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Is not enclosing class java

Не является заключающим классом Java

Я пытаюсь сделать игру в Тетрис и получаю ошибку компилятора

Shape is not an enclosing class

когда я пытаюсь создать объект

Я использую внутренние классы для каждой фигуры. Вот часть моего кода

Что я делаю не так?

11 Ответов

ZShape не является статическим, поэтому он требует экземпляр внешнего класса.

Самое простое решение-сделать ZShape и любой вложенный класс static , если вы можете.

Я бы также сделал любые поля final или static final , которые вы также можете использовать.

Предположим, что RetailerProfileModel — это ваш основной класс, а RetailerPaymentModel-внутренний класс внутри него. Вы можете создать объект внутреннего класса вне класса следующим образом:

Я бы предложил не преобразовывать нестатический класс в статический, потому что в этом случае ваш внутренний класс не может получить доступ к нестатическим членам внешнего класса.

Так что, в таком случае, вы можете сделать что-то вроде:

Иногда нам нужно создать новый экземпляр внутреннего класса, который не может быть статическим, потому что он зависит от некоторых глобальных переменных родительского класса. В этом случае при попытке создать экземпляр внутреннего класса, который не является статическим, возникает ошибка not an enclosing class .

Возьмем пример вопроса, Что делать, если ZShape не может быть статическим, потому что ему нужна глобальная переменная класса Shape ?

Как вы можете создать новый экземпляр ZShape ? Вот как это делается:

Добавьте getter в родительский класс:

Доступ к нему вот так:

Я столкнулся с той же проблемой. Я решил эту проблему, создав экземпляр для каждого внутреннего публичного класса. что касается вашей ситуации, я предлагаю вам использовать наследование, отличное от внутренних классов.

тогда вы можете новый Shape(); и посетите ZShape через shape.zShape;

Нет необходимости делать вложенный класс статическим но он должен быть общедоступным

В случае если родительский класс равен singleton используйте следующий способ:

где getInstance() вернет объект родительского класса singleton.

Одна вещь, которую я сначала не понял, читая принятый ответ, заключалась в том, что сделать внутренний класс статичным-это в основном то же самое, что переместить его в свой собственный отдельный класс.

Таким образом, при получении ошибки

xxx не является замкнутым классом

Вы можете решить эту проблему одним из следующих способов:

  • Добавьте ключевое слово static во внутренний класс, или
  • Переместите его в свой отдельный класс.

Для того чтобы достигнуть требования от вопроса, мы можем положить классы в интерфейс:

а потом использовать как автор пробовал раньше:

Если мы ищем правильное решение «logical», следует использовать fabric шаблон дизайна

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Во время чтения java Doc я получил следующие вещи . Внутренний класс C является прямым внутренним классом класса O, если O является непосредственно лексически заключающим классом C и объявление C не.

Is not enclosing class java

November 2018

248k time

I’m trying to make a tetris game and I’m getting «Shape is not an enclosing class» when I try to create an object

I’m using inner classes for each shape. Here’s part of my code

What am I doing wrong ?

13 answers

Я столкнулся с той же проблемой. Я решена путем создания экземпляра для каждого внутреннего общественного класса. как и для вас ситуации, я предлагаю вам использовать наследование, кроме внутренних классов.

то вы можете новый Shape (); и посетить ZShape через shape.zShape;

Пусть RetailerProfileModel Ваш главный класс и RetailerPaymentModel является внутренним классом внутри него. Вы можете создать объект внутреннего класса вне класса следующим образом:

Одна вещь, которую я не понимаю, в первом чтении, когда принятый ответ был, что делает внутренний класс статический, в основном то же самое, перемещая его в свой собственный отдельный класс.

Таким образом, при получении ошибки

Вы можете решить в одном из следующих способов:

  • Добавьте static ключевое слово к внутреннему классу, или
  • Переместить его на свой собственный отдельный класс.

Я столкнулся с той же проблемой. Я решена путем создания экземпляра для каждого внутреннего общественного класса. как и для вас ситуации, я предлагаю вам использовать наследование, кроме внутренних классов.

то вы можете новый Shape (); и посетить ZShape через shape.zShape;

Иногда нам нужно создать новый экземпляр внутреннего класса , который не может быть статическим , поскольку это зависит от некоторых глобальных переменных родительского класса. В этой ситуации, если вы пытаетесь создать экземпляр внутреннего класса , который не является статическим, not an enclosing class выбрасывается ошибка.

Возьмем в качестве примера вопрос, что , если ZShape не может быть статическим , поскольку она нужна глобальная переменная Shape класса?

Как вы можете создать новый экземпляр ZShape ? Это как:

Добавить поглотитель в родительском классе:

I’m trying to make a tetris game and I’m getting “Shape is not an enclosing class” when I try to create an object

I’m using inner classes for each shape. Here’s part of my code

What am I doing wrong ?

ZShape is not static so it requires an instance of the outer class.

The simplest solution is to make ZShape and any nested class static if you can.

I would also make any fields final or static final that you can as well.

Suppose RetailerProfileModel is your Main class and RetailerPaymentModel is an inner class within it. You can create an object of the Inner class outside the class as follows:

What I would suggest is not converting the non-static class to a static class because in that case, your inner class can’t access the non-static members of outer class.

So, in such case, you can do something like:

No need to make the nested class as static but it must be public

In case if Parent class is singleton use following way:

where getInstance() will return parent class singleton object.

I have encountered the same problem.
I solved by creating an instance for every inner public Class.
as for you situation, i suggest you use inheritance other than inner classes.

then you can new Shape();
and visit ZShape through shape.zShape;

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I’m trying to make a Tetris game and I’m getting the compiler error

Shape is not an enclosing class

when I try to create an object

I’m using inner classes for each shape. Here’s part of my code

What am I doing wrong ?

11 Answers 11

ZShape is not static so it requires an instance of the outer class.

The simplest solution is to make ZShape and any nested class static if you can.

I would also make any fields final or static final that you can as well.

Suppose RetailerProfileModel is your Main class and RetailerPaymentModel is an inner class within it. You can create an object of the Inner class outside the class as follows:

What I would suggest is not converting the non-static class to a static class because in that case, your inner class can’t access the non-static members of outer class.

So, in such case, you can do something like:

As stated in the docs:

Sometimes, we need to create a new instance of an inner class that can’t be static because it depends on some global variables of the parent class. In that situation, if you try to create the instance of an inner class that is not static, a not an enclosing class error is thrown.

Taking the example of the question, what if ZShape can’t be static because it need global variable of Shape class?

How can you create new instance of ZShape ? This is how:

Add a getter in the parent class:

Access it like this:

I have encountered the same problem. I solved by creating an instance for every inner public Class. as for you situation, i suggest you use inheritance other than inner classes.

then you can new Shape(); and visit ZShape through shape.zShape;

No need to make the nested class as static but it must be public

One thing I didn’t realize at first when reading the accepted answer was that making an inner class static is basically the same thing as moving it to its own separate class.

Thus, when getting the error

xxx is not an enclosing class

You can solve it in either of the following ways:

  • Add the static keyword to the inner class, or
  • Move it out to its own separate class.

Is not enclosing class java

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Is not enclosing class java

631 просмотра

4 ответа

353 Репутация автора

Considering the java code below:

I am reading a book which tells me that Inner.method() will hide both versions of Enclosing.method() , which means it is an error if I call method(aString) somewhere in class Inner .

Why is the language designed like that?

Update:
According to the answer given by @Debosmit Ray, it is related to shadowing. I have read the docs and understood what it is.

What still confusing me is why method shadowing is based on method name not method signature?

Ответы (4)

плюса

35821 Репутация автора

Because in that Scope, Method, Variables and constants make shadow over global method and variables.

2 плюса

4008 Репутация автора

Non-static nested class or inner classes are used as a way to logically group classes that are only used in one place; it makes the code more readable and promotes encapsulation.

If a declaration of a type (such as a member variable or a parameter name) in a particular scope (such as an inner class or a method definition) has the same name as another declaration in the enclosing scope, then the declaration shadows the declaration of the enclosing scope.

Shadowing here would mean that if you have a variable x in the outer class and another variable x in the inner class, modifying x in the inner class would not affect the x in the outer class.

I quite liked this question and the point you brought up. Does my explanantion help you understand?

1 плюс

8549 Репутация автора

Had you found the correct tag for your question, you would have understood the concept better already! Take look at the tag info for shadowing right on SO.

In computer programming, shadowing occurs when a variable declared within a certain scope (decision block, method or inner class) has the same name as a variable declared in an outer scope. This can lead to confusion, as it may be unclear which variable subsequent uses of the shadowed variable name refer to, which depends on the name resolution rules of the language.

One of the first languages to introduce variable shadowing was ALGOL, which first introduced blocks to establish scopes. It was also permitted by many of the derivative programming languages including C++ and Java.

The C# language breaks this tradition, allowing variable shadowing between an inner and an outer class, and between a method and its containing class, but not between an if-block and its containing method, or between case statements in a switch block.

Wikipedia link (doesn’t provide much though)

Why is the language designed like that?

Let me give you a real world analogy to might help you understand.

Think of a building (1) that has a button named Turn on lights (3). When you press that button, it turns on all the lights in the building. Now think of a cubicle (2) inside that building. There is a small lamp in that cubicle, and a similar button named Turn on lights. Now when you press that button, what do you want it to do — turn on all the lights of the building, or just the lamp in the cubicle? Probably the latter. Though both the buttons have the same name (4), they behave differently depending on their place (5).

Now apply this analogy to OOP. Look at the words in italics one more time, and match up!

  1. building —> Enclosing class
  2. cubicle —> Inner class
  3. Turn on lights —> Method
  4. name —> Method name/signature
  5. place —> Scope

Please note that the analogy doesn’t take into consideration many other concepts of OOP, but I think it might help you understand the why part of your question.

Reply to Update:

What still confusing me is why method shadowing is based on method name not method signature?

Your question doesn’t seem to be a valid one. You will understand this shortly.

You said shadowing is not based on method signature. If what you mean by this is: «if inner class method has same signature as enclosing class method, then shadowing doesn’t take place», then you are wrong. Try it out by making a another method inside the inner class like void method(String str) and then calling that method inside the inner class. You will see that it is obviously shadowed.

And the reason why you got an error when you invoked method(aString) inside the inner class is completely something else— the method method(String str) doesn’t even exist inside the inner class scope.

Feel free if you need further clarification.

1 плюс

503 Репутация автора

This is called shadowing.

Some declarations may be shadowed in part of their scope by another declaration of the same name, in which case a simple name cannot be used to refer to the declared entity.

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It’s convenient to have shadowing in programming language. For example, in constructors, you can have both parameters and class field variables the same name, and use this to distinguish them.

In some other languages, variables may be shadowed by code blocks, for example, in C++ you can write code like:

The variable i inside the loop is different from the outside i .

Why is the language designed like that?

As you can see in the constructor example, sometimes variables may really mean the same thing. Shadowing makes you can use the same names without creating a new name, which is kind of bothering because naming a variable is not quiet simple.

And in the loop sample, which is not supported in Java but is a great sample to show the advantage of shadowing, sometimes in code block you may declared some temporary variables without modifying other variables outside the block.

Why method shadowing is based on method name not method signature?

In JLS 15.12, there’s a explanation about method invocation.

You can see that in step 1, compiler will search for the scope that can call this method. In the end, it found Enclosing.Inner .

And at step 2, compiler will check signature of the methods.

Linked Lists

Introduction

A linked list is a linear data structure where each element is a separate object.
Each element (we will call it a node) of a list is comprising of two items — the data and a reference to the next node. The last node has a reference to null . The entry point into a linked list is called the head of the list. It should be noted that head is not a separate node, but the reference to the first node. If the list is empty then the head is a null reference.

A linked list is a dynamic data structure. The number of nodes in a list is not fixed and can grow and shrink on demand. Any application which has to deal with an unknown number of objects will need to use a linked list.

One disadvantage of a linked list against an array is that it does not allow direct access to the individual elements. If you want to access a particular item then you have to start at the head and follow the references until you get to that item.

Another disadvantage is that a linked list uses more memory compare with an array — we extra 4 bytes (on 32-bit CPU) to store a reference to the next node.

Types of Linked Lists

A singly linked list is described above

A doubly linked list is a list that has two references, one to the next node and another to previous node.
Another important type of a linked list is called a circular linked list where last node of the list points back to the first node (or the head) of the list.

The Node class

We implement the LinkedList class with two inner classes: static Node class and non-static LinkedListIterator class. See LinkedList.java for a complete implementation.

Examples
Let us assume the singly linked list above and trace down the effect of each fragment below. The list is restored to its initial state before each line executes

    Linked List Operations

    The method creates a node and prepends it at the beginning of the list.

    Start with the head and access each node until you reach null. Do not change the head reference.

    The method appends the node to the end of the list. This requires traversing, but make sure you stop at the last node

    Find a node containing «key» and insert a new node after it. In the picture below, we insert a new node after «e»:

    Find a node containing «key» and insert a new node before that node. In the picture below, we insert a new node before «a»:
    For the sake of convenience, we maintain two references prev and cur . When we move along the list we shift these two references, keeping prev one step before cur . We continue until cur reaches the node before which we need to make an insertion. If cur reaches null, we don’t insert, otherwise we insert a new node between prev and cur .

    Examine this implementation

    Find a node containing «key» and delete it. In the picture below we delete a node containing «A» The algorithm is similar to insert «before» algorithm. It is convinient to use two references prev and cur . When we move along the list we shift these two references, keeping prev one step before cur . We continue until cur reaches the node which we need to delete. There are three exceptional cases, we need to take care of:

    1. list is empty
    2. delete the head node
    3. node is not in the list

    Iterator

    The whole idea of the iterator is to provide an access to a private aggregated data and at the same moment hiding the underlying representation. An iterator is Java is an object, and therefore it’s implementation requires creating a class that implements the Iterator interface. Usually such class is implemented as a private inner class. The Iterator interface contains the following methods:

    • AnyType next() — returns the next element in the container
    • boolean hasNext() — checks if there is a next element
    • void remove() — (optional operation).removes the element returned by next()

    In this section we implement the Iterator in the LinkedList class. First of all we add a new method to the LinkedList class:

    Here LinkedListIterator is a private class inside the LinkedList class The LinkedListIterator class must provide implementations for next() and hasNext() methods. Here is the next() method:

    Cloning

    Like for any other objects, we need to learn how to clone linked lists. If we simply use the clone() method from the Object class, we will get the following structure called a «shallow» copy:

    The Object’s clone() will create a copy of the first node, and share the rest. This is not exactly what we mean by «a copy of the object». What we actually want is a copy represented by the picture below

    Since out data is immutable it’s ok to have data shared between two linked lists. There are a few ideas to implement linked list copying. The simplest one is to traverse the original list and copy each node by using the addFirst() method. When this is finished, you will have a new list in the reverse order. Finally, we will have to reverse the list:

    A better way involves using a tail reference for the new list, adding each new node after the last node.

    Applications

    Polynomial Algebra

    The biggest integer that we can store in a variable of the type int is 2 31 — 1 on 32-but CPU. You can easily verify this by the following operations:

    This code doesn’t produce an error, it produces a result! The printed value is a negative integer -2147483648 = -2 31 . If the value becomes too large, Java saves only the low order 32 (or 64 for longs) bits and throws the rest away.

    In real life applications we need to deal with integers that are larger than 64 bits (the size of a long). To manipulate with such big numbers, we will be using a linked list data structure. First we observe that each integer can be expressed in the decimal system of notation. 937 = 9*10 2 + 3*10 1 + 7*10 0 2011 = 2*10 3 + 0*10 2 + 1*10 1 + 1*10 0

    Now, if we replace a decimal base 10 by a character, say ‘x’, we obtain a univariate polynomial, such as

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