Что такое Instance (инстансы) в Java?
Что такое INSTANCE в программирований ?
Вот читаю книгу на английском, но не могу понять INSTANCE, что это значит в программирований ? .
Как узнать что это за инстансы SQL Server?
Не пойму к каким сервисам относятся эти инстансы (см. аттач). Можно это как-то узнать?
Что такое |= в java?
Уважаемые киберфорумцы, встретил такой код:// ставим флаг, чтобы уведомление пропало после нажатия.
Сообщение было отмечено mininvit как решение
Решение
"Что такое Instance в Java и для чего они вообще нужны"?
Это экземпляры классов.
Есть примитивные типы (boolean, byte, char, short, int, long, float и double), которые не превратили в классы, потому что и без этого программы виснут и бьют рекорды по производительности.
Есть другие типы — классы.
Достаточно объявить переменную примитивного типа и значение может сразу хранится в этой переменной. Поэтому пишут:
int n;
Если нужна переменная типа конкретного класса, то она уже не может вместить в себя все допустимые свойства класса, т.к. в java она хранит некий указатель на область памяти, где хранятся все свойства конкретного экземпляра класса. Поэтому в начале объявляют такую переменную:
Box myBox;
а потом выделяют место в памяти и записывают некую ссылку на эту область в переменную:
myBox = new Box();
И да! java строго типизированный язык. Поэтому требуется указывать тип каждой создаваемой переменной.
Сообщение было отмечено mininvit как решение
Решение
Когда Вы создаете переменную, то в первую очередь намерены использовать конкретное значение, которое будет хранить переменная.
Для int это могут быть простые числа (например 10 или 11), а для переменной типа какого-то класса конкретным значением станет instance(экземпляр) этого класса (в котором уже есть свойства, которые можно менять).
Когда объявляется переменная типа класса:
Box myBox;
то instance еще не существует (и через переменную не возможно поменять свойств). Переменная просто "не связана со значением". Значение нужно создать:
myBox = new Box();
java — это упрощения языка с++, для широкого использования. Вам могут быть не ясны конструкции, потому что это фрагменты конструкций другого языка. Упрощение, за которым теряется суть происходящего.
Переменная типа класса в java — это сложный объект, единственная задача которого хранить внутри ссылку(адрес расположения) реального объекта (который создается отдельно командой new) и предоставлять к нему доступ. Если реальный объект еще не создан, то переменная не может хранить его ссылку и предоставить к нему доступ. С примитивными типами все проще, потому что они являются тем, что мы о них думаем — конкретные простые значения, представимые в виде двоичного кода.
What exactly is an instance in Java?
What is the difference between an object, instance, and reference? They say that they have to create an instance to their application? What does that mean?
12 Answers 12
An object and an instance are the same thing.
Personally I prefer to use the word «instance» when referring to a specific object of a specific type, for example «an instance of type Foo». But when talking about objects in general I would say «objects» rather than «instances».
A reference either refers to a specific object or else it can be a null reference.
They say that they have to create an instance to their application. What does it mean?
They probably mean you have to write something like this:
If you are unsure what type you should instantiate you should contact the developers of the application and ask for a more complete example.
«instance to an application» means nothing.
«object» and «instance» are the same thing. There is a «class» that defines structure, and instances of that class (obtained with new ClassName() ). For example there is the class Car , and there are instance with different properties like mileage, max speed, horse-power, brand, etc.
Reference is, in the Java context, a variable* — it is something pointing to an object/instance. For example, String s = null; — s is a reference, that currently references no instance, but can reference an instance of the String class.
*Jon Skeet made a note about the difference between a variable and a reference. See his comment. It is an important distinction about how Java works when you invoke a method — pass-by-value.
The value of s is a reference. It’s very important to distinguish between variables and values, and objects and references.
When you use the keyword new for example JFrame j = new JFrame(); you are creating an instance of the class JFrame .
The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory.
Note: The phrase «instantiating a class» means the same thing as «creating an object.» When you create an object, you are creating an «instance» of a class, therefore «instantiating» a class.
The types of the Java programming language are divided into two categories: primitive types and reference types.
The reference types are class types, interface types, and array types.
There is also a special null type.
An object is a dynamically created instance of a class type or a dynamically created array .
The values of a reference type are references to objects.
I think that Object = Instance. Reference is a «link» to an Object.
variable c stores a reference to an object of type Car.
Here an object is created from the Computer class. A reference named c allows the programmer to access the object.
The main differnece is when you say ClassName obj = null; you are just creating an object for that class. It’s not an instance of that class.
This statement will just allot memory for the static meber variables, not for the normal member variables.
But when you say ClassName obj = new ClassName(); you are creating an instance of the class. This staement will allot memory all member variables.
basically object and instance are the two words used interchangeably. A class is template for an object and an object is an instance of a class.
«creating an instance of a class» how about, «you are taking a class and making a new variable of that class that WILL change depending on an input that changes»
Class in the library called Nacho
variable Libre to hold the «instance» that will change
Nacho Libre = new Nacho(Variable, Scanner Input, or whatever goes here, This is the place that accepts the changes then puts the value in «Libre» on the left side of the equals sign (you know «Nacho Libre = new Nacho(Scanner.in)» «Nacho Libre» is on the left of the = (that’s not tech talk, that’s my way of explaining it)
I think that is better than saying «instance of type» or «instance of class». Really the point is it just needs to be detailed out more. «instance of type or class» is not good enough for the beginner. wow, its like a tongue twister and your brain cannot focus on tongue twisters very well. that «instance» word is very annoying and the mere sound of it drives me nuts. it begs for more detail. it begs to be broken down better. I had to google what «instance» meant just to get my bearings straight. try saying «instance of class» to your grandma. yikes!
Know the Difference Between Reference, Object, Instance, and Class?
Get clear with these terminologies used in programming.
In this article, I’m going to explain about most frequently used terminologies in the java programming language. Classes, objects, instances, and references are a few terms that you may have heard on a day-to-day basis while writing codes. After reading this article you will get to know about these terms and their different usage.
What is a Class?
Class is a blueprint/template/representation or user-defined data type for the objects. We write only one class for hundreds of objects. A class is defined using the class keyword followed by the name of the class and then the class body is defined.
Example:
The Student is defined as a class using the class keyword and then inside curly brackets, instance variables, and instance methods are defined.
What is an Object?
An object is a real-world or software entry that has attributes(instance fields) and behavior(instance methods). The object is created with a new operator in the heap. e.g. new ClassName(); . Objects are instantiated when the class is loaded into memory. Objects are also called Instances.
Example:
Here the object of type Student is created using a new operator in heap and the address is returned in variable s1, then default construct student() is called.
What is a Reference?
Reference holds the address of an object or instance. Whenever we want to call instance methods, we use this reference which holds the address of the object. References are like C++ pointers.
s1 is a reference of type Student and points to the object of type Student and will be used to access instance variables and methods.
Below I’ve written Student class with the instance variable and methods. And created objects and references for this class in Main (driver class).
OUTPUT :
When Does the Java Compiler Add the Default Constructor?
If a class doesn’t have any constructor provided by the programmer, then the java compiler will add a default constructor without parameters that will call a superclass constructor internally with a super() call. This is called a default constructor.
You can see that I haven’t added a default constructor in the Student class hence compiler will create one default constructor and will add it to the class.
Example:
Note: Inside the default constructor, it will add a super() call also, to call the superclass constructor.[In the case of the Student class, the superclass is the Object class] and every class internally inherits the object class.
Purpose of adding default constructor:
The constructor’s duty is to initialize instance variables. If there are no instance variables then you could choose to remove the constructor from your class.
But when you are inheriting some class it is your responsibility to call the superclass constructor to make sure that the superclass initializes all its instance variables properly.
That’s why if there are no constructors, the java compiler will add a default constructor and calls a superclass constructor.
Note: super() call inside the default constructor is generally hidden.
That’s all for this article. Hope you have enjoyed this article.
What Are Java Instance Variables & Why Do They Matter?
What is an instance variable in Java? For starters, they are more valuable than they may initially sound. They are invariably vital for your Java software. But what are they really, and how do they work?
This post will explain the concepts behind an instance variable, how they work and how you can use them. You will see some code examples of syntax and learn of some ways you can use them in your development to build better, more efficient software.