Java 8-可选类 - 芒果文档

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📜 Java 8-可选类

📅 最后修改于: 2021-01-05 04:40:29 🧑 作者: Mango

可选的是用于包含非空对象的容器对象。可选对象用于表示缺少值的null。此类具有各种实用程序方法，以方便代码将值处理为“可用”或“不可用”，而不是检查空值。它是Java 8中引入的，类似于Guava中的Optional。

类声明

以下是java.util.Optional 类的声明-

public final class Optional extends Object

类方法

Sr.No.	Method & Description
1	static Optional empty() Returns an empty Optional instance.
2	boolean equals(Object obj) Indicates whether some other object is “equal to” this Optional.
3	Optional filter(Predicate predicate) If a value is present and the value matches a given predicate, it returns an Optional describing the value, otherwise returns an empty Optional.
4	Optional flatMap(Function> mapper) If a value is present, it applies the provided Optional-bearing mapping function to it, returns that result, otherwise returns an empty Optional.
5	T get() If a value is present in this Optional, returns the value, otherwise throws NoSuchElementException.
6	int hashCode() Returns the hash code value of the present value, if any, or 0 (zero) if no value is present.
7	void ifPresent(Consumer consumer) If a value is present, it invokes the specified consumer with the value, otherwise does nothing.
8	boolean isPresent() Returns true if there is a value present, otherwise false.
9	Optional map(Function mapper) If a value is present, applies the provided mapping function to it, and if the result is non-null, returns an Optional describing the result.
10	static Optional of(T value) Returns an Optional with the specified present non-null value.
11	static Optional ofNullable(T value) Returns an Optional describing the specified value, if non-null, otherwise returns an empty Optional.
12	T orElse(T other) Returns the value if present, otherwise returns other.
13	T orElseGet(Supplier other) Returns the value if present, otherwise invokes other and returns the result of that invocation.
14	T orElseThrow(Supplier exceptionSupplier) Returns the contained value, if present, otherwise throws an exception to be created by the provided supplier.
15	String toString() Returns a non-empty string representation of this Optional suitable for debugging.

此类从以下类继承方法-

java.lang.Object

可选示例

使用您选择的任何编辑器(例如，C：\> JAVA)创建以下Java程序。

Java8Tester.java

现场演示

import java.util.Optional;

public class Java8Tester {

   public static void main(String args[]) {
      Java8Tester java8Tester = new Java8Tester();
      Integer value1 = null;
      Integer value2 = new Integer(10);
        
      //Optional.ofNullable - allows passed parameter to be null.
      Optional a = Optional.ofNullable(value1);
        
      //Optional.of - throws NullPointerException if passed parameter is null
      Optional b = Optional.of(value2);
      System.out.println(java8Tester.sum(a,b));
   }
    
   public Integer sum(Optional a, Optional b) {
      //Optional.isPresent - checks the value is present or not
        
      System.out.println("First parameter is present: " + a.isPresent());
      System.out.println("Second parameter is present: " + b.isPresent());
        
      //Optional.orElse - returns the value if present otherwise returns
      //the default value passed.
      Integer value1 = a.orElse(new Integer(0));
        
      //Optional.get - gets the value, value should be present
      Integer value2 = b.get();
      return value1 + value2;
   }
}

验证结果

使用javac编译器编译类，如下所示：

C:\JAVA>javac Java8Tester.java

现在如下运行Java8Tester-

C:\JAVA>java Java8Tester

它应该产生以下输出-

First parameter is present: false
Second parameter is present: true
10