testInstanceMethod() {
System.out.println("Instance " + " method in Animal.");
}
}
Then we have a subclass Cat:
public class Cat extends Animal {
public static void testClassMethod() {
System.out.println("The class method" + " in Cat.");
}
public void testInstanceMethod() {
System.out.println("The instance method" + " in Cat.");
}
public static void main(String[] args) {
Cat myCat = new Cat();
Animal myAnimal = myCat;
Animal.testClassMethod();
myAnimal.testInstanceMethod();
}
}
Then they say:
The output from this program is as follows:
Class method in Animal.
The instance method in Cat.
To me, the fact that calling a class method testClassMethod() directly from the Animal class executes the method in Animal class is pretty obvious, nothing special there. Then they call the testInstanceMethod() from a reference to myCat, so again pretty obvious that the method executed then is the one in the instance of Cat.
From what I see, the call hiding behaves just like overriding, so why make that distinction? If I run this code using the classes above:
Cat.testClassMethod();
I'll get:
The class method in Cat.
But if I remove the testClassMethod() from Cat, then I'll get:
The class method in Animal.
Which shows me that writing a static method, with the same signature as in the parent, in a subclass pretty much does an override.
Hopefully I'm making clear my where I'm confused and someone can shed some light. Thanks very much in advance!
Overriding basically supports late binding. Therefore, it's decided at run time which method will be called. It is for non-static methods.
Hiding is for all other members (static methods, instance members, static members). It is based on the early binding. More clearly, the method or member to be called or used is decided during compile time.
In your example, the first call, Animal.testClassMethod() is a call to a static method, hence it is pretty sure which method is going to be called.
In the second call, myAnimal.testInstanceMethod(), you call a non-static method. This is what you call run-time polymorphism. It is not decided until run time which method is to be called.
For further clarification, read Overriding Vs Hiding.
Static methods are hidden, non-static methods are overriden.
The difference is notable when calls are not qualified "something()" vs "this.something()".
I can't really seem to put it down on words, so here goes an example:
public class Animal {
public static void something() {
System.out.println("animal.something");
}
public void eat() {
System.out.println("animal.eat");
}
public Animal() {
// This will always call Animal.something(), since it can't be overriden, because it is static.
something();
// This will call the eat() defined in overriding classes.
eat();
}
}
public class Dog extends Animal {
public static void something() {
// This method merely hides Animal.something(), making it uncallable, but does not override it, or alter calls to it in any way.
System.out.println("dog.something");
}
public void eat() {
// This method overrides eat(), and will affect calls to eat()
System.out.println("dog.eat");
}
public Dog() {
super();
}
public static void main(String[] args) {
new Dog();
}
}
OUTPUT:
animal.something
dog.eat
This is the difference between overrides and hiding,
If both method in parent class and child class are an instance method, it called overrides.
If both method in parent class and child class are static method, it called hiding.
One method cant be static in parent and as an instance in the child. and visa versa.
If I understand your question properly then the answer is "you already are overriding".
"Which shows me that writing a static method, with the same name as in the parent, in a subclass pretty much does an override."
If you write a method in a subclass with exactly the same name as a method in a superclass it will override the superclass's method. The #Override annotation is not required to override a method. It does however make your code more readable and forces the compiler to check that you are actually overriding a method (and didn't misspell the subclass method for example).
Overriding happens only with instance methods.
When the type of the reference variable is Animal and the object is Cat then the instance method is called from Cat (this is overriding). For the same acat object the class method of Animal is used.
public static void main(String[] args) {
Animal acat = new Cat();
acat.testInstanceMethod();
acat.testClassMethod();
}
Output is:
The instance method in Cat.
Class method in Animal.
public class First {
public void Overriding(int i) { /* will be overridden in class Second */ }
public static void Hiding(int i) { /* will be hidden in class Second
because it's static */ }
}
public class Second extends First {
public void Overriding(int i) { /* overridden here */ }
public static void Hiding(int i) { /* hides method in class First
because it's static */ }
}
The rule for memorizing is simple: a method in an extending class
can't change static to void and
can't change void to static.
It will cause of compile-error.
But if void Name is changed to void Name it's Overriding.
And if static Name is changed to static Name it's Hiding. (Both the static method of the subclass as well as the one of the superclass can be called, depending on the type of the reference used to call the method.)
In this code snippet I use 'private' access modifier instead of 'static' to show you difference between hiding methods and overriding methods.
class Animal {
// Use 'static' or 'private' access modifiers to see how method hiding work.
private void testInstancePrivateMethod(String source) {
System.out.println("\tAnimal: instance Private method calling from "+source);
}
public void testInstanceMethodUsingPrivateMethodInside() {
System.out.println("\tAnimal: instance Public method with using of Private method.");
testInstancePrivateMethod( Animal.class.getSimpleName() );
}
// Use default, 'protected' or 'public' access modifiers to see how method overriding work.
protected void testInstanceProtectedMethod(String source) {
System.out.println("\tAnimal: instance Protected method calling from "+source);
}
public void testInstanceMethodUsingProtectedMethodInside() {
System.out.println("\tAnimal: instance Public method with using of Protected method.");
testInstanceProtectedMethod( Animal.class.getSimpleName() );
}
}
public class Cat extends Animal {
private void testInstancePrivateMethod(String source) {
System.out.println("Cat: instance Private method calling from " + source );
}
public void testInstanceMethodUsingPrivateMethodInside() {
System.out.println("Cat: instance Public method with using of Private method.");
testInstancePrivateMethod( Cat.class.getSimpleName());
System.out.println("Cat: and calling parent after:");
super.testInstanceMethodUsingPrivateMethodInside();
}
protected void testInstanceProtectedMethod(String source) {
System.out.println("Cat: instance Protected method calling from "+ source );
}
public void testInstanceMethodUsingProtectedMethodInside() {
System.out.println("Cat: instance Public method with using of Protected method.");
testInstanceProtectedMethod(Cat.class.getSimpleName());
System.out.println("Cat: and calling parent after:");
super.testInstanceMethodUsingProtectedMethodInside();
}
public static void main(String[] args) {
Cat myCat = new Cat();
System.out.println("----- Method hiding -------");
myCat.testInstanceMethodUsingPrivateMethodInside();
System.out.println("\n----- Method overriding -------");
myCat.testInstanceMethodUsingProtectedMethodInside();
}
}
Output:
----- Method hiding -------
Cat: instance Public method with using of Private method.
Cat: instance Private method calling from Cat
Cat: and calling parent after:
Animal: instance Public method with using of Private method.
Animal: instance Private method calling from Animal
----- Method overriding -------
Cat: instance Public method with using of Protected method.
Cat: instance Protected method calling from Cat
Cat: and calling parent after:
Animal: instance Public method with using of Protected method.
Cat: instance Protected method calling from Animal
I think this is not yet fully explained.
Please see the following example.
class Animal {
public static void testClassMethod() {
System.out.println("The static method in Animal");
}
public void testInstanceMethod() {
System.out.println("The instance method in Animal");
}
}
public class Cat extends Animal {
public static void testClassMethod() {
System.out.println("The static method in Cat");
}
public void testInstanceMethod() {
System.out.println("The instance method in Cat");
}
public static void main(String[] args) {
Animal myCat = new Cat();
Cat myCat2 = new Cat();
myCat.testClassMethod();
myCat2.testClassMethod();
myCat.testInstanceMethod();
myCat2.testInstanceMethod();
}
}
The output will be as follows.
The static method in Animal
The static method in Cat
The instance method in Cat
The instance method in Cat
Based on my recent Java studies
method overriding, when the subclass have the same method with the same signature in the subclass.
Method hiding, when the subclass have the same method name, but different parameter. In this case, you're not overriding the parent method, but hiding it.
Example from OCP Java 7 book, page 70-71:
public class Point {
private int xPos, yPos;
public Point(int x, int y) {
xPos = x;
yPos = y;
}
public boolean equals(Point other){
.... sexy code here ......
}
public static void main(String []args) {
Point p1 = new Point(10, 20);
Point p2 = new Point(50, 100);
Point p3 = new Point(10, 20);
System.out.println("p1 equals p2 is " + p1.equals(p2));
System.out.println("p1 equals p3 is " + p1.equals(p3));
//point's class equals method get invoked
}
}
but if we write the following main:
public static void main(String []args) {
Object p1 = new Point(10, 20);
Object p2 = new Point(50, 100);
Object p3 = new Point(10, 20);
System.out.println("p1 equals p2 is " + p1.equals(p2));
System.out.println("p1 equals p3 is " + p1.equals(p3));
//Object's class equals method get invoked
}
In the second main, we using the Object class as static type, so when we calling the equal method in Point object, it's waiting a Point class to arrive as a parameter,but Object coming. So the Object class equals method getting run, because we have an equals(Object o) there. In this case, the Point's class equals dosen't overrides, but hides the Object class equals method.
public class Parent {
public static void show(){
System.out.println("Parent");
}
}
public class Child extends Parent{
public static void show(){
System.out.println("Child");
}
}
public class Main {
public static void main(String[] args) {
Parent parent=new Child();
parent.show(); // it will call parent show method
}
}
// We can call static method by reference ( as shown above) or by using class name (Parent.show())
The linked java tutorial page explains the concept of overriding and hiding
An instance method in a subclass with the same signature (name, plus the number and the type of its parameters) and return type as an instance method in the superclass overrides the superclass's method.
If a subclass defines a static method with the same signature as a static method in the superclass, then the method in the subclass hides the one in the superclass.
The distinction between hiding a static method and overriding an instance method has important implications:
The version of the overridden instance method that gets invoked is the one in the subclass.
The version of the hidden static method that gets invoked depends on whether it is invoked from the superclass or the subclass.
Coming back to your example:
Animal myAnimal = myCat;
/* invokes static method on Animal, expected. */
Animal.testClassMethod();
/* invokes child class instance method (non-static - it's overriding) */
myAnimal.testInstanceMethod();
Above statement does not show hiding yet.
Now change the code as below to get different output:
Animal myAnimal = myCat;
/* Even though myAnimal is Cat, Animal class method is invoked instead of Cat method*/
myAnimal.testClassMethod();
/* invokes child class instance method (non-static - it's overriding) */
myAnimal.testInstanceMethod();
In addition to the examples listed above, here is a small sample code to clarify the distinction between hiding and overriding:
public class Parent {
// to be hidden (static)
public static String toBeHidden() {
return "Parent";
}
// to be overridden (non-static)
public String toBeOverridden() {
return "Parent";
}
public void printParent() {
System.out.println("to be hidden: " + toBeHidden());
System.out.println("to be overridden: " + toBeOverridden());
}
}
public class Child extends Parent {
public static String toBeHidden() {
return "Child";
}
public String toBeOverridden() {
return "Child";
}
public void printChild() {
System.out.println("to be hidden: " + toBeHidden());
System.out.println("to be overridden: " + toBeOverridden());
}
}
public class Main {
public static void main(String[] args) {
Child child = new Child();
child.printParent();
child.printChild();
}
}
The call of child.printParent() outputs:
to be hidden: Parent
to be overridden: Child
The call of child.printChild() outputs:
to be hidden: Child
to be overridden: Child
As wee can see from the outputs above (especially the bold marked outputs), method hiding behaves differently from overriding.
Java allows both hiding and overriding only for methods. The same rule does not apply to variables. Overriding variables is not permitted, so variables can only be hidden (no difference between static or non-static variable). The example below shows how the method getName() is overriden and the variable name is hidden:
public class Main {
public static void main(String[] args) {
Parent p = new Child();
System.out.println(p.name); // prints Parent (since hiding)
System.out.println(p.getName()); // prints Child (since overriding)
}
}
class Parent {
String name = "Parent";
String getName() {
return name;
}
}
class Child extends Parent {
String name = "Child";
String getName() {
return name;
}
}
At runtime the child version of an overridden method is always executed for an instance
regardless of whether the method call is defi ned in a parent or child class method. In this
manner, the parent method is never used unless an explicit call to the parent method is
referenced, using the syntax
ParentClassName.method().
Alternatively, at runtime the parent
version of a hidden method is always executed if the call to the method is defined in the
parent class.
In method overriding, method resolution is done by the JVM on the basis of runtime object. Whereas in method hiding, method resolution is done by the compiler on the basis of reference.
Thus,
If the code would have been written as,
public static void main(String[] args) {
Animal myCat = new Cat();
myCat.testClassMethod();
}
The Output would be as below:
Class method in Animal.
It is called hiding because the compiler hides the super class method implementation, when subclass has the same static method.
Compiler has no restricted visibility for overridden methods and it’s only during runtime that it’s decided which one is used.
This is the difference between overriding and hiding:
Animal a = new Cat();
a.testClassMethod() will call the method in parent class since it is an example of method hiding. The method to be called is determined by the type of the reference variable and decided at compile time.
a.testInstanceMethod() will call the method in child class since it is an example of method overriding. The method to be called is determined by the object which is used to call the method at runtime.
How is static method hiding happening in java?
Cat class is extending Animal class. So in Cat class will have both static methods (i mean Child class's static method and Parent class's static method)
But how JVM hiding Parent static method? How it's dealing in Heap and Stack?