Экземпляром какого класса является поле system err

Among the facilities provided by the System class are standard input, standard output, and error output streams; access to externally defined properties and environment variables; a means of loading files and libraries; and a utility method for quickly copying a portion of an array.

Nested Class Summary

Field Summary

Method Summary

Methods inherited from class java.lang.Object

Field Detail

For simple stand-alone Java applications, a typical way to write a line of output data is:

See the println methods in class PrintStream .

Typically this stream corresponds to display output or another output destination specified by the host environment or user. By convention, this output stream is used to display error messages or other information that should come to the immediate attention of a user even if the principal output stream, the value of the variable out , has been redirected to a file or other destination that is typically not continuously monitored.

Method Detail

setIn

First, if there is a security manager, its checkPermission method is called with a RuntimePermission(«setIO») permission to see if it’s ok to reassign the «standard» input stream.

setOut

First, if there is a security manager, its checkPermission method is called with a RuntimePermission(«setIO») permission to see if it’s ok to reassign the «standard» output stream.

setErr

First, if there is a security manager, its checkPermission method is called with a RuntimePermission(«setIO») permission to see if it’s ok to reassign the «standard» error output stream.

console

inheritedChannel

This method returns the channel obtained by invoking the inheritedChannel method of the system-wide default SelectorProvider object.

In addition to the network-oriented channels described in inheritedChannel , this method may return other kinds of channels in the future.

setSecurityManager

If there is a security manager already installed, this method first calls the security manager’s checkPermission method with a RuntimePermission(«setSecurityManager») permission to ensure it’s ok to replace the existing security manager. This may result in throwing a SecurityException .

Otherwise, the argument is established as the current security manager. If the argument is null and no security manager has been established, then no action is taken and the method simply returns.

getSecurityManager

currentTimeMillis

See the description of the class Date for a discussion of slight discrepancies that may arise between «computer time» and coordinated universal time (UTC).

nanoTime

This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time. The value returned represents nanoseconds since some fixed but arbitrary origin time (perhaps in the future, so values may be negative). The same origin is used by all invocations of this method in an instance of a Java virtual machine; other virtual machine instances are likely to use a different origin.

This method provides nanosecond precision, but not necessarily nanosecond resolution (that is, how frequently the value changes) — no guarantees are made except that the resolution is at least as good as that of currentTimeMillis() .

Differences in successive calls that span greater than approximately 292 years (2 63 nanoseconds) will not correctly compute elapsed time due to numerical overflow.

The values returned by this method become meaningful only when the difference between two such values, obtained within the same instance of a Java virtual machine, is computed.

For example, to measure how long some code takes to execute:

To compare elapsed time against a timeout, use instead of because of the possibility of numerical overflow.

arraycopy

If the src and dest arguments refer to the same array object, then the copying is performed as if the components at positions srcPos through srcPos+length-1 were first copied to a temporary array with length components and then the contents of the temporary array were copied into positions destPos through destPos+length-1 of the destination array.

If dest is null , then a NullPointerException is thrown.

If src is null , then a NullPointerException is thrown and the destination array is not modified.

The src argument refers to an object that is not an array.
The dest argument refers to an object that is not an array.
The src argument and dest argument refer to arrays whose component types are different primitive types.
The src argument refers to an array with a primitive component type and the dest argument refers to an array with a reference component type.
The src argument refers to an array with a reference component type and the dest argument refers to an array with a primitive component type.

The srcPos argument is negative.
The destPos argument is negative.
The length argument is negative.
srcPos+length is greater than src.length , the length of the source array.
destPos+length is greater than dest.length , the length of the destination array.

Otherwise, if any actual component of the source array from position srcPos through srcPos+length-1 cannot be converted to the component type of the destination array by assignment conversion, an ArrayStoreException is thrown. In this case, let k be the smallest nonnegative integer less than length such that src[srcPos+ k ] cannot be converted to the component type of the destination array; when the exception is thrown, source array components from positions srcPos through srcPos+ k -1 will already have been copied to destination array positions destPos through destPos+ k -1 and no other positions of the destination array will have been modified. (Because of the restrictions already itemized, this paragraph effectively applies only to the situation where both arrays have component types that are reference types.)

identityHashCode

getProperties

First, if there is a security manager, its checkPropertiesAccess method is called with no arguments. This may result in a security exception.

The current set of system properties for use by the getProperty(String) method is returned as a Properties object. If there is no current set of system properties, a set of system properties is first created and initialized. This set of system properties always includes values for the following keys:

Shows property keys and associated values

Key	Description of Associated Value
java.version	Java Runtime Environment version which may be interpreted as a Runtime.Version
java.vendor	Java Runtime Environment vendor
java.vendor.url	Java vendor URL
java.home	Java installation directory
java.vm.specification.version	Java Virtual Machine specification version which may be interpreted as a Runtime.Version
java.vm.specification.vendor	Java Virtual Machine specification vendor
java.vm.specification.name	Java Virtual Machine specification name
java.vm.version	Java Virtual Machine implementation version which may be interpreted as a Runtime.Version
java.vm.vendor	Java Virtual Machine implementation vendor
java.vm.name	Java Virtual Machine implementation name
java.specification.version	Java Runtime Environment specification version which may be interpreted as a Runtime.Version
java.specification.vendor	Java Runtime Environment specification vendor
java.specification.name	Java Runtime Environment specification name
java.class.version	Java class format version number
java.class.path	Java class path
java.library.path	List of paths to search when loading libraries
java.io.tmpdir	Default temp file path
java.compiler	Name of JIT compiler to use
os.name	Operating system name
os.arch	Operating system architecture
os.version	Operating system version
file.separator	File separator («/» on UNIX)
path.separator	Path separator («:» on UNIX)
line.separator	Line separator («\n» on UNIX)
user.name	User’s account name
user.home	User’s home directory
user.dir	User’s current working directory

Multiple paths in a system property value are separated by the path separator character of the platform.

Note that even if the security manager does not permit the getProperties operation, it may choose to permit the getProperty(String) operation.

lineSeparator

On UNIX systems, it returns «\n» ; on Microsoft Windows systems it returns «\r\n» .

setProperties

First, if there is a security manager, its checkPropertiesAccess method is called with no arguments. This may result in a security exception.

The argument becomes the current set of system properties for use by the getProperty(String) method. If the argument is null , then the current set of system properties is forgotten.

getProperty

First, if there is a security manager, its checkPropertyAccess method is called with the key as its argument. This may result in a SecurityException.

If there is no current set of system properties, a set of system properties is first created and initialized in the same manner as for the getProperties method.

getProperty

First, if there is a security manager, its checkPropertyAccess method is called with the key as its argument.

If there is no current set of system properties, a set of system properties is first created and initialized in the same manner as for the getProperties method.

setProperty

First, if a security manager exists, its SecurityManager.checkPermission method is called with a PropertyPermission(key, «write») permission. This may result in a SecurityException being thrown. If no exception is thrown, the specified property is set to the given value.

clearProperty

getenv

If a security manager exists, its checkPermission method is called with a RuntimePermission («getenv.»+name) permission. This may result in a SecurityException being thrown. If no exception is thrown the value of the variable name is returned.

System properties and environment variables are both conceptually mappings between names and values. Both mechanisms can be used to pass user-defined information to a Java process. Environment variables have a more global effect, because they are visible to all descendants of the process which defines them, not just the immediate Java subprocess. They can have subtly different semantics, such as case insensitivity, on different operating systems. For these reasons, environment variables are more likely to have unintended side effects. It is best to use system properties where possible. Environment variables should be used when a global effect is desired, or when an external system interface requires an environment variable (such as PATH ).

On UNIX systems the alphabetic case of name is typically significant, while on Microsoft Windows systems it is typically not. For example, the expression System.getenv(«FOO»).equals(System.getenv(«foo»)) is likely to be true on Microsoft Windows.

getenv

If the system does not support environment variables, an empty map is returned.

The returned map will never contain null keys or values. Attempting to query the presence of a null key or value will throw a NullPointerException . Attempting to query the presence of a key or value which is not of type String will throw a ClassCastException .

The returned map and its collection views may not obey the general contract of the Object.equals(java.lang.Object) and Object.hashCode() methods.

The returned map is typically case-sensitive on all platforms.

If a security manager exists, its checkPermission method is called with a RuntimePermission («getenv.*») permission. This may result in a SecurityException being thrown.

When passing information to a Java subprocess, system properties are generally preferred over environment variables.

getLogger

This method calls the exit method in class Runtime . This method never returns normally.

The call System.exit(n) is effectively equivalent to the call:

Calling the gc method suggests that the Java Virtual Machine expend effort toward recycling unused objects in order to make the memory they currently occupy available for quick reuse. When control returns from the method call, the Java Virtual Machine has made a best effort to reclaim space from all discarded objects.

The call System.gc() is effectively equivalent to the call:

runFinalization

Calling this method suggests that the Java Virtual Machine expend effort toward running the finalize methods of objects that have been found to be discarded but whose finalize methods have not yet been run. When control returns from the method call, the Java Virtual Machine has made a best effort to complete all outstanding finalizations.

The call System.runFinalization() is effectively equivalent to the call:

runFinalizersOnExit

If there is a security manager, its checkExit method is first called with 0 as its argument to ensure the exit is allowed. This could result in a SecurityException.

The call System.load(name) is effectively equivalent to the call:

loadLibrary

The call System.loadLibrary(name) is effectively equivalent to the call

mapLibraryName

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.
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All rights reserved. Use is subject to license terms and the documentation redistribution policy.

Class System

For simple stand-alone Java applications, a typical way to write a line of output data is:

See the println methods in class PrintStream .

Method Details

setIn

setOut

setErr

console

inheritedChannel

In addition to the network-oriented channels described in inheritedChannel , this method may return other kinds of channels in the future.

setSecurityManager

Otherwise, the argument is established as the current security manager. If the argument is null and no security manager has been established, then no action is taken and the method simply returns.

getSecurityManager

currentTimeMillis

See the description of the class Date for a discussion of slight discrepancies that may arise between «computer time» and coordinated universal time (UTC).

nanoTime

Differences in successive calls that span greater than approximately 292 years (2 63 nanoseconds) will not correctly compute elapsed time due to numerical overflow.

The values returned by this method become meaningful only when the difference between two such values, obtained within the same instance of a Java virtual machine, is computed.

For example, to measure how long some code takes to execute:

To compare elapsed time against a timeout, use instead of because of the possibility of numerical overflow.

arraycopy

If dest is null , then a NullPointerException is thrown.

If src is null , then a NullPointerException is thrown and the destination array is not modified.

The src argument refers to an object that is not an array.
The dest argument refers to an object that is not an array.
The src argument and dest argument refer to arrays whose component types are different primitive types.
The src argument refers to an array with a primitive component type and the dest argument refers to an array with a reference component type.
The src argument refers to an array with a reference component type and the dest argument refers to an array with a primitive component type.

The srcPos argument is negative.
The destPos argument is negative.
The length argument is negative.
srcPos+length is greater than src.length , the length of the source array.
destPos+length is greater than dest.length , the length of the destination array.

identityHashCode

getProperties

Shows property keys and associated values

Key	Description of Associated Value
java.version	Java Runtime Environment version, which may be interpreted as a Runtime.Version
java.version.date	Java Runtime Environment version date, in ISO-8601 YYYY-MM-DD format, which may be interpreted as a LocalDate
java.vendor	Java Runtime Environment vendor
java.vendor.url	Java vendor URL
java.vendor.version	Java vendor version (optional)
java.home	Java installation directory
java.vm.specification.version	Java Virtual Machine specification version, whose value is the feature element of the runtime version
java.vm.specification.vendor	Java Virtual Machine specification vendor
java.vm.specification.name	Java Virtual Machine specification name
java.vm.version	Java Virtual Machine implementation version which may be interpreted as a Runtime.Version
java.vm.vendor	Java Virtual Machine implementation vendor
java.vm.name	Java Virtual Machine implementation name
java.specification.version	Java Runtime Environment specification version, whose value is the feature element of the runtime version
java.specification.vendor	Java Runtime Environment specification vendor
java.specification.name	Java Runtime Environment specification name
java.class.version	Java class format version number
java.class.path	Java class path (refer to ClassLoader.getSystemClassLoader() for details)
java.library.path	List of paths to search when loading libraries
java.io.tmpdir	Default temp file path
java.compiler	Name of JIT compiler to use
os.name	Operating system name
os.arch	Operating system architecture
os.version	Operating system version
file.separator	File separator («/» on UNIX)
path.separator	Path separator («:» on UNIX)
line.separator	Line separator («\n» on UNIX)
user.name	User’s account name
user.home	User’s home directory
user.dir	User’s current working directory
native.encoding	Character encoding name derived from the host environment and/or the user’s settings. Setting this system property has no effect.

Multiple paths in a system property value are separated by the path separator character of the platform.

Note that even if the security manager does not permit the getProperties operation, it may choose to permit the getProperty(String) operation.

lineSeparator

On UNIX systems, it returns «\n» ; on Microsoft Windows systems it returns «\r\n» .

setProperties

The argument becomes the current set of system properties for use by the getProperty(String) method. If the argument is null , then the current set of system properties is forgotten.

getProperty

If there is no current set of system properties, a set of system properties is first created and initialized in the same manner as for the getProperties method.

getProperty

If there is no current set of system properties, a set of system properties is first created and initialized in the same manner as for the getProperties method.

setProperty

clearProperty

getenv

If a security manager exists, its checkPermission method is called with a RuntimePermission(«getenv.»+name) permission. This may result in a SecurityException being thrown. If no exception is thrown the value of the variable name is returned.

getenv

If the system does not support environment variables, an empty map is returned.

The returned map and its collection views may not obey the general contract of the Object.equals(java.lang.Object) and Object.hashCode() methods.

The returned map is typically case-sensitive on all platforms.

If a security manager exists, its checkPermission method is called with a RuntimePermission(«getenv.*») permission. This may result in a SecurityException being thrown.

When passing information to a Java subprocess, system properties are generally preferred over environment variables.

Работа с классом Java System (java.lang.System)

Класс Java System является одним из базовых и используется практически в любом проекте. Он располагает множеством полезных методов для работы, некоторые из которых мы сегодня и рассмотрим.

Кратко о классе Java System

Класс java.lang.System является final, все поля и методы являются статическими (static), поэтому мы не можем создать подкласс и переопределить его методы используя наследование. Класс Java System не предоставляет каких-либо публичных конструкторов, поэтому мы не можем создать экземпляр этого класса.

Пример работы с классом

Вызов метода из класса java.lang.System выглядит так: System.out.print() — это один из самых простых способов логгирования информации.

Далее рассмотрим различные функции, предоставляемые классом java.lang.System .

Пример использования Array Copy класса Java System

Класс System обеспечивает нативный метод для копирования данных из одного массива в другой. Это встроенный метод, поэтому должен работать быстрее, чем другие способы копирования массива.

Метод System.arraycopy бросает исключение IndexOutOfBoundsException если копирование обратится к данным за пределами границ массива.

Также он бросает ArrayStoreException если элемент в исходном массиве не может быть сохранен в массиве назначения из-за несоответствия типа.

Еще может быть выброшен NullPointerException, если массив источник или массив назначения null .

Класс System в Java

Java предоставляет нам полный набор готовых классов и библиотек, что снижает необходимость в служебном кодировании. Одним из таких классов является класс System в Java.

Что такое класс System в Java?

Системный является одним из базовых классов в Java и принадлежит пакету java.lang. Класс System является финальным и не предоставляет общедоступных конструкторов. Из-за этого все члены и методы, содержащиеся в этом классе, являются статическими по природе.

Таким образом, вы не можете наследовать этот класс для переопределения его методов. Поскольку класс System имеет множество ограничений, существуют различные предварительно созданные поля и методы класса. Ниже я перечислил несколько важных функций, поддерживаемых этим классом:

Стандартный ввод и вывод.
Ошибка вывода потоков.
Доступ к внешним свойствам и переменным среды.
Встроенная утилита для копирования части массива.
Предоставляет средства для загрузки файлов и библиотек.

Объявление

Поля класса

Класс java.lang.System поставляется с тремя полями:

public static final InputStream in: Это стандартный поток ввода в Java-программировании. Этот поток уже открыт и доступен для ввода входных данных. Этот поток ввода в основном соответствует вводу с клавиатуры или другим источникам ввода, которые указываются хост-средой или пользователем.
public static final PrintStream out: Это стандартный поток вывода в Java-программировании . Этот поток уже открыт и доступен для принятия выходных данных. Этот выходной поток в основном соответствует отображению выходного или другого выходного пункта назначения, который указан хост-средой или пользователем.
public static final PrintStream err: Это стандартный поток вывода ошибок в Java-программировании. Этот поток уже открыт и доступен для принятия выходных данных. Этот выходной поток в основном соответствует отображению выходного или другого выходного пункта назначения, который указан хост-средой или пользователем. Технически этот выходной поток используется для отображения сообщений об ошибках или другой информации, которая требует немедленного внимания пользователя.

Методы системного класса

Всего в классе java.lang.System объявлено 28 встроенных методов.

Метод	Описание
static void arraycopy (Object src, int srcPos, Object dest, int destPos, int length)	Помогает копировать массив из указанного исходного массива, начиная с указанной позиции до указанной позиции целевого массива.
static String clearProperty (ключ String)	Помогает удалить системное свойство, указанное указанным ключом.
Консоль static static ()	Помогает вернуть любой доступный уникальный объект консоли, связанный с текущей JVM.
статическое длинное currentTimeMillis()	Помогает вернуть текущее время в миллисекундах
выход из статической пустоты (статус int)	Помогает завершить текущую JVM
статическая пустота gc()	Этот метод помогает в запуске сборщика мусора
статическая карта <String, String> getenv()	Помогает в возврате неизменяемой строковой карты текущей системы
static String getenv (имя строки)	Помогает в получении значения указанной переменной среды
статические свойства getProperties ()	Помогает в определении текущих свойств системы
статическая строка getProperty (строковый ключ)	Помогает получить системное свойство, указанное указанным ключом.
статическая строка getProperty (ключ строки, строка определения)	Помогает получить системное свойство, указанное указанным ключом.
статический SecurityManager getSecurityManager ()	Помогает получить интерфейс безопасности системы
static int identityHashCode (Object x)	Помогает возвращать тот же хэш-код для данного объекта, значение которого будет похоже на метод по умолчанию hashCode (), независимо от переопределения класса данного объекта hashCode ()
статический Канал унаследованный канал ()	Помогает в возврате канала, который унаследован от объекта, создавшего JVM.
статическая строка lineSeparator ()	Помогает в возврате системно-зависимой строки разделителя строк.
статическая пустая нагрузка (строковое имя файла)	Помогает в загрузке файла кода с указанным именем файла из локальной файловой системы в виде динамической библиотеки.
static void loadLibrary (строковое имя_библиотеки)	Помогает в загрузке системной библиотеки, указанной аргументом libname
статическая строка mapLibraryName (строковое имя_библиотеки)	Помогает в отображении имени библиотеки в специфическую для платформы строку, представляющую собственную библиотеку
статический длинный nanoTime ()	Помогает возвращать текущее значение работающего источника времени высокого разрешения JVM в течение наносекунд
static void runFinalization ()	Помогает в выполнении методов завершения любых объектов, ожидающих завершения
static void setErr (PrintStream err)	Помогает переназначить «стандартный» поток вывода ошибок
статическая пустота setIn (InputStream in)	Помогает переназначить «стандартный» поток ввода
static void setOut (PrintStream out)	Помогает переназначить «стандартный» поток вывода
static void setProperties (Свойства реквизита)	Помогает в настройке системных свойств для аргумента Свойства
статическая строка setProperty (строковый ключ, строковое значение)	Помогает в настройке системного свойства, указанного указанным ключом
static void setSecurityManager (SecurityManager s)	Помогает в настройке безопасности системы
static void runFi nalizersOnExit (логическое значение)	Устаревшее

Реализация

В следующем примере я реализовал несколько из рассмотренных выше методов.

Вывод

Вы можете попробовать реализовать остальные методы, и если вы где-то застряли, вы можете оставить комментарий, и мы поможем вам с этим.

Экземпляром какого класса является поле system err