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這篇文章主要介紹“如何理解Java的枚舉與注解”，在日常操作中，相信很多人在如何理解Java的枚舉與注解問題上存在疑惑，小編查閱了各式資料，整理出簡單好用的操作方法，希望對大家解答”如何理解Java的枚舉與注解”的疑惑有所幫助！接下來，請跟著小編一起來學習吧！
JDK1.5之前需要自定義枚舉類；JDK 1.5 新增的 enum 關鍵字用于定義枚舉類。若枚舉只有一個成員, 則可以作為一種單例模式的實現方式
枚舉類對象的屬性不應允許被改動, 所以應該使用 private final 修飾。
枚舉類的使用 private final 修飾的屬性應該在構造器中為其賦值。
若枚舉類顯式的定義了帶參數的構造器, 則在列出枚舉值時也必須對應的傳入參數。
必須在枚舉類的第一行聲明枚舉類對象。 JDK 1.5 中可以在 switch 表達式中使用Enum定義的枚舉類的對象作為表達式, case 子句可以直接使用枚舉值的名字, 無需添加枚舉類作為限定。
枚舉類和普通類的區別：
使用 enum 定義的枚舉類默認繼承了 java.lang.Enum 類
枚舉類的構造器只能使用 private 訪問控制符
枚舉類的所有實例必須在枚舉類中顯式列出(, 分隔 ; 結尾). 列出的實例系統會自動添加 public static final 修飾
package org.springframework.http;
import org.springframework.lang.Nullable;
/**
* Enumeration of HTTP status codes.
*
* <p>The HTTP status code series can be retrieved via {@link #series()}.
*
* @author Arjen Poutsma
* @author Sebastien Deleuze
* @author Brian Clozel
* @since 3.0
* @see HttpStatus.Series
* @see <a href="https://www.iana.org/assignments/http-status-codes">HTTP Status Code Registry</a>
* @see <a href="https://en.wikipedia.org/wiki/List_of_HTTP_status_codes">List of HTTP status codes - Wikipedia</a>
*/
public enum HttpStatus {
// 1xx Informational
/**
* {@code 100 Continue}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.2.1">HTTP/1.1: Semantics and Content, section 6.2.1</a>
*/
CONTINUE(100, "Continue"),
/**
* {@code 101 Switching Protocols}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.2.2">HTTP/1.1: Semantics and Content, section 6.2.2</a>
*/
SWITCHING_PROTOCOLS(101, "Switching Protocols"),
/**
* {@code 102 Processing}.
* @see <a href="https://tools.ietf.org/html/rfc2518#section-10.1">WebDAV</a>
*/
PROCESSING(102, "Processing"),
/**
* {@code 103 Checkpoint}.
* @see <a href="https://code.google.com/p/gears/wiki/ResumableHttpRequestsProposal">A proposal for supporting
* resumable POST/PUT HTTP requests in HTTP/1.0</a>
*/
CHECKPOINT(103, "Checkpoint"),
// 2xx Success
/**
* {@code 200 OK}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.3.1">HTTP/1.1: Semantics and Content, section 6.3.1</a>
*/
OK(200, "OK"),
/**
* {@code 201 Created}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.3.2">HTTP/1.1: Semantics and Content, section 6.3.2</a>
*/
CREATED(201, "Created"),
/**
* {@code 202 Accepted}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.3.3">HTTP/1.1: Semantics and Content, section 6.3.3</a>
*/
ACCEPTED(202, "Accepted"),
/**
* {@code 203 Non-Authoritative Information}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.3.4">HTTP/1.1: Semantics and Content, section 6.3.4</a>
*/
NON_AUTHORITATIVE_INFORMATION(203, "Non-Authoritative Information"),
/**
* {@code 204 No Content}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.3.5">HTTP/1.1: Semantics and Content, section 6.3.5</a>
*/
NO_CONTENT(204, "No Content"),
/**
* {@code 205 Reset Content}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.3.6">HTTP/1.1: Semantics and Content, section 6.3.6</a>
*/
RESET_CONTENT(205, "Reset Content"),
/**
* {@code 206 Partial Content}.
* @see <a href="https://tools.ietf.org/html/rfc7233#section-4.1">HTTP/1.1: Range Requests, section 4.1</a>
*/
PARTIAL_CONTENT(206, "Partial Content"),
/**
* {@code 207 Multi-Status}.
* @see <a href="https://tools.ietf.org/html/rfc4918#section-13">WebDAV</a>
*/
MULTI_STATUS(207, "Multi-Status"),
/**
* {@code 208 Already Reported}.
* @see <a href="https://tools.ietf.org/html/rfc5842#section-7.1">WebDAV Binding Extensions</a>
*/
ALREADY_REPORTED(208, "Already Reported"),
/**
* {@code 226 IM Used}.
* @see <a href="https://tools.ietf.org/html/rfc3229#section-10.4.1">Delta encoding in HTTP</a>
*/
IM_USED(226, "IM Used"),
// 3xx Redirection
/**
* {@code 300 Multiple Choices}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.4.1">HTTP/1.1: Semantics and Content, section 6.4.1</a>
*/
MULTIPLE_CHOICES(300, "Multiple Choices"),
/**
* {@code 301 Moved Permanently}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.4.2">HTTP/1.1: Semantics and Content, section 6.4.2</a>
*/
MOVED_PERMANENTLY(301, "Moved Permanently"),
/**
* {@code 302 Found}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.4.3">HTTP/1.1: Semantics and Content, section 6.4.3</a>
*/
FOUND(302, "Found"),
/**
* {@code 302 Moved Temporarily}.
* @see <a href="https://tools.ietf.org/html/rfc1945#section-9.3">HTTP/1.0, section 9.3</a>
* @deprecated in favor of {@link #FOUND} which will be returned from {@code HttpStatus.valueOf(302)}
*/
@Deprecated
MOVED_TEMPORARILY(302, "Moved Temporarily"),
/**
* {@code 303 See Other}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.4.4">HTTP/1.1: Semantics and Content, section 6.4.4</a>
*/
SEE_OTHER(303, "See Other"),
/**
* {@code 304 Not Modified}.
* @see <a href="https://tools.ietf.org/html/rfc7232#section-4.1">HTTP/1.1: Conditional Requests, section 4.1</a>
*/
NOT_MODIFIED(304, "Not Modified"),
/**
* {@code 305 Use Proxy}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.4.5">HTTP/1.1: Semantics and Content, section 6.4.5</a>
* @deprecated due to security concerns regarding in-band configuration of a proxy
*/
@Deprecated
USE_PROXY(305, "Use Proxy"),
/**
* {@code 307 Temporary Redirect}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.4.7">HTTP/1.1: Semantics and Content, section 6.4.7</a>
*/
TEMPORARY_REDIRECT(307, "Temporary Redirect"),
/**
* {@code 308 Permanent Redirect}.
* @see <a href="https://tools.ietf.org/html/rfc7238">RFC 7238</a>
*/
PERMANENT_REDIRECT(308, "Permanent Redirect"),
// --- 4xx Client Error ---
/**
* {@code 400 Bad Request}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.1">HTTP/1.1: Semantics and Content, section 6.5.1</a>
*/
BAD_REQUEST(400, "Bad Request"),
/**
* {@code 401 Unauthorized}.
* @see <a href="https://tools.ietf.org/html/rfc7235#section-3.1">HTTP/1.1: Authentication, section 3.1</a>
*/
UNAUTHORIZED(401, "Unauthorized"),
/**
* {@code 402 Payment Required}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.2">HTTP/1.1: Semantics and Content, section 6.5.2</a>
*/
PAYMENT_REQUIRED(402, "Payment Required"),
/**
* {@code 403 Forbidden}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.3">HTTP/1.1: Semantics and Content, section 6.5.3</a>
*/
FORBIDDEN(403, "Forbidden"),
/**
* {@code 404 Not Found}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.4">HTTP/1.1: Semantics and Content, section 6.5.4</a>
*/
NOT_FOUND(404, "Not Found"),
/**
* {@code 405 Method Not Allowed}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.5">HTTP/1.1: Semantics and Content, section 6.5.5</a>
*/
METHOD_NOT_ALLOWED(405, "Method Not Allowed"),
/**
* {@code 406 Not Acceptable}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.6">HTTP/1.1: Semantics and Content, section 6.5.6</a>
*/
NOT_ACCEPTABLE(406, "Not Acceptable"),
/**
* {@code 407 Proxy Authentication Required}.
* @see <a href="https://tools.ietf.org/html/rfc7235#section-3.2">HTTP/1.1: Authentication, section 3.2</a>
*/
PROXY_AUTHENTICATION_REQUIRED(407, "Proxy Authentication Required"),
/**
* {@code 408 Request Timeout}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.7">HTTP/1.1: Semantics and Content, section 6.5.7</a>
*/
REQUEST_TIMEOUT(408, "Request Timeout"),
/**
* {@code 409 Conflict}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.8">HTTP/1.1: Semantics and Content, section 6.5.8</a>
*/
CONFLICT(409, "Conflict"),
/**
* {@code 410 Gone}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.9">
* HTTP/1.1: Semantics and Content, section 6.5.9</a>
*/
GONE(410, "Gone"),
/**
* {@code 411 Length Required}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.10">
* HTTP/1.1: Semantics and Content, section 6.5.10</a>
*/
LENGTH_REQUIRED(411, "Length Required"),
/**
* {@code 412 Precondition failed}.
* @see <a href="https://tools.ietf.org/html/rfc7232#section-4.2">
* HTTP/1.1: Conditional Requests, section 4.2</a>
*/
PRECONDITION_FAILED(412, "Precondition Failed"),
/**
* {@code 413 Payload Too Large}.
* @since 4.1
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.11">
* HTTP/1.1: Semantics and Content, section 6.5.11</a>
*/
PAYLOAD_TOO_LARGE(413, "Payload Too Large"),
/**
* {@code 413 Request Entity Too Large}.
* @see <a href="https://tools.ietf.org/html/rfc2616#section-10.4.14">HTTP/1.1, section 10.4.14</a>
* @deprecated in favor of {@link #PAYLOAD_TOO_LARGE} which will be
* returned from {@code HttpStatus.valueOf(413)}
*/
@Deprecated
REQUEST_ENTITY_TOO_LARGE(413, "Request Entity Too Large"),
/**
* {@code 414 URI Too Long}.
* @since 4.1
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.12">
* HTTP/1.1: Semantics and Content, section 6.5.12</a>
*/
URI_TOO_LONG(414, "URI Too Long"),
/**
* {@code 414 Request-URI Too Long}.
* @see <a href="https://tools.ietf.org/html/rfc2616#section-10.4.15">HTTP/1.1, section 10.4.15</a>
* @deprecated in favor of {@link #URI_TOO_LONG} which will be returned from {@code HttpStatus.valueOf(414)}
*/
@Deprecated
REQUEST_URI_TOO_LONG(414, "Request-URI Too Long"),
/**
* {@code 415 Unsupported Media Type}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.13">
* HTTP/1.1: Semantics and Content, section 6.5.13</a>
*/
UNSUPPORTED_MEDIA_TYPE(415, "Unsupported Media Type"),
/**
* {@code 416 Requested Range Not Satisfiable}.
* @see <a href="https://tools.ietf.org/html/rfc7233#section-4.4">HTTP/1.1: Range Requests, section 4.4</a>
*/
REQUESTED_RANGE_NOT_SATISFIABLE(416, "Requested range not satisfiable"),
/**
* {@code 417 Expectation Failed}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.5.14">
* HTTP/1.1: Semantics and Content, section 6.5.14</a>
*/
EXPECTATION_FAILED(417, "Expectation Failed"),
/**
* {@code 418 I'm a teapot}.
* @see <a href="https://tools.ietf.org/html/rfc2324#section-2.3.2">HTCPCP/1.0</a>
*/
I_AM_A_TEAPOT(418, "I'm a teapot"),
/**
* @deprecated See
* <a href="https://tools.ietf.org/rfcdiff?difftype=--hwdiff&url2=draft-ietf-webdav-protocol-06.txt">
* WebDAV Draft Changes</a>
*/
@Deprecated
INSUFFICIENT_SPACE_ON_RESOURCE(419, "Insufficient Space On Resource"),
/**
* @deprecated See
* <a href="https://tools.ietf.org/rfcdiff?difftype=--hwdiff&url2=draft-ietf-webdav-protocol-06.txt">
* WebDAV Draft Changes</a>
*/
@Deprecated
METHOD_FAILURE(420, "Method Failure"),
/**
* @deprecated
* See <a href="https://tools.ietf.org/rfcdiff?difftype=--hwdiff&url2=draft-ietf-webdav-protocol-06.txt">
* WebDAV Draft Changes</a>
*/
@Deprecated
DESTINATION_LOCKED(421, "Destination Locked"),
/**
* {@code 422 Unprocessable Entity}.
* @see <a href="https://tools.ietf.org/html/rfc4918#section-11.2">WebDAV</a>
*/
UNPROCESSABLE_ENTITY(422, "Unprocessable Entity"),
/**
* {@code 423 Locked}.
* @see <a href="https://tools.ietf.org/html/rfc4918#section-11.3">WebDAV</a>
*/
LOCKED(423, "Locked"),
/**
* {@code 424 Failed Dependency}.
* @see <a href="https://tools.ietf.org/html/rfc4918#section-11.4">WebDAV</a>
*/
FAILED_DEPENDENCY(424, "Failed Dependency"),
/**
* {@code 425 Too Early}.
* @since 5.2
* @see <a href="https://tools.ietf.org/html/rfc8470">RFC 8470</a>
*/
TOO_EARLY(425, "Too Early"),
/**
* {@code 426 Upgrade Required}.
* @see <a href="https://tools.ietf.org/html/rfc2817#section-6">Upgrading to TLS Within HTTP/1.1</a>
*/
UPGRADE_REQUIRED(426, "Upgrade Required"),
/**
* {@code 428 Precondition Required}.
* @see <a href="https://tools.ietf.org/html/rfc6585#section-3">Additional HTTP Status Codes</a>
*/
PRECONDITION_REQUIRED(428, "Precondition Required"),
/**
* {@code 429 Too Many Requests}.
* @see <a href="https://tools.ietf.org/html/rfc6585#section-4">Additional HTTP Status Codes</a>
*/
TOO_MANY_REQUESTS(429, "Too Many Requests"),
/**
* {@code 431 Request Header Fields Too Large}.
* @see <a href="https://tools.ietf.org/html/rfc6585#section-5">Additional HTTP Status Codes</a>
*/
REQUEST_HEADER_FIELDS_TOO_LARGE(431, "Request Header Fields Too Large"),
/**
* {@code 451 Unavailable For Legal Reasons}.
* @see <a href="https://tools.ietf.org/html/draft-ietf-httpbis-legally-restricted-status-04">
* An HTTP Status Code to Report Legal Obstacles</a>
* @since 4.3
*/
UNAVAILABLE_FOR_LEGAL_REASONS(451, "Unavailable For Legal Reasons"),
// --- 5xx Server Error ---
/**
* {@code 500 Internal Server Error}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.6.1">HTTP/1.1: Semantics and Content, section 6.6.1</a>
*/
INTERNAL_SERVER_ERROR(500, "Internal Server Error"),
/**
* {@code 501 Not Implemented}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.6.2">HTTP/1.1: Semantics and Content, section 6.6.2</a>
*/
NOT_IMPLEMENTED(501, "Not Implemented"),
/**
* {@code 502 Bad Gateway}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.6.3">HTTP/1.1: Semantics and Content, section 6.6.3</a>
*/
BAD_GATEWAY(502, "Bad Gateway"),
/**
* {@code 503 Service Unavailable}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.6.4">HTTP/1.1: Semantics and Content, section 6.6.4</a>
*/
SERVICE_UNAVAILABLE(503, "Service Unavailable"),
/**
* {@code 504 Gateway Timeout}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.6.5">HTTP/1.1: Semantics and Content, section 6.6.5</a>
*/
GATEWAY_TIMEOUT(504, "Gateway Timeout"),
/**
* {@code 505 HTTP Version Not Supported}.
* @see <a href="https://tools.ietf.org/html/rfc7231#section-6.6.6">HTTP/1.1: Semantics and Content, section 6.6.6</a>
*/
HTTP_VERSION_NOT_SUPPORTED(505, "HTTP Version not supported"),
/**
* {@code 506 Variant Also Negotiates}
* @see <a href="https://tools.ietf.org/html/rfc2295#section-8.1">Transparent Content Negotiation</a>
*/
VARIANT_ALSO_NEGOTIATES(506, "Variant Also Negotiates"),
/**
* {@code 507 Insufficient Storage}
* @see <a href="https://tools.ietf.org/html/rfc4918#section-11.5">WebDAV</a>
*/
INSUFFICIENT_STORAGE(507, "Insufficient Storage"),
/**
* {@code 508 Loop Detected}
* @see <a href="https://tools.ietf.org/html/rfc5842#section-7.2">WebDAV Binding Extensions</a>
*/
LOOP_DETECTED(508, "Loop Detected"),
/**
* {@code 509 Bandwidth Limit Exceeded}
*/
BANDWIDTH_LIMIT_EXCEEDED(509, "Bandwidth Limit Exceeded"),
/**
* {@code 510 Not Extended}
* @see <a href="https://tools.ietf.org/html/rfc2774#section-7">HTTP Extension Framework</a>
*/
NOT_EXTENDED(510, "Not Extended"),
/**
* {@code 511 Network Authentication Required}.
* @see <a href="https://tools.ietf.org/html/rfc6585#section-6">Additional HTTP Status Codes</a>
*/
NETWORK_AUTHENTICATION_REQUIRED(511, "Network Authentication Required");
private final int value;
private final String reasonPhrase;
HttpStatus(int value, String reasonPhrase) {
this.value = value;
this.reasonPhrase = reasonPhrase;
}
/**
* Return the integer value of this status code.
*/
public int value() {
return this.value;
}
/**
* Return the reason phrase of this status code.
*/
public String getReasonPhrase() {
return this.reasonPhrase;
}
/**
* Return the HTTP status series of this status code.
* @see HttpStatus.Series
*/
public Series series() {
return Series.valueOf(this);
}
/**
* Whether this status code is in the HTTP series
* {@link org.springframework.http.HttpStatus.Series#INFORMATIONAL}.
* This is a shortcut for checking the value of {@link #series()}.
* @since 4.0
* @see #series()
*/
public boolean is1xxInformational() {
return (series() == Series.INFORMATIONAL);
}
/**
* Whether this status code is in the HTTP series
* {@link org.springframework.http.HttpStatus.Series#SUCCESSFUL}.
* This is a shortcut for checking the value of {@link #series()}.
* @since 4.0
* @see #series()
*/
public boolean is2xxSuccessful() {
return (series() == Series.SUCCESSFUL);
}
/**
* Whether this status code is in the HTTP series
* {@link org.springframework.http.HttpStatus.Series#REDIRECTION}.
* This is a shortcut for checking the value of {@link #series()}.
* @since 4.0
* @see #series()
*/
public boolean is3xxRedirection() {
return (series() == Series.REDIRECTION);
}
/**
* Whether this status code is in the HTTP series
* {@link org.springframework.http.HttpStatus.Series#CLIENT_ERROR}.
* This is a shortcut for checking the value of {@link #series()}.
* @since 4.0
* @see #series()
*/
public boolean is4xxClientError() {
return (series() == Series.CLIENT_ERROR);
}
/**
* Whether this status code is in the HTTP series
* {@link org.springframework.http.HttpStatus.Series#SERVER_ERROR}.
* This is a shortcut for checking the value of {@link #series()}.
* @since 4.0
* @see #series()
*/
public boolean is5xxServerError() {
return (series() == Series.SERVER_ERROR);
}
/**
* Whether this status code is in the HTTP series
* {@link org.springframework.http.HttpStatus.Series#CLIENT_ERROR} or
* {@link org.springframework.http.HttpStatus.Series#SERVER_ERROR}.
* This is a shortcut for checking the value of {@link #series()}.
* @since 5.0
* @see #is4xxClientError()
* @see #is5xxServerError()
*/
public boolean isError() {
return (is4xxClientError() || is5xxServerError());
}
/**
* Return a string representation of this status code.
*/
@Override
public String toString() {
return this.value + " " + name();
}
/**
* Return the enum constant of this type with the specified numeric value.
* @param statusCode the numeric value of the enum to be returned
* @return the enum constant with the specified numeric value
* @throws IllegalArgumentException if this enum has no constant for the specified numeric value
*/
public static HttpStatus valueOf(int statusCode) {
HttpStatus status = resolve(statusCode);
if (status == null) {
throw new IllegalArgumentException("No matching constant for [" + statusCode + "]");
}
return status;
}
/**
* Resolve the given status code to an {@code HttpStatus}, if possible.
* @param statusCode the HTTP status code (potentially non-standard)
* @return the corresponding {@code HttpStatus}, or {@code null} if not found
* @since 5.0
*/
@Nullable
public static HttpStatus resolve(int statusCode) {
for (HttpStatus status : values()) {
if (status.value == statusCode) {
return status;
}
}
return null;
}
/**
* Enumeration of HTTP status series.
* <p>Retrievable via {@link HttpStatus#series()}.
*/
public enum Series {
INFORMATIONAL(1),
SUCCESSFUL(2),
REDIRECTION(3),
CLIENT_ERROR(4),
SERVER_ERROR(5);
private final int value;
Series(int value) {
this.value = value;
}
/**
* Return the integer value of this status series. Ranges from 1 to 5.
*/
public int value() {
return this.value;
}
/**
* Return the enum constant of this type with the corresponding series.
* @param status a standard HTTP status enum value
* @return the enum constant of this type with the corresponding series
* @throws IllegalArgumentException if this enum has no corresponding constant
*/
public static Series valueOf(HttpStatus status) {
return valueOf(status.value);
}
/**
* Return the enum constant of this type with the corresponding series.
* @param statusCode the HTTP status code (potentially non-standard)
* @return the enum constant of this type with the corresponding series
* @throws IllegalArgumentException if this enum has no corresponding constant
*/
public static Series valueOf(int statusCode) {
Series series = resolve(statusCode);
if (series == null) {
throw new IllegalArgumentException("No matching constant for [" + statusCode + "]");
}
return series;
}
/**
* Resolve the given status code to an {@code HttpStatus.Series}, if possible.
* @param statusCode the HTTP status code (potentially non-standard)
* @return the corresponding {@code Series}, or {@code null} if not found
* @since 5.1.3
*/
@Nullable
public static Series resolve(int statusCode) {
int seriesCode = statusCode / 100;
for (Series series : values()) {
if (series.value == seriesCode) {
return series;
}
}
return null;
}
}
} 枚舉類的主要方法：
values()方法：返回枚舉類型的對象數組。該方法可以很方便地遍歷所有的枚舉值。
valueOf(String str)：可以把一個字符串轉為對應的枚舉類對象。要求字符串必須是枚舉類對象的“名字”。如不是，會有運行時異常。
和普通 Java 類一樣，枚舉類可以實現一個或多個接口。若需要每個枚舉值在調用實現的接口方法呈現出不同的行為方式, 則可以讓每個枚舉值分別來實現該方法。
從 JDK 5.0 開始, Java 增加了對元數據(MetaData) 的支持, 也就是 Annotation(注解)。Annotation 其實就是代碼里的特殊標記, 這些標記可以在編譯, 類加載, 運行時被讀取, 并執行相應的處理. 通過使用 Annotation, 程序員可以在不改變原有邏輯的情況下, 在源文件中嵌入一些補充信息。Annotation 可以像修飾符一樣被使用, 可用于修飾包,類, 構造器, 方法, 成員變量, 參數, 局部變量的聲明, 這些信息被保存在 Annotation 的 “name=value” 對中。Annotation 能被用來為程序元素(類, 方法, 成員變量等) 設置元數據。
使用 Annotation 時要在其前面增加 @ 符號, 并把該 Annotation 當成一個修飾符使用。用于修飾它支持的程序元素
三個基本的 Annotation:
@Override: 限定重寫父類方法, 該注釋只能用于方法
@Deprecated: 用于表示某個程序元素(類, 方法等)已過時
@SuppressWarnings: 抑制編譯器警告
定義新的 Annotation 類型使用 @interface 關鍵字。
Annotation 的成員變量在 Annotation 定義中以無參數方法的形式來聲明. 其方法名和返回值定義了該成員的名字和類型。
可以在定義 Annotation 的成員變量時為其指定初始值, 指定成員變量的初始值可使用 default 關鍵字
public @interface MyAnnotation{
String name() default "相逢何必曾相識";
}
沒有成員定義的 Annotation 稱為標記; 包含成員變量的 Annotation 稱為元數據 Annotation。
JDK 5.0 在 java.lang.reflect 包下新增了 AnnotatedElement 接口, 該接口代表程序中可以接受注解的程序元素。
當一個 Annotation 類型被定義為運行時 Annotation 后, 該注釋才是運行時可見, 當 class 文件被載入時保存在 class 文件中的 Annotation 才會被虛擬機讀取。
程序可以調用 AnnotationElement 對象的如下方法來訪問 Annotation 信息。
JDK 的元 Annotation 用于修飾其他 Annotation 定義；JDK5.0提供了專門在注解上的注解類型，分別是：
Retention
Target
Documented
Inherited
@Retention: 只能用于修飾一個 Annotation 定義, 用于指定該 Annotation 可以保留多長時間, @Rentention 包含一個 RetentionPolicy 類型的成員變量, 使用 @Rentention 時必須為該 value 成員變量指定值:
RetentionPolicy.SOURCE: 編譯器直接丟棄這種策略的注釋
RetentionPolicy.CLASS: 編譯器將把注釋記錄在 class 文件中. 當運行 Java 程序時, JVM 不會保留注解。 這是默認值
RetentionPolicy.RUNTIME:編譯器將把注釋記錄在 class 文件中. 當運行 Java 程序時, JVM 會保留注釋. 程序可以通過反射獲取該注釋
@Target: 用于修飾 Annotation 定義, 用于指定被修飾的 Annotation 能用于修飾哪些程序元素. @Target 也包含一個名為 value 的成員變量.
@Documented: 用于指定被該元 Annotation 修飾的 Annotation 類將被 javadoc 工具提取成文檔.
定義為Documented的注解必須設置Retention值為RUNTIME。
@Inherited: 被它修飾的 Annotation 將具有繼承性.如果某個類使用了被 @Inherited 修飾的 Annotation, 則其子類將自動具有該注解
到此，關于“如何理解Java的枚舉與注解”的學習就結束了，希望能夠解決大家的疑惑。理論與實踐的搭配能更好的幫助大家學習，快去試試吧！若想繼續學習更多相關知識，請繼續關注億速云網站，小編會繼續努力為大家帶來更多實用的文章！
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