Что такое object в java
Класс Object
В Java есть специальный суперкласс Object и все классы являются его подклассами. Поэтому ссылочная переменная класса Object может ссылаться на объект любого другого класса. Так как массивы являются тоже классами, то переменная класса Object может ссылаться и на любой массив.
В таком виде объект обычно не используют. Чтобы с объектом что-то сделать, нужно выполнить приведение типов.
У класса есть несколько важных методов.
Методы getClass(), notify(), notifyAll(), wait() являются финальными и их нельзя переопределять.
Метод hashCode()
Для вычисления хеш-кода в классе String применяется следующий алгоритм.
У любого объекта имется хеш-код, определяемый по умолчанию, который вычисляется по адресу памяти, занимаемой объектом.
Значение хеш-кода возвращает целочисленное значение, в том числ и отрицательное.
Если в вашем классе переопределяется метод equals(), то следует переопределить и метод hashCode().
Метод toString()
Очень важный метод, возвращающий значение объекта в виде символьной строки.
Очень часто при использовании метода toString() для получения описания объекта можно получить набор бессмысленных символов, например, [I@421199e8. На самом деле в них есть смысл, доступный специалистом. Он сразу может сказать, что мы имеем дело с одномерным массивом (одна квадратная скобка), который имеет тип int (символ I). Остальные символы тоже что-то означают, но вам знать это не обязательно.
Если же вам нужно научное объяснение, то метод работает по следующему алгоритму (из документации).
Обычно принято переопределять метод, чтобы он выводил результат в читаемом виде.
Что такое object в java
The actual result type is Class where |X| is the erasure of the static type of the expression on which getClass is called. For example, no cast is required in this code fragment:
Number n = 0;
Class c = n.getClass();
hashCode
As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (The hashCode may or may not be implemented as some function of an object’s memory address at some point in time.)
equals
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.
clone
By convention, the object returned by this method should be independent of this object (which is being cloned). To achieve this independence, it may be necessary to modify one or more fields of the object returned by super.clone before returning it. Typically, this means copying any mutable objects that comprise the internal «deep structure» of the object being cloned and replacing the references to these objects with references to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually the case that no fields in the object returned by super.clone need to be modified.
toString
The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character ` @ ‘, and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:
notify
The awakened thread will not be able to proceed until the current thread relinquishes the lock on this object. The awakened thread will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened thread enjoys no reliable privilege or disadvantage in being the next thread to lock this object.
Only one thread at a time can own an object’s monitor.
notifyAll
The awakened threads will not be able to proceed until the current thread relinquishes the lock on this object. The awakened threads will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened threads enjoy no reliable privilege or disadvantage in being the next thread to lock this object.
This method should only be called by a thread that is the owner of this object’s monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
In all respects, this method behaves as if wait(0L, 0) had been called. See the specification of the wait(long, int) method for details.
In all respects, this method behaves as if wait(timeout, 0) had been called. See the specification of the wait(long, int) method for details.
The current thread must own this object’s monitor lock. See the notify method for a description of the ways in which a thread can become the owner of a monitor lock.
This method causes the current thread (referred to here as T ) to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. Note that only the locks on this object are relinquished; any other objects on which the current thread may be synchronized remain locked while the thread waits.
A thread can wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. See the example below.
For more information on this topic, see section 14.2, «Condition Queues,» in Brian Goetz and others’ Java Concurrency in Practice (Addison-Wesley, 2006) or Item 69 in Joshua Bloch’s Effective Java, Second Edition (Addison-Wesley, 2008).
If the current thread is interrupted by any thread before or while it is waiting, then an InterruptedException is thrown. The interrupted status of the current thread is cleared when this exception is thrown. This exception is not thrown until the lock status of this object has been restored as described above.
finalize
The finalize method of class Object performs no special action; it simply returns normally. Subclasses of Object may override this definition.
The Java programming language does not guarantee which thread will invoke the finalize method for any given object. It is guaranteed, however, that the thread that invokes finalize will not be holding any user-visible synchronization locks when finalize is invoked. If an uncaught exception is thrown by the finalize method, the exception is ignored and finalization of that object terminates.
After the finalize method has been invoked for an object, no further action is taken until the Java virtual machine has again determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, including possible actions by other objects or classes which are ready to be finalized, at which point the object may be discarded.
The finalize method is never invoked more than once by a Java virtual machine for any given object.
Any exception thrown by the finalize method causes the finalization of this object to be halted, but is otherwise ignored.
Report a bug or suggest an enhancement
For further API reference and developer documentation see the Java SE Documentation, which contains more detailed, developer-targeted descriptions with conceptual overviews, definitions of terms, workarounds, and working code examples.
Java is a trademark or registered trademark of Oracle and/or its affiliates in the US and other countries.
Copyright © 1993, 2018, Oracle and/or its affiliates, 500 Oracle Parkway, Redwood Shores, CA 94065 USA.
All rights reserved. Use is subject to license terms and the documentation redistribution policy.
Что такое object в java
The actual result type is Class where |X| is the erasure of the static type of the expression on which getClass is called. For example, no cast is required in this code fragment:
Number n = 0;
Class c = n.getClass();
hashCode
As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the Java TM programming language.)
equals
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.
clone
By convention, the object returned by this method should be independent of this object (which is being cloned). To achieve this independence, it may be necessary to modify one or more fields of the object returned by super.clone before returning it. Typically, this means copying any mutable objects that comprise the internal «deep structure» of the object being cloned and replacing the references to these objects with references to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually the case that no fields in the object returned by super.clone need to be modified.
toString
The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character ` @ ‘, and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:
notify
The awakened thread will not be able to proceed until the current thread relinquishes the lock on this object. The awakened thread will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened thread enjoys no reliable privilege or disadvantage in being the next thread to lock this object.
Only one thread at a time can own an object’s monitor.
notifyAll
The awakened threads will not be able to proceed until the current thread relinquishes the lock on this object. The awakened threads will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened threads enjoy no reliable privilege or disadvantage in being the next thread to lock this object.
This method should only be called by a thread that is the owner of this object’s monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
The current thread must own this object’s monitor.
A thread can also wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. In other words, waits should always occur in loops, like this one: (For more information on this topic, see Section 3.2.3 in Doug Lea’s «Concurrent Programming in Java (Second Edition)» (Addison-Wesley, 2000), or Item 50 in Joshua Bloch’s «Effective Java Programming Language Guide» (Addison-Wesley, 2001).
If the current thread is interrupted by any thread before or while it is waiting, then an InterruptedException is thrown. This exception is not thrown until the lock status of this object has been restored as described above.
Note that the wait method, as it places the current thread into the wait set for this object, unlocks only this object; any other objects on which the current thread may be synchronized remain locked while the thread waits.
This method should only be called by a thread that is the owner of this object’s monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
This method is similar to the wait method of one argument, but it allows finer control over the amount of time to wait for a notification before giving up. The amount of real time, measured in nanoseconds, is given by:
The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop: This method should only be called by a thread that is the owner of this object’s monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
The current thread must own this object’s monitor. The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object’s monitor to wake up either through a call to the notify method or the notifyAll method. The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop: This method should only be called by a thread that is the owner of this object’s monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
finalize
The finalize method of class Object performs no special action; it simply returns normally. Subclasses of Object may override this definition.
The Java programming language does not guarantee which thread will invoke the finalize method for any given object. It is guaranteed, however, that the thread that invokes finalize will not be holding any user-visible synchronization locks when finalize is invoked. If an uncaught exception is thrown by the finalize method, the exception is ignored and finalization of that object terminates.
After the finalize method has been invoked for an object, no further action is taken until the Java virtual machine has again determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, including possible actions by other objects or classes which are ready to be finalized, at which point the object may be discarded.
The finalize method is never invoked more than once by a Java virtual machine for any given object.
Any exception thrown by the finalize method causes the finalization of this object to be halted, but is otherwise ignored.
Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
Copyright © 1993, 2020, Oracle and/or its affiliates. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.
Что такое object в java
The actual result type is Class where |X| is the erasure of the static type of the expression on which getClass is called. For example, no cast is required in this code fragment:
Number n = 0;
Class c = n.getClass();
hashCode
As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the Java™ programming language.)
equals
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.
clone
By convention, the object returned by this method should be independent of this object (which is being cloned). To achieve this independence, it may be necessary to modify one or more fields of the object returned by super.clone before returning it. Typically, this means copying any mutable objects that comprise the internal «deep structure» of the object being cloned and replacing the references to these objects with references to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually the case that no fields in the object returned by super.clone need to be modified.
toString
The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character ` @ ‘, and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:
notify
The awakened thread will not be able to proceed until the current thread relinquishes the lock on this object. The awakened thread will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened thread enjoys no reliable privilege or disadvantage in being the next thread to lock this object.
Only one thread at a time can own an object’s monitor.
notifyAll
The awakened threads will not be able to proceed until the current thread relinquishes the lock on this object. The awakened threads will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened threads enjoy no reliable privilege or disadvantage in being the next thread to lock this object.
This method should only be called by a thread that is the owner of this object’s monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
The current thread must own this object’s monitor.
A thread can also wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. In other words, waits should always occur in loops, like this one: (For more information on this topic, see Section 3.2.3 in Doug Lea’s «Concurrent Programming in Java (Second Edition)» (Addison-Wesley, 2000), or Item 50 in Joshua Bloch’s «Effective Java Programming Language Guide» (Addison-Wesley, 2001).
If the current thread is interrupted by any thread before or while it is waiting, then an InterruptedException is thrown. This exception is not thrown until the lock status of this object has been restored as described above.
Note that the wait method, as it places the current thread into the wait set for this object, unlocks only this object; any other objects on which the current thread may be synchronized remain locked while the thread waits.
This method should only be called by a thread that is the owner of this object’s monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
This method is similar to the wait method of one argument, but it allows finer control over the amount of time to wait for a notification before giving up. The amount of real time, measured in nanoseconds, is given by:
The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop: This method should only be called by a thread that is the owner of this object’s monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
The current thread must own this object’s monitor. The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object’s monitor to wake up either through a call to the notify method or the notifyAll method. The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop: This method should only be called by a thread that is the owner of this object’s monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.
finalize
The finalize method of class Object performs no special action; it simply returns normally. Subclasses of Object may override this definition.
The Java programming language does not guarantee which thread will invoke the finalize method for any given object. It is guaranteed, however, that the thread that invokes finalize will not be holding any user-visible synchronization locks when finalize is invoked. If an uncaught exception is thrown by the finalize method, the exception is ignored and finalization of that object terminates.
After the finalize method has been invoked for an object, no further action is taken until the Java virtual machine has again determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, including possible actions by other objects or classes which are ready to be finalized, at which point the object may be discarded.
The finalize method is never invoked more than once by a Java virtual machine for any given object.
Any exception thrown by the finalize method causes the finalization of this object to be halted, but is otherwise ignored.
Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
Copyright © 1993, 2021, Oracle and/or its affiliates. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.
Объекты в Java
1. Объекты
Все в Java является объектом.
Вернее, очень мало чего в Java объектом не является. Например, примитивные типы. Но это скорее редкое исключение, чем правило.
Что же такое объект?
Объект — это сгруппированные вместе данные и методы для того, чтобы эти данные обрабатывать. Когда мы говорим о данных, имеем в виду переменные, конечно.
Про переменные объекта говорят, что это «данные объекта» или «состояние объекта».
Про методы объекта говорят: это «поведение объекта». Состояние объекта (переменные объекта) принято менять только с помощью методов того же объекта. Менять переменные объекта напрямую (не через методы объекта) считается дурным тоном.
У каждого объекта, как и у каждой переменной, есть тип. Этот тип определяется один раз при создании объекта и поменять его в дальнейшем нельзя. Типом объекта считается его класс.
У каждого объекта есть собственная копия переменных класса (полей класса). Если в классе была объявлена нестатическая переменная int a, и ваша программа во время работы создала 10 объектов этого класса, теперь в каждом объекте есть собственная переменная int a.
Взаимодействие с объектом
Самый удобный способ работы с объектом — сохранить ссылку на объект в переменную, и потом вызывать методы у этой переменной. Выглядит это для вас знакомо:
Где переменная — это переменная, которая хранит в себе ссылку на объект, а метод — это метод класса объекта.
Если вы хотите обратиться к полю (переменной) объекта, то тоже нужно использовать оператор точка :
Где переменная — это переменная, которая хранит в себе ссылку на объект, а поле — это переменная класса (поле объекта).
2. Оператор new
Где Класс — это имя класса для объявления переменной и имя класса создаваемого объекта. Переменная — это переменная, в которую сохраняется ссылка на созданный объект. А параметры — это параметры метода создания объекта.
Какие именно могут быть параметры у объекта, решают программисты, которые пишут класс объекта.
Вы уже создавали объекты ранее, и даже пользовались именно этой конструкцией. Надеюсь, вы не забыли?
Примеры создания объектов:
| Код | Описание |
|---|---|
| Создает объект типа String | |
| Создает объект типа Scanner | |
| Создает объект типа int[] : контейнер на 10 элементов типа int |