{"id":10006635,"date":"2022-09-29T19:06:13","date_gmt":"2022-09-29T11:06:13","guid":{"rendered":"https:\/\/ityet.com\/?p=10006635"},"modified":"2022-09-29T19:06:13","modified_gmt":"2022-09-29T11:06:13","slug":"optional%e6%ba%90%e7%a0%81%e8%a7%a3%e6%9e%90%e4%b8%8e%e5%ae%9e%e8%b7%b5","status":"publish","type":"post","link":"https:\/\/itmax.cn\/?p=10006635","title":{"rendered":"Optional\u6e90\u7801\u89e3\u6790\u4e0e\u5b9e\u8df5"},"content":{"rendered":"

1 \u5bfc\u8bfb<\/h3>\n

NullPointerException\u5728\u5f00\u53d1\u8fc7\u7a0b\u4e2d\u7ecf\u5e38\u9047\u5230\uff0c\u7a0d\u6709\u4e0d\u614e\u5c0fBUG\u5c31\u51fa\u73b0\u4e86\uff0c\u5982\u679c\u907f\u514d\u8fd9\u4e2a\u95ee\u9898\u5462\uff0cOptional\u5c31\u662f\u4e13\u95e8\u89e3\u51b3\u8fd9\u4e2a\u95ee\u9898\u7684\u7c7b\uff0c\u90a3\u4e48Optional\u5982\u4f55\u4f7f\u7528\u5462\uff1f\u8ba9\u6211\u4eec\u4e00\u8d77\u63a2\u7d22\u4e00\u4e0b\u5427\uff01<\/p>\n

2 \u6e90\u7801\u89e3\u6790<\/h3>\n

2.1 Optional\u5b9a\u4e49<\/h4>\n

Optional\u7c7b\u662fJava8\u4e3a\u4e86\u89e3\u51b3null\u503c\u5224\u65ad\u95ee\u9898\u800c\u521b\u5efa\u7684\u5bb9\u5668\u7c7b\uff0c\u5728java.util \u4e0b,\u4f7f\u7528Optional\u7c7b\u53ef\u4ee5\u907f\u514d\u663e\u5f0f\u7684null\u503c\u5224\u65ad\uff0c\u907f\u514dnull\u5bfc\u81f4\u7684NullPointerException\u3002\u9996\u5148\uff0cOptional\u662f\u4e00\u4e2a\u5bb9\u5668\uff0c\u5b83\u53ef\u4ee5\u4fdd\u5b58\u7c7b\u578bT\u7684\u503c\uff0c\u4e5f\u53ef\u4ee5\u4e3anull\u7684\u5bb9\u5668\u5bf9\u8c61\u3002Optional\u5bb9\u5668\u53ea\u80fd\u5b58\u4e00\u4e2a\u503c\u3002<\/p>\n

2.2 Optional\u7684\u5c5e\u6027<\/h4>\n

1\uff09\u6e90\u7801\uff1a<\/p>\n

\/**\n * Common instance for {@code empty()}.\n *\/\nprivate static final Optional<?> EMPTY = new Optional<>();\n \n \n\/**\n * If non-null, the value; if null, indicates no value is present\n *\/\nprivate final T value;<\/code><\/pre>\n
\u6839\u636e\u6e90\u7801\u53ef\u4ee5\u770b\u5230Optional\u6709\u4e24\u4e2a\u5c5e\u6027\uff0c\u4e00\u4e2a\u662f\u4e3a\u7a7a\u503c\u51c6\u5907\u7684EMPTY\u548c\u6cdb\u578b\u503cvalue;<\/p>\n
2.3 Optional\u7684\u65b9\u6cd5<\/h4>\n
Optional\u9664toString()\u3001hashCode() \u3001equals()\u7b49Object\u7684\u65b9\u6cd5\u5916\uff0c\u8fd8\u5305\u542b\u4ee5\u4e0b\u65b9\u6cd5\u3002<\/p>\n
2.3.1 \u79c1\u6709\u6784\u9020\u65b9\u6cd5<\/strong><\/p>\n
\/**\n * Constructs an empty instance.\n *\n * @implNote Generally only one empty instance, {@link Optional#EMPTY},\n * should exist per VM.\n *\/\nprivate Optional() {\n    this.value = null;\n}\n \n \n\/**\n* Constructs an instance with the value present.\n*\n* @param value the non-null value to be present\n* @throws NullPointerException if value is null\n*\/\nprivate Optional(T value) {\n    this.value = Objects.requireNonNull(value);\n}<\/code><\/pre>\n
\u5206\u522b\u662f\u521b\u5efa\u4e00\u4e2a\u7a7a\u5b9e\u4f8b\u548c\u6784\u9020\u4e00\u4e2a\u5177\u6709\u5f53\u524d\u503c\u7684\u5b9e\u4f8b\u3002<\/p>\n
2.3.2 \u521b\u5efa\u65b9\u6cd5<\/strong>
1\uff09\u6e90\u7801<\/p>\n
\npublic static<T> Optional<T> empty() {\n    @SuppressWarnings("unchecked")\n    Optional<T> t = (Optional<T>) EMPTY;\n    return t;\n}\npublic static <T> Optional<T> of(T value) {\n     return new Optional<>(value);\n}\npublic static <T> Optional<T> ofNullable(T value) {\n     return value == null ? empty() : of(value);\n}<\/code><\/pre>\n
2\uff09\u65b9\u6cd5\u8bf4\u660e<\/p>\n
    \n
  • empty(\uff09: \u521b\u5efa\u4e00\u4e2a\u7a7a\u7684 Optional \u5b9e\u4f8b<\/li>\n
  • of(T t) : \u521b\u5efa\u4e00\u4e2a Optional \u5b9e\u4f8b\uff0c\u5f53 t\u4e3anull\u65f6\u629b\u51fa\u5f02\u5e38<\/li>\n
  • ofNullable(T t): \u521b\u5efa\u4e00\u4e2a Optional \u5b9e\u4f8b\uff0c\u4f46\u5f53 t\u4e3anull\u65f6\u4e0d\u4f1a\u629b\u51fa\u5f02\u5e38\uff0c\u800c\u662f\u8fd4\u56de\u4e00\u4e2a\u7a7a\u7684\u5b9e\u4f8b<\/li>\n<\/ul>\n
    3\uff09\u6d4b\u8bd5\u4ee3\u7801<\/p>\n
    \npublic static void main(String[] args) {\n    Integer value1 = null;\n    Integer value2 = 1;\n    try {\n        Optional<Integer> optional1 = Optional.empty();\n        System.out.println("optional1\u521b\u5efa\u4e86");\n    }catch (Exception e){\n        System.out.println("optional1\u5931\u8d25\u4e86");\n    }\n    try {\n        Optional<Integer> optional2 = Optional.of(value1);\n        System.out.println("optional2\u521b\u5efa\u4e86");\n    }catch (Exception e){\n        System.out.println("optional2\u5931\u8d25\u4e86");\n    }\n    try {\n        Optional<Integer> optional3 = Optional.ofNullable(value1);\n        System.out.println("optional3\u521b\u5efa\u4e86");\n    }catch (Exception e){\n        System.out.println("optional3\u5931\u8d25\u4e86");\n    }\n    try {\n        Optional<Integer> optional4 = Optional.of(value2);\n        System.out.println("optional4\u521b\u5efa\u4e86");\n    }catch (Exception e){\n        System.out.println("optional4\u5931\u8d25\u4e86");\n    }\n    try {\n        Optional<Integer> optional5 = Optional.ofNullable(value2);\n        System.out.println("optional5\u521b\u5efa\u4e86");\n    }catch (Exception e){\n        System.out.println("optional5\u5931\u8d25\u4e86");\n    }\n}\n<\/code><\/pre>\n
    4\uff09\u8fd0\u884c\u7ed3\u679c
    \"image.png\"<\/p>\n
    2.3.3 \u503c\u83b7\u53d6\u65b9\u6cd5<\/strong>
    1\uff09\u6e90\u7801<\/p>\n
    \npublic T get() {\n    if (value == null) {\n        throw new NoSuchElementException("No value present");\n    }\n    return value;\n}<\/code><\/pre>\n
    2\uff09\u65b9\u6cd5\u8bf4\u660e<\/p>\n
    get(): \u5982\u679cOptional\u4e0d\u4e3a\u7a7a\uff0c\u5219\u8fd4\u56de\u8be5Optional\u5bb9\u5668\u4e2d\u7684\u503c\uff0c\u5426\u5219\u629b\u51faNoSuchElementExceptio \u3002<\/p>\n
    3\uff09\u6d4b\u8bd5\u4ee3\u7801<\/p>\n
    \npublic static void main(String[] args) {\n    Integer value1 = null;\n    Integer value2 = 1;\n    Optional<Integer> optional1 = Optional.ofNullable(value1);\n    Optional<Integer> optional2 = Optional.of(value2);\n    try {\n        Integer result=optional1.get();\n        System.out.println("optional1\u7684\u503c\u662f\uff1a"+result);\n    }catch (Exception e){\n        System.out.println("optional1\u7684\u503c\u83b7\u53d6\u5931\u8d25,\u539f\u56e0:"+e.getMessage());\n    }\n    try {\n        Integer result=optional2.get();\n        System.out.println("optional2\u7684\u503c\u662f\uff1a"+result);\n    }catch (Exception e){\n        System.out.println("optional2\u7684\u503c\u83b7\u53d6\u5931\u8d25,\u539f\u56e0:"+e.getMessage());\n    }\n}\n<\/code><\/pre>\n
    4\uff09\u8fd0\u884c\u7ed3\u679c
    \"image.png\"<\/p>\n
    2.3.4 \u5224\u65ad\u65b9\u6cd5<\/strong>
    1\uff09\u6e90\u7801<\/p>\n
    \npublic boolean isPresent() {\n    return value != null;\n}\npublic void ifPresent(Consumer<? super T> consumer) {\n    if (value != null)\n        consumer.accept(value);\n}\npublic T orElse(T other) {\n    return value != null ? value : other;\n }   \npublic T orElseGet(Supplier<? extends T> other) {\n     return value != null ? value : other.get();\n}\npublic <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {\n   if (value != null) {\n      return value;\n   } else {\n      throw exceptionSupplier.get();\n   }\n}\n<\/code><\/pre>\n
    2\uff09\u65b9\u6cd5\u8bf4\u660e<\/p>\n
      \n
    • isPresent(): \u5224\u65adoptional\u662f\u5426\u4e3a\u7a7a\uff0c\u5982\u679c\u7a7a\u5219\u8fd4\u56defalse\uff0c\u5426\u5219\u8fd4\u56detrue<\/li>\n
    • ifPresent(Consumer c): \u5982\u679coptional\u4e0d\u4e3a\u7a7a\uff0c\u5219\u5c06optional\u4e2d\u7684\u5bf9\u8c61\u4f20\u7ed9Comsumer\u51fd\u6570<\/li>\n
    • orElse(T other): \u5982\u679coptional\u4e0d\u4e3a\u7a7a\uff0c\u5219\u8fd4\u56deoptional\u4e2d\u7684\u5bf9\u8c61\uff1b\u5982\u679c\u4e3anull\uff0c\u5219\u8fd4\u56de other \u8fd9\u4e2a\u5bf9\u8c61\u3002<\/li>\n
    • orElseGet(Supplier other): \u5982\u679coptional\u4e0d\u4e3a\u7a7a\uff0c\u5219\u8fd4\u56deoptional\u4e2d\u7684\u5bf9\u8c61\uff1b\u5982\u679c\u4e3anull\uff0c\u5426\u5219\u8c03\u7528\u5176\u4ed6\u51fd\u6570\u5e76\u8fd4\u56de\u8c03\u7528\u7684\u7ed3\u679c<\/li>\n
    • orElseThrow(Supplier exception): \u5982\u679coptional\u4e0d\u4e3a\u7a7a\uff0c\u5219\u8fd4\u56deoptional\u4e2d\u7684\u5bf9\u8c61\uff1b\u5982\u679c\u4e3anull\uff0c\u5219\u629b\u51faSupplier\u51fd\u6570\u751f\u6210\u7684\u5f02\u5e38<\/li>\n<\/ul>\n
      3\uff09\u6d4b\u8bd5\u4ee3\u7801<\/p>\n
      \npublic static void main(String[] args) {\n    Integer value1 = null;\n    Integer value2 = 1;\n    Optional<Integer> optional1 = Optional.ofNullable(value1);\n    Optional<Integer> optional2 = Optional.of(value2);\n    try {\n        if(optional1.isPresent()){\n            System.out.println("optional1\u7684isPresent\u7ed3\u679c\u4e0d\u4e3a\u7a7a");\n        }else{\n            System.out.println("optional1\u7684isPresent\u7ed3\u679c\u4e3a\u7a7a");\n        }\n    }catch (Exception e){\n        System.out.println("optional1\u7684isPresent\u5224\u7a7a\u5931\u8d25,\u539f\u56e0:"+e.getMessage());\n    }\n    try {\n        if(optional2.isPresent()){\n            System.out.println("optional2\u7684isPresent\u7ed3\u679c\u4e0d\u4e3a\u7a7a");\n        }else{\n            System.out.println("optional2\u7684isPresent\u7ed3\u679c\u4e3a\u7a7a");\n        }\n    }catch (Exception e){\n        System.out.println("optional2\u7684isPresent\u5224\u7a7a\u5931\u8d25,\u539f\u56e0:"+e.getMessage());\n    }\n \n \n    optional1.ifPresent(t->{\n        int i =t+1;\n        System.out.println("optional1\u5904\u7406\u540e\u7684\u503c\u662f"+i);\n    });\n    optional2.ifPresent(t->{\n        int i =t+1;\n        System.out.println("optional2\u5904\u7406\u540e\u7684\u503c\u662f"+i);});\n \n \n    Integer value3 = 2;\n    Integer result = optional1.orElse(value3);\n    System.out.println("optional1\u6267\u884corElse\u5904\u7406\u540e\u7684\u503c\u662f"+result);\n \n \n    result = optional2.orElse(value3);\n    System.out.println("optional2\u6267\u884corElse\u5904\u7406\u540e\u7684\u503c\u662f"+result);\n \n \n    result = optional1.orElseGet(()-> new Integer(-1));\n    System.out.println("optional1\u6267\u884corElseGet\u5904\u7406\u540e\u7684\u503c\u662f"+result);\n \n \n    result = optional2.orElseGet(()-> new Integer(-1));\n    System.out.println("optional2\u6267\u884corElseGet\u5904\u7406\u540e\u7684\u503c\u662f"+result);\n    try {\n    result = optional1.orElseThrow (()-> new RuntimeException("\u503c\u662f\u7a7a\u7684"));\n    System.out.println("optional1\u6267\u884corElseThrow\u5904\u7406\u540e\u7684\u503c\u662f"+result);\n    }catch (Exception e){\n        System.out.println("optional1\u7684orElseThrow\u629b\u51fa\u5f02\u5e38:"+e.getMessage());\n    }\n    try {\n    result = optional2.orElseThrow (()-> new RuntimeException("\u503c\u662f\u7a7a\u7684"));\n    System.out.println("optional2\u6267\u884corElseThrow\u5904\u7406\u540e\u7684\u503c\u662f"+result);\n    }catch (Exception e){\n        System.out.println("optional2\u7684orElseThrow\u629b\u51fa\u5f02\u5e38:"+e.getMessage());\n<\/code><\/pre>\n
      4\uff09\u8fd0\u884c\u7ed3\u679c
      \"image.png\"<\/p>\n
      2.3.5 \u8fc7\u6ee4\u65b9\u6cd5
      1\uff09\u6e90\u7801<\/p>\n
      \npublic Optional<T> filter(Predicate<? super T> predicate) {\n    Objects.requireNonNull(predicate);\n    if (!isPresent())\n        return this;\n    else\n        return predicate.test(value) ? this : empty();\n}<\/code><\/pre>\n
      2\uff09\u65b9\u6cd5\u8bf4\u660e<\/p>\n
      filter(Predicate p): \u5982\u679coptional\u4e0d\u4e3a\u7a7a\uff0c\u5219\u6267\u884cPredicate p\uff0c\u5982\u679cp\u7684\u7ed3\u679c\u4e3atrue\uff0c\u5219\u8fd4\u56de\u539f\u672c\u7684optional\uff0c\u5426\u5219\u8fd4\u56de\u7a7a\u7684optional<\/p>\n
      3\uff09\u6d4b\u8bd5\u4ee3\u7801<\/p>\n
      \npublic static void main(String[] args) {\n    Integer value1 = 5;\n    Integer value2 = 6;\n    Optional<Integer> optional1 = Optional.ofNullable(value1);\n    Optional<Integer> optional2 = Optional.of(value2);\n \n \n    Optional<Integer> result =optional1.filter(t->t > 5);\n    System.out.println("optional1\u7684filter\u540e\u7684\u503c:"+result);\n     result =optional2.filter(t->t > 5);\n    System.out.println("optional2\u7684filter\u540e\u7684\u503c:"+result);<\/code><\/pre>\n
      4\uff09\u8fd0\u884c\u7ed3\u679c
      \"image.png\"<\/p>\n
      2.3.6 \u6620\u5c04\u65b9\u6cd5<\/strong>
      1\uff09\u6e90\u7801<\/p>\n
      \npublic<U> Optional<U> map(Function<? super T, ? extends U> mapper) {\n    Objects.requireNonNull(mapper);\n    if (!isPresent())\n        return empty();\n    else {\n        return Optional.ofNullable(mapper.apply(value));\n    }\n}\npublic<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) {\n    Objects.requireNonNull(mapper);\n    if (!isPresent())\n        return empty();\n    else {\n        return Objects.requireNonNull(mapper.apply(value));\n    }\n}\n<\/code><\/pre>\n
      2\uff09\u65b9\u6cd5\u8bf4\u660e<\/p>\n