Guava-选修课 - 芒果文档

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📜 Guava-选修课

📅 最后修改于: 2020-11-16 06:50:46 🧑 作者: Mango

可选的是不可变对象，用于包含非空对象。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，以方便代码处理可用值或不可用值，而不是检查空值。

类声明

以下是com.google.common.base.Optional 类的声明-

@GwtCompatible(serializable = true)
public abstract class Optional
   extends Object
      implements Serializable

类方法

Sr.No	Method & Description
1	static Optionalabsent() Returns an Optional instance with no contained reference.
2	abstract Set asSet() Returns an immutable singleton Set whose only element is the contained instance if it is present; an empty immutable Set otherwise.
3	abstract boolean equals(Object object) Returns true if object is an Optional instance, and either the contained references are equal to each other or both are absent.
4	static OptionalfromNullable(T nullableReference) If nullableReference is non-null, returns an Optional instance containing that reference; otherwise returns absent().
5	abstract Tget() Returns the contained instance, which must be present.
6	abstract int hashCode() Returns a hash code for this instance.
7	abstract boolean isPresent() Returns true if this holder contains a (non-null) instance.
8	static Optionalof(T reference) Returns an Optional instance containing the given non-null reference.
9	abstract Optional or(Optional secondChoice) Returns this Optional if it has a value present; secondChoice otherwise.
10	abstract T or(Supplier supplier) Returns the contained instance if it is present; supplier.get() otherwise.
11	abstract T or(T defaultValue) Returns the contained instance if it is present; defaultValue otherwise.
12	abstract T orNull() Returns the contained instance if it is present; null otherwise.
13	static Iterable presentInstances(Iterable> optionals) Returns the value of each present instance from the supplied optionals, in order, skipping over occurrences of absent().
14	abstract String toString() Returns a string representation for this instance.
15	abstract Optional transform(Function function) If the instance is present, it is transformed with the given Function; otherwise, absent() is returned.

继承的方法

此类从以下类继承方法-

java.lang.Object

选修课的例子

使用您选择的任何编辑器在C：/> Guava中创建以下Java程序。

GuavaTester.java

import com.google.common.base.Optional;

public class GuavaTester {
   public static void main(String args[]) {
      GuavaTester guavaTester = new GuavaTester();

      Integer value1 =  null;
      Integer value2 =  new Integer(10);
      
      //Optional.fromNullable - allows passed parameter to be null.
      Optional a = Optional.fromNullable(value1);
      
      //Optional.of - throws NullPointerException if passed parameter is null
      Optional b = Optional.of(value2);        

      System.out.println(guavaTester.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.or - returns the value if present otherwise returns
      //the default value passed.
      Integer value1 = a.or(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:\Guava>javac GuavaTester.java

现在运行GuavaTester以查看结果。

C:\Guava>java GuavaTester

查看结果。

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