I have a class, which has some private static final fields (unsure if it is correct to call these constants), and one other instance variable set just private, which is direct copy of one of the private static final fields.
Every time the object is created from other classes, I want it to have the same variables, as they are assigned in the class, and without creating a constructor, Java allows me access all of the methods with the correct returns as I would expect.
Is it okay to not create a constructor in this case?
There are at least two answers to your question:
It's fine not to include a constructor in your class. The Java compiler will add a default zero-parameters constructor for you.
It sounds like you shouldn't be constructing instances of your class in the first place. :-)
You've said
Every time the object is created from other classes, I want it to have the same variables, as they are assigned in the class, and without creating a constructor, Java allows me access all of the methods with the correct returns as I would expect.
It sounds like you have roughly:
class TheClass {
public final SomeType variable = /*...*/;
// ...
}
...and you're doing this:
TheClass instance = new TheClass();
doSomethingWith(instance.variable);
There's no reason to create an instance there, just use the class name directly:
doSomethingWith(TheClass.variable);
Java is somewhat interesting in that it allows you to access static members via instance references (instance.variable), but the normal way to access them is through the class (TheClass.variable).
Also, if your static members aren't final, you can get some very confusing-seeming behavior:
TheClass a = new TheClass();
a.variable = 1;
TheClass b = new TheClass();
b.variable = 2;
System.out.println(a.variable); // 2?!?!?!
So in general, best to avoid accessing static members via instance references.
Alternately, make the class a singleton with instance (non-static) members instead.
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.
I didn't understand this part:
Instance variables can be accessed directly by calling the variable name inside the class. However, within static methods (when instance variables are given accessibility), they should be called using the fully qualified name. ObjectReference.VariableName.
Can you give an example please ?
Static methods are methods that are called without a reference to an instance of that object. So instance variables cannot be called statically, as each instance will have its own values. So in a static method, you need a specific instance of an object in order to know which value of the instance variable you are trying to use.
The difference is how you access these variables:
class myClass {
public static int staticVar;
public int nonStaticVar;
//Constructor initialises both
}
Static approach:
int otherVariable = MyClass.staticVar;
As you can see, for the static variable you do not need to make an object to access it. Note that you can imagine a static variable to have the trait "once per class", which means, that you can not have 2 versions of staticVar.
Non Static (instance variable):
MyClass instanceOfMyClass = new myClass();
int otherVariable2 = instanceOfMyClass.nonStaticVar;
To have 2 versions of nonStaticVar you can simply make 2 objects and give this variable different values in the 2 objects. Note that in this case you have to make an object.
I am brand new to Java from a little of C++ and I am wondering what happens when I call a static method or access a static field without having an instance of the class:
class Foo {
public final static Scanner _input = new Scanner(System.in);
}
class Bar {
public final static Scanner _input = new Scanner(System.in);
}
...
SomeCode[Foo._input.nextInt()];
I can't imagine a temp object is created, but have no idea.
Thanks
I am wondering what happens when I call a static method or access a static field without having an instance of the class
It just accesses the field... it's as simple as that. It doesn't create an instance of Foo - it wouldn't need to.
The field isn't associated with any instance of the type - it's associated with the type itself. That's what static means in Java, basically. You should try to ignore any of the meanings of static in C++ which are somewhat different, I'm afraid.
One thing to be aware of is effectively a problem in the design of Java - you can access static members via references, but it doesn't mean what you'd expect. For example:
Foo foo = null;
Scanner scanner = foo._input;
That looks like it will go bang with a NullPointerException - but it doesn't. That code is equivalent to:
Foo foo = null;
Scanner scanner = Foo._input;
It's worth avoiding the first form of code as far as possible. It can be really misleading, particularly when you're calling methods - it looks like the call relies on the instance (and can be polymorphic) but actually both of those are incorrect :(
I can't imagine a temp object is created,
Nope. You use the class object to call the method.
temp object is created???
static keyword to create fields and methods that belong to the class, rather than to an instance of the class.
Heavily recommending
static fields are associated with the class itself. Their resolution is done at compile time; initialization of static variables (and static initialization blocks) are run when the class is loaded by the JVM.
As to how to access such variables (or methods), you can either use the class name, as you do; but Java also allows you to do that using instance variables of the class. Including null ones!
All these three expressions allow to access a static variable named bar in class Foo (provided that bar is visible by the caller):
Foo.bar; // using the class name
new Foo().bar; // using an instance
((Foo) null).bar; // using a null instance
You have used both static and final keyword , Both have different meaning.
When you declare any variable using static keyword then its declared as a class variable ,Means its common to all instance of that class. access using class name no need to use instance .
E.g.
Class Bycicle
{
static String Type ='exercise';
String Owner;
}
If you create 10 instance of this class then 1o copy of owner will created , while type will remain only one common copy for all 10 object. One object change type value then it will effect to all other object.
if you are declaring static with final then it common to all and also not allowed to change once it declared and initialize at compile time.
Go here for more interesting details click here
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.
I have a simple question here.
If I declare a variable inside an object which was made [declared] in the main class, like this:
public static int number;
(
usually I declare it like this :
private int number;
)
can it be used in a different object which was also made [declared] in the main class?
btw I do not care about security atm, I just want to make something work, don't care about protection)
Here's a telling quote from Java Language Specification:
JLS 8.3.1.1 static Fields
If a field is declared static, there exists exactly one incarnation of the field, no matter how many instances (possibly zero) of the class may eventually be created. A static field, sometimes called a class variable, is incarnated when the class is initialized.
A field that is not declared static (sometimes called a non-static field) is called an instance variable. Whenever a new instance of a class is created, a new variable associated with that instance is created for every instance variable declared in that class or any of its superclasses.
[Example program follows...]
In short, a static field is a class variable: it belongs to the class, as opposed to the instances of the class. In a way, you can think of a static field as a variable shared by instances of the class, but it's much more consistent to think of static fields as belonging to the class, just like static method also belongs to the class, etc.
Since they belong to the class, they do not require an instance of said class to access (assuming adequate visibility), and in fact it's considered bad programming practice to access static members through an instance instead of a type expression.
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Static variables and methods
If the class holding 'number' is called MyClass
you can refer to it as MyClass.number from any method.
Doing so for a variable is not good design though.
There are really two issues here: public vs. private in the context of inner classes, and static variables.
Part 1:
static means that you don't need an instance of the class to access that variable. Suppose you have some code like:
class MyClass {
public static String message = "Hello, World!";
}
You can access the property this way:
System.out.println(MyClass.message);
If you remove the static keyword, you would instead do:
System.out.println(new MyClass().message);
You are accessing the property in the context of an instance, which is a copy of the class created by the new keyword.
Part 2:
If you define two classes in the same java file, one of them must be an inner class. An inner class can have a static keyword, just like a property. If static, it can be used separately. If not-static, it can only be used in the context of a class instance.
Ex:
class MyClass {
public static class InnerClass {
}
}
Then you can do:
new MyClass.InnerClass();
Without the 'static', you would need:
new MyClass().new InnerClass(); //I think
If an inner class is static, it can only access static properties from the outer class. If the inner class is non-static, it can access any property. An inner class doesn't respect the rules of public, protected, or private. So the following is legal:
class MyClass {
private String message;
private class InnerClass {
public InnerClass() {
System.out.println(message);
}
}
}
If the inner class has keyword static, this would not work, since message is not static.
static variables are shared by all instances of a given class. If it's public, it is visible to everything.
non-static variables belong to only one instance.
Since your main method is static, it can only see static variables. But you should avoid working statically - make an instance of a class, and pass the data around as method/constructor parameters, rather than sharing it via static variables.