Well, I must admit that I'm very new to Java Programming and very hesitate to post the question here because there are lots of question similar to my question. I have viewed those question but STILL I can't understand what's the logic behind "protected" modifier. So I think it is better to post my own question here.
Here is class A in package PackOne
package PackOne;
public class A {
protected void protectedMethod() {
System.out.println("A's protectedMethod");
}
}
Here is Class B in package PackTwo. However, it is a subclass of class A.
package PackTwo;
import PackOne.A;
public class B extends A {
public static void main(String[] args) {
//Test 1
protectedMethod(); //error: non-static method protectedMethod()
// cannot be referenced from a static context.
//Test 2
A instanceofA = new A();
instanceofA.protectedMethod();//error: protectedMethod()
//has protected access in PackOne.A
}
public void anotherMethodOfB() {
//Test 3
protectedMethod();//Pass
}
//Test 4
A instanceofA = new A();
instanceofA.protectedMethod();//error: package instanceofA does not existed.
}
Please explain Why only Test 3's call to protected method in class A is passed but the other 3 Tests(1,2,4) yield errors?
This has nothing to do with subclasses.
You cannot call a non-static method from a static function. Now
public static void main
is static, while
protectedMethod()
is not.
Second, you cannot call protected methods from "outside" a class. If you are in class B, you cannot call a protected method of another class that isn't B.
Finally, with point 4, this code is not part of a method, so it doesn't make any sense at all.
Test 1: This is nothing to do with protected/private/public - you need an object instance of A to call this method.
Test 2: You are NOT in an object instance of A or B, you are in a static method. You need to be calling the protected method from within A or B - being in a static method is not within the class, only instances count.
Test 3: You are in the instance.
Test 4: Same as Test 2 - this is an anonymous static method.
It's not so much that you're having trouble understanding protected access as you are having trouble understanding where you can call instance methods.
protectedMethod() is a method you can call on instances of class A.
The first method call in main() is invalid because you aren't attempting to call the method on an instance of A - main() is a static method and thus it belongs to the class of B rather than an instance of B.
The fourth is not valid because you cannot call methods as statements outside of a method body.
A protected method in a parent class allows a subclass to use it internally, which is why 'Test 3' passes.
Test 1 fails because main is a static method and has no access to non-static fields and functions.
Test 2 fails because you are trying invoke a protected method via an instance, which is not allowed. Same with Test 4.
To understand your problem, you first need to understand the access modifiers:
Public: Any class can access this function/variable. (You still have to create an instance first)
Private: No other class can access this function/variable. It can only be accessed inside the class. You can still use public functions from that class to access it's private functions and variables, but direct access is not possible. Note that private functions and variables are NOT inherited (i.e. not part of the subclass).
Protected: Protected functions and variables can only be accessed by that class and any of it's subclasses (any class that directly/indirectly inherits from it)
Static: allows you to call functions without having to create an object first. The Math class is a good example as it only contains static methods and variables (and can't even be instantiated). (note: Static is not an access modifier. It also does a bit more than just this, look it up if you want to know more)
As for your example:
Test 1: Your method is inherited from class A, and thus available. However, since it is not a static method, it cannot be directly accessed: You either need to make it static, or create an instance of class B, then call the function through that instance.
Test 2: This doesn't work because A & B are in different packages. Access to protected methods is only allowed within the same package (even if one inherits from the other). If A and B where in the same package, this would work.
Test 3: Object B inherits public and protected methods of A. It's not the method in A that is accessed, but the inherited method in B. To see this, change A's code to the following:
<!-- language: java -->
protected void protectedMethod() {
System.out.println(getClass().getName() + ("'s Protected method"));
}
Executing this will give B's Protected method as a result.
Test 4: Executing code outside of a function doesn't work. Don't do this (ever).
Note: You can access private and protected variables through reflection, though this is a far more advanced thing. It's usually also not a very idea, since variables are made private/protected for a reason (i.e. to prevent unauthorized access/modifications)
Related
I was just reading over the text given to me in my textbook and I'm not really sure I understand what it is saying. It's basically telling me that static methods or class methods include the "modifier" keyword static. But I don't really know what that means?
Could someone please explain to me in really simple terms what Static or Class Methods are?
Also, could I get a simple explanation on what Instance methods are?
This is what they give me in the textbook:
There are important practical implications of the presence or absence of the static modifier. A public class method may be invoked and executed as soon as Java processes the definition of the class to which it belongs. That is not the case for an instance method. Before a public instance method may be invoked and executed, an instance must be created of the class to which it belongs. To use a public class method, you just need the class. On the other hand, before you can use a public instance method you must have an instance of the class.
The manner in which a static method is invoked within the definition of another method varies according to whether or not the two methods belong to the same class. In the example above, factorial and main are both methods of the MainClass class. As a result, the invocation of factorial in the definition of main simply references the method name, "factorial".
The basic paradigm in Java is that you write classes, and that those classes are instantiated. Instantiated objects (an instance of a class) have attributes associated with them (member variables) that affect their behavior; when the instance has its method executed it will refer to these variables.
However, all objects of a particular type might have behavior that is not dependent at all on member variables; these methods are best made static. By being static, no instance of the class is required to run the method.
You can do this to execute a static method:
MyClass.staticMethod(); // Simply refers to the class's static code
But to execute a non-static method, you must do this:
MyClass obj = new MyClass(); //Create an instance
obj.nonstaticMethod(); // Refer to the instance's class's code
On a deeper level the compiler, when it puts a class together, collects pointers to methods and attaches them to the class. When those methods are executed it follows the pointers and executes the code at the far end. If a class is instantiated, the created object contains a pointer to the "virtual method table", which points to the methods to be called for that particular class in the inheritance hierarchy. However, if the method is static, no "virtual method table" is needed: all calls to that method go to the exact same place in memory to execute the exact same code. For that reason, in high-performance systems it's better to use a static method if you are not reliant on instance variables.
Methods and variables that are not declared as static are known as instance methods and instance variables. To refer to instance methods and variables, you must instantiate the class first means you should create an object of that class first.For static you don't need to instantiate the class u can access the methods and variables with the class name using period sign which is in (.)
for example:
Person.staticMethod(); //accessing static method.
for non-static method you must instantiate the class.
Person person1 = new Person(); //instantiating
person1.nonStaticMethod(); //accessing non-static method.
Difference between Static methods and Instance methods
Instance method are methods which require an object of its class to be created before it can be called. Static methods are the methods in Java that can be called without creating an object of class.
Static method is declared with static keyword. Instance method is not with static keyword.
Static method means which will exist as a single copy for a class. But instance methods exist as multiple copies depending on the number of instances created for that class.
Static methods can be invoked by using class reference. Instance or non static methods are invoked by using object reference.
Static methods can’t access instance methods and instance variables directly. Instance method can access static variables and static methods directly.
Reference : geeksforgeeks
Static methods, variables belongs to the whole class, not just an object instance. A static method, variable is associated with the class as a whole rather than with specific instances of a class. Each object will share a common copy of the static methods, variables. There is only one copy per class, no matter how many objects are created from it.
Instance methods => invoked on specific instance of a specific class. Method wants to know upon which class it was invoked. The way it happens there is a invisible parameter called 'this'. Inside of 'this' we have members of instance class already set with values. 'This' is not a variable. It's a value, you cannot change it and the value is reference to the receiver of the call.
Ex: You call repairmen(instance method) to fix your TV(actual program). He comes with tools('this' parameter). He comes with specific tools needed for fixing TV and he can fix other things also.
In static methods => there is no such thing as 'this'.
Ex: The same repairman (static method). When you call him you have to specify which repairman to call(like electrician). And he will come and fix your TV only. But, he doesn't have tools to fix other things (there is no 'this' parameter).
Static methods are usually useful for operations that don't require any data from an instance of the class (from 'this') and can perform their intended purpose solely using their arguments.
In short, static methods and static variables are class level where as instance methods and instance variables are instance or object level.
This means whenever a instance or object (using new ClassName()) is created, this object will retain its own copy of instace variables. If you have five different objects of same class, you will have five different copies of the instance variables. But the static variables and methods will be the same for all those five objects. If you need something common to be used by each object created make it static. If you need a method which won't need object specific data to work, make it static. The static method will only work with static variable or will return data on the basis of passed arguments.
class A {
int a;
int b;
public void setParameters(int a, int b){
this.a = a;
this.b = b;
}
public int add(){
return this.a + this.b;
}
public static returnSum(int s1, int s2){
return (s1 + s2);
}
}
In the above example, when you call add() as:
A objA = new A();
objA.setParameters(1,2); //since it is instance method, call it using object
objA.add(); // returns 3
B objB = new B();
objB.setParameters(3,2);
objB.add(); // returns 5
//calling static method
// since it is a class level method, you can call it using class itself
A.returnSum(4,6); //returns 10
class B{
int s=8;
int t = 8;
public addition(int s,int t){
A.returnSum(s,t);//returns 16
}
}
In first class, add() will return the sum of data passed by a specific object. But the static method can be used to get the sum from any class not independent if any specific instance or object. Hence, for generic methods which only need arguments to work can be made static to keep it all DRY.
If state of a method is not supposed to be changed or its not going to use any instance variables.
You want to call method without instance.
If it only works on arguments provided to it.
Utility functions are good instance of static methods. i.e math.pow(), this function is not going to change the state for different values. So it is static.
The behavior of an object depends on the variables and the methods of that class. When we create a class we create an object for it. For static methods, we don't require them as static methods means all the objects will have the same copy so there is no need of an object.
e.g:
Myclass.get();
In instance method each object will have different behaviour so they have to call the method using the object instance.
e.g:
Myclass x = new Myclass();
x.get();
Static Methods vs Instance methods
Constructor:
const Person = function (birthYear) {
this.birthYear = birthYear;
}
Instance Method -> Available
Person.prototype.calcAge = function () {
2037 - this.birthYear);
}
Static Method -> Not available
Person.hey = function(){
console.log('Hey')
}
Class:
class PersonCl {
constructor(birthYear) {
this.birthYear = birthYear;
}
/**
* Instance Method -> Available to instances
*/
calcAge() {
console.log(2037 - this.birthYear);
}
/**
* Static method -> Not available to instances
*/
static hey() {
console.log('Static HEY ! ');
}
}
The static modifier when placed in front of a function implies that only one copy of that function exists. If the static modifier is not placed in front of the function then with every object or instance of that class a new copy of that function is made. :)
Same is the case with variables.
In a java book I'm reading, a practice program takes the protected clone() method in the Object class and overrides it. When they do this tho, they expand the visibility modifier to public so it can be used in any package. I'm confused why this would need to be done though. If by definition, everything is a subclass of Object and a method is declared protected in the Object class. Wouldn't that mean every class would have access to it. What is the need to make the method public when overriding it? I'm confused.
You need to override it as public because you may wish to call it from a third class. Let's assume you have a Base class which has a subclass Child that overrides the method clone. Now I may have a third class as below.
public class Third {
public void m1(Child c) {
Child d = c.clone();
}
}
Here, class, Third is calling clone method, hence it needs to be declared public to be called.
I was just reading over the text given to me in my textbook and I'm not really sure I understand what it is saying. It's basically telling me that static methods or class methods include the "modifier" keyword static. But I don't really know what that means?
Could someone please explain to me in really simple terms what Static or Class Methods are?
Also, could I get a simple explanation on what Instance methods are?
This is what they give me in the textbook:
There are important practical implications of the presence or absence of the static modifier. A public class method may be invoked and executed as soon as Java processes the definition of the class to which it belongs. That is not the case for an instance method. Before a public instance method may be invoked and executed, an instance must be created of the class to which it belongs. To use a public class method, you just need the class. On the other hand, before you can use a public instance method you must have an instance of the class.
The manner in which a static method is invoked within the definition of another method varies according to whether or not the two methods belong to the same class. In the example above, factorial and main are both methods of the MainClass class. As a result, the invocation of factorial in the definition of main simply references the method name, "factorial".
The basic paradigm in Java is that you write classes, and that those classes are instantiated. Instantiated objects (an instance of a class) have attributes associated with them (member variables) that affect their behavior; when the instance has its method executed it will refer to these variables.
However, all objects of a particular type might have behavior that is not dependent at all on member variables; these methods are best made static. By being static, no instance of the class is required to run the method.
You can do this to execute a static method:
MyClass.staticMethod(); // Simply refers to the class's static code
But to execute a non-static method, you must do this:
MyClass obj = new MyClass(); //Create an instance
obj.nonstaticMethod(); // Refer to the instance's class's code
On a deeper level the compiler, when it puts a class together, collects pointers to methods and attaches them to the class. When those methods are executed it follows the pointers and executes the code at the far end. If a class is instantiated, the created object contains a pointer to the "virtual method table", which points to the methods to be called for that particular class in the inheritance hierarchy. However, if the method is static, no "virtual method table" is needed: all calls to that method go to the exact same place in memory to execute the exact same code. For that reason, in high-performance systems it's better to use a static method if you are not reliant on instance variables.
Methods and variables that are not declared as static are known as instance methods and instance variables. To refer to instance methods and variables, you must instantiate the class first means you should create an object of that class first.For static you don't need to instantiate the class u can access the methods and variables with the class name using period sign which is in (.)
for example:
Person.staticMethod(); //accessing static method.
for non-static method you must instantiate the class.
Person person1 = new Person(); //instantiating
person1.nonStaticMethod(); //accessing non-static method.
Difference between Static methods and Instance methods
Instance method are methods which require an object of its class to be created before it can be called. Static methods are the methods in Java that can be called without creating an object of class.
Static method is declared with static keyword. Instance method is not with static keyword.
Static method means which will exist as a single copy for a class. But instance methods exist as multiple copies depending on the number of instances created for that class.
Static methods can be invoked by using class reference. Instance or non static methods are invoked by using object reference.
Static methods can’t access instance methods and instance variables directly. Instance method can access static variables and static methods directly.
Reference : geeksforgeeks
Static methods, variables belongs to the whole class, not just an object instance. A static method, variable is associated with the class as a whole rather than with specific instances of a class. Each object will share a common copy of the static methods, variables. There is only one copy per class, no matter how many objects are created from it.
Instance methods => invoked on specific instance of a specific class. Method wants to know upon which class it was invoked. The way it happens there is a invisible parameter called 'this'. Inside of 'this' we have members of instance class already set with values. 'This' is not a variable. It's a value, you cannot change it and the value is reference to the receiver of the call.
Ex: You call repairmen(instance method) to fix your TV(actual program). He comes with tools('this' parameter). He comes with specific tools needed for fixing TV and he can fix other things also.
In static methods => there is no such thing as 'this'.
Ex: The same repairman (static method). When you call him you have to specify which repairman to call(like electrician). And he will come and fix your TV only. But, he doesn't have tools to fix other things (there is no 'this' parameter).
Static methods are usually useful for operations that don't require any data from an instance of the class (from 'this') and can perform their intended purpose solely using their arguments.
In short, static methods and static variables are class level where as instance methods and instance variables are instance or object level.
This means whenever a instance or object (using new ClassName()) is created, this object will retain its own copy of instace variables. If you have five different objects of same class, you will have five different copies of the instance variables. But the static variables and methods will be the same for all those five objects. If you need something common to be used by each object created make it static. If you need a method which won't need object specific data to work, make it static. The static method will only work with static variable or will return data on the basis of passed arguments.
class A {
int a;
int b;
public void setParameters(int a, int b){
this.a = a;
this.b = b;
}
public int add(){
return this.a + this.b;
}
public static returnSum(int s1, int s2){
return (s1 + s2);
}
}
In the above example, when you call add() as:
A objA = new A();
objA.setParameters(1,2); //since it is instance method, call it using object
objA.add(); // returns 3
B objB = new B();
objB.setParameters(3,2);
objB.add(); // returns 5
//calling static method
// since it is a class level method, you can call it using class itself
A.returnSum(4,6); //returns 10
class B{
int s=8;
int t = 8;
public addition(int s,int t){
A.returnSum(s,t);//returns 16
}
}
In first class, add() will return the sum of data passed by a specific object. But the static method can be used to get the sum from any class not independent if any specific instance or object. Hence, for generic methods which only need arguments to work can be made static to keep it all DRY.
If state of a method is not supposed to be changed or its not going to use any instance variables.
You want to call method without instance.
If it only works on arguments provided to it.
Utility functions are good instance of static methods. i.e math.pow(), this function is not going to change the state for different values. So it is static.
The behavior of an object depends on the variables and the methods of that class. When we create a class we create an object for it. For static methods, we don't require them as static methods means all the objects will have the same copy so there is no need of an object.
e.g:
Myclass.get();
In instance method each object will have different behaviour so they have to call the method using the object instance.
e.g:
Myclass x = new Myclass();
x.get();
Static Methods vs Instance methods
Constructor:
const Person = function (birthYear) {
this.birthYear = birthYear;
}
Instance Method -> Available
Person.prototype.calcAge = function () {
2037 - this.birthYear);
}
Static Method -> Not available
Person.hey = function(){
console.log('Hey')
}
Class:
class PersonCl {
constructor(birthYear) {
this.birthYear = birthYear;
}
/**
* Instance Method -> Available to instances
*/
calcAge() {
console.log(2037 - this.birthYear);
}
/**
* Static method -> Not available to instances
*/
static hey() {
console.log('Static HEY ! ');
}
}
The static modifier when placed in front of a function implies that only one copy of that function exists. If the static modifier is not placed in front of the function then with every object or instance of that class a new copy of that function is made. :)
Same is the case with variables.
My Assumptions:
Static method cannot cannot call non-static methods.
Constructors are kind of a method with no return type.
Given this example...
public class Main {
public static void main(String[] args) {
Main p = new Main(); // constructor call
k(); // [implicit] `this` reference
}
protected Main() {
System.out.print("1234");
}
protected void k() {
}
}
this line prints 1234: Main p = new Main()
this line throws an Exception: k()
Why did the example code do those two things? Don't they conflict with my above Assumptions? Are my Assumptions correct?
1 - Static method cannot cannot call non-static methods.
Sure they can, but they need an object to call the method on.
In a static method, there's no this reference available, so foo() (which is equivalent to this.foo()) is illegal.
2 - Constructors are kind of a method with no return type.
If they should be compared to methods, I would say constructors are closer to non-static methods (since there is indeed a this reference inside a constructor).
Given this view, it should be clear to you why a static method can call a constructor without any problems.
So, to sum it up:
Main p = new Main();
is okay, since new Main() does not rely on any existing object.
k();
is not okay since it is equivalent to this.k() and this is not available in your (static) main method.
No. Constructors aren't ordinary methods in this respect. The whole point of the constructor is to, well, construct a new instance of the class.
So it can be invoked in static scope too. Just think about it: if you needed an existing instance of your class in order to create a new instance of it, you would simply never be able to instantiate it ever.
A few clarifications:
Static method cannot cannot call non-static methods.
Not quite. You can call a nonstatic method from inside a static method, just you need to scope it to a specific object of that class. I.e.
p.k();
would work perfectly in your code sample above.
The call
k();
would be fine inside an instance (nonstatic) method. And it would be equivalent to
this.k();
The implied this refers to the current instance of the class. Whenever the compiler sees an unqualified call like k() within an instance method, it will automatically scope it with this. . However, since static methods aren't tied to any instance of the class, you (and the compiler) can't refer to this inside a static method. Hence you need to explicitly name an instance of the class to call an instance method on.
Rules are simple:
1 - Static method cannot cannot call non-static methods.
That's simply not true. A static method can call a non-static method, just via a "target" reference. For example, this is fine in a static method:
Integer x = Integer.valueOf(10);
int y = x.intValue(); // Instance method!
The real point is "there's no this reference within a static method".
2 - Constructors are kind of a method with no return type.
That's not a really useful model, to be honest. It makes more sense (from the caller's point of view) to consider a constructor as a static method with a return type that's the same as the declaring class, but even that's not a perfect model by any means.
I suggest you think of a constructor as a different type of member. Embrace the differences between constructors and methods, instead of trying to hide them.
Many articles say objects get created only after the class's constructor gets called. But I found this snippet and it works fine.
public class A {
public A(){
this.foo();//line #1
}
private void foo() {
System.out.print("without an instance..!!!!");
}
}
class B extends A
{
public static void main(String[] args){
A a = new A(); //line #2
}
}
Here you see, I'm trying to create an object of its super class in line #2 and in its constructor how come its method got called without an instance. What's going on here, is Constructer of instance of A is getting called here.?
The constructor is always called when an object is created. Even if you don't explicitly define a constructor, the compiler will generate one for you with an empty body.
You may call other methods of the class from the constructor. All non-static methods get an implicit (compiler generated) parameter to this, the actual class instance. However, it is important to know that while executing the constructor, the object is not yet fully created, although all data members of the class in question (if there are such) have already been initialized, at least to some default value. Because of this, you
should not publish this (i.e. pass it to other objects / threads) before exiting the constructor call, and
you should not call non-final, non-private methods from the constructor.
Doing either of these (in a non-final class) means that you give access to an object not yet fully constructed, which may result in subtle, hard to find bugs later. E.g. if the virtual method in question is overridden in a subclass and the implementation depends on some member defined and initialized only in the subclass constructor, the method gets called before the subclass member is correctly initialized, thus it won't have the value you would expect.
Because public static void main is the entry point of the programm. So you do not require to create instance of the class B.
The method signature for the main() method contains three modifiers:
* public indicates that the main() method can be called by any object.
* static indicates that the main() method is a class method.
* void indicates that the main() method has no return value.
Read more : Understanding public static void main function
So that Constructor of A is get called when the programm get executed and that it calls the foo method of the super class A.
It's not called without an instance.
You call it on this - that's an instance.
The constructor of A is getting called because you called it directly. However, if you wished to call A through B, within Main [which is called without a current instance of the containing class B (1 because its static, and 2. its reserved for the beginning of the application)] you would just change the "new A()" to "new B()"
Since you're using a constructor with no parameters, a default constructor in automatically generated at compile time.
Whether main() is the entry point or not is not the explanation. The reason is that main() is static, therefore doesn't require an instance of its class.
No instance of B is ever created by this program.
class A is public, and its inherited by class B. class B can instantiate class A, with
A object=new A()
and the object initialization is done by Constructor method defined automatically. Constructor of A in turn calls method foo(), which is private to class A. As far as i know, to call a method from a class which is in the same class scope, no instance is needed.