Java 8深度可选

谁借莪１个温暖的怀抱￠ 2023-02-15 05:07 2阅读 0赞

Java 8在java.util包中引入了新的Optional类。 它用来表示存在或不存在的值。 这种新结构的主要优点是，没有太多的空检查和`NullPointerException` 。 它避免了任何运行时`NullPointerExceptions`并支持我们开发整洁的Java API或应用程序。 像集合和数组一样，它也是一个最多容纳一个值的容器。 让我们通过一些有用的示例来探索这种新结构。

Java 8可选的优点：

1.  不需要空检查。
2.  在运行时不再有NullPointerException。
3.  我们可以开发干净整洁的API。
4.  没有更多的锅炉板代码

## 1.可选的基本示例 ##

如果给定对象中存在值，则`Optional.ofNullable()`方法将返回Non-empty可选。 否则返回空可选。

`Optionaal.empty()`方法对于创建空的Optional对象很有用。

OptionalBasicExample.java

package com.mkyong;
    
    import java.util.Optional;
    
    public class OptionalBasicExample {
    
        public static void main(String[] args) {
    
            Optional<String> gender = Optional.of("MALE");
            String answer1 = "Yes";
            String answer2 = null;
    
            System.out.println("Non-Empty Optional:" + gender);
            System.out.println("Non-Empty Optional: Gender value : " + gender.get());
            System.out.println("Empty Optional: " + Optional.empty());
    
            System.out.println("ofNullable on Non-Empty Optional: " + Optional.ofNullable(answer1));
            System.out.println("ofNullable on Empty Optional: " + Optional.ofNullable(answer2));
            
            // java.lang.NullPointerException
            System.out.println("ofNullable on Non-Empty Optional: " + Optional.of(answer2));
    
        }
    
    }

输出量

Non-Empty Optional:Optional[MALE]
    Non-Empty Optional: Gender value : MALE
    Empty Optional: Optional.empty
    
    ofNullable on Non-Empty Optional: Optional[Yes]
    ofNullable on Empty Optional: Optional.empty
    
    Exception in thread "main" java.lang.NullPointerException
    	at java.util.Objects.requireNonNull(Objects.java:203)
    	at java.util.Optional.<init>(Optional.java:96)
    	at java.util.Optional.of(Optional.java:108)
    	//...

## 2. Optional.map和flatMap ##

OptionalMapFlapMapExample.java

package com.mkyong;
    
    import java.util.Optional;
    
    public class OptionalMapFlapMapExample {
    
        public static void main(String[] args) {
    
            Optional<String> nonEmptyGender = Optional.of("male");
            Optional<String> emptyGender = Optional.empty();
    
            System.out.println("Non-Empty Optional:: " + nonEmptyGender.map(String::toUpperCase));
            System.out.println("Empty Optional    :: " + emptyGender.map(String::toUpperCase));
    
            Optional<Optional<String>> nonEmptyOtionalGender = Optional.of(Optional.of("male"));
            System.out.println("Optional value   :: " + nonEmptyOtionalGender);
            System.out.println("Optional.map     :: " + nonEmptyOtionalGender.map(gender -> gender.map(String::toUpperCase)));
            System.out.println("Optional.flatMap :: " + nonEmptyOtionalGender.flatMap(gender -> gender.map(String::toUpperCase)));
            
        }
    
    }

输出量

Non-Empty Optional:: Optional[MALE]
    Empty Optional    :: Optional.empty
    Optional value   :: Optional[Optional[male]]
    Optional.map     :: Optional[Optional[MALE]]
    Optional.flatMap :: Optional[MALE]

## 3. Optional.filter ##

OptionalFilterExample.java

package com.mkyong;
    
    import java.util.Optional;
    
    public class OptionalFilterExample {
    
        public static void main(String[] args) {
    
            Optional<String> gender = Optional.of("MALE");
            Optional<String> emptyGender = Optional.empty();
    
            //Filter on Optional
            System.out.println(gender.filter(g -> g.equals("male"))); //Optional.empty
            System.out.println(gender.filter(g -> g.equalsIgnoreCase("MALE"))); //Optional[MALE]
            System.out.println(emptyGender.filter(g -> g.equalsIgnoreCase("MALE"))); //Optional.empty
    
        }
    
    }

输出量

Optional.empty
    Optional[MALE]
    Optional.empty

## 4.可选的isPresent和ifPresent ##

如果给定的Optional对象为非空，则`Optional.isPresent()`返回true。 否则，它返回false。

如果给定的Optional对象为非空，则`Optional.ifPresent()`执行给定的操作。 否则，它返回false。

OptionalIfPresentExample.java

package com.mkyong;
    
    import java.util.Optional;
    
    public class OptionalIfPresentExample {
    
        public static void main(String[] args) {
    
            Optional<String> gender = Optional.of("MALE");
            Optional<String> emptyGender = Optional.empty();
    
            if (gender.isPresent()) {
                System.out.println("Value available.");
            } else {
                System.out.println("Value not available.");
            }
    
            gender.ifPresent(g -> System.out.println("In gender Option, value available."));
    
            //condition failed, no output print
            emptyGender.ifPresent(g -> System.out.println("In emptyGender Option, value available."));
    
        }
    
    }

输出量

Value available.
    In gender Option, value available.

## 5.可选的orElse方法 ##

如果存在于可选容器中，则返回值。 否则，返回给定的默认值。

OptionalOrElseExample.java

package com.mkyong;
    
    import java.util.Optional;
    
    public class OptionalOrElseExample {
    
        public static void main(String[] args) {
    
            Optional<String> gender = Optional.of("MALE");
            Optional<String> emptyGender = Optional.empty();
    
            System.out.println(gender.orElse("<N/A>")); //MALE
            System.out.println(emptyGender.orElse("<N/A>")); //<N/A>
    
            System.out.println(gender.orElseGet(() -> "<N/A>")); //MALE
            System.out.println(emptyGender.orElseGet(() -> "<N/A>")); //<N/A>
    
        }
    
    }

输出量

MALE
    <N/A>
    MALE
    <N/A>

## 6.没有Java 8可选 ##

每个人都熟悉在线购物。 让我们假设我们要为著名的电子商务网站实现移动产品模块。

让我们实现不带Java 8 Optional的Mobile Domain模块。

ScreenResolution.java

package com.mkyong.without.optional;
    
    public class ScreenResolution {
    
    	private int width;
    	private int height;
    	
    	public ScreenResolution(int width, int height){
    		this.width = width;
    		this.height = height;
    	}
    	
    	public int getWidth() {
    		return width;
    	}
    
    	public int getHeight() {
    		return height;
    	}
    	
    }

DisplayFeatures.java

package com.mkyong.without.optional;
    
    public class DisplayFeatures {
    
    	private String size; // In inches
    	private ScreenResolution resolution;
    	
    	public DisplayFeatures(String size, ScreenResolution resolution){
    		this.size = size;
    		this.resolution = resolution;
    	}
    	
    	public String getSize() {
    		return size;
    	}
    	public ScreenResolution getResolution() {
    		return resolution;
    	}
    	
    }

Mobile.java

package com.mkyong.without.optional;
    
    public class Mobile {
    
    	private long id;
    	private String brand;
    	private String name;
    	private DisplayFeatures displayFeatures;
    	// Likewise we can see Memory Features, Camera Features etc.
    	
    	public Mobile(long id, String brand, String name, 
                                DisplayFeatures displayFeatures){
    		this.id = id;
    		this.brand = brand;
    		this.name = name;
    		this.displayFeatures = displayFeatures;
    	}
    
    	public long getId() {
    		return id;
    	}
    
    	public String getBrand() {
    		return brand;
    	}
    
    	public String getName() {
    		return name;
    	}
    
    	public DisplayFeatures getDisplayFeatures() {
    		return displayFeatures;
    	}
    	
    }

在这里，如果我们观察到`getMobileScreenWidth()`方法，它有很多样板代码，并且有很多空检查。 在Java 8之前，我们应该做所有这些无意义的工作，以避免Runtime NullPointerExceptions。

MobileService.java

package com.mkyong.without.optional;
    
    public class MobileService {
    	
    	public int getMobileScreenWidth(Mobile mobile){
    
    		if(mobile != null){
    			DisplayFeatures dfeatures = mobile.getDisplayFeatures();
    			if(dfeatures != null){
    				ScreenResolution resolution = dfeatures.getResolution();
    				if(resolution != null){
    					return resolution.getWidth();
    				}
    			}
    		}
    		return 0;
    
    	}
    
    }

开发一个测试应用程序以测试这些Domain对象。

MobileTesterWithoutOptional.java

package com.mkyong.without.optional;
    
    public class MobileTesterWithoutOptional {
    	
    	public static void main(String[] args) {
    
    		ScreenResolution resolution = new ScreenResolution(750,1334);
    		DisplayFeatures dfeatures = new DisplayFeatures("4.7", resolution);
    		Mobile mobile = new Mobile(2015001, "Apple", "iPhone 6s", dfeatures);
    		
    		MobileService mService = new MobileService();
    		
    		int mobileWidth = mService.getMobileScreenWidth(mobile);
    		System.out.println("Apple iPhone 6s Screen Width = " + mobileWidth);
    		
    		ScreenResolution resolution2 = new ScreenResolution(0,0);
    		DisplayFeatures dfeatures2 = new DisplayFeatures("0", resolution2);
    		Mobile mobile2 = new Mobile(2015001, "Apple", "iPhone 6s", dfeatures2);		
    		int mobileWidth2 = mService.getMobileScreenWidth(mobile2);
    		System.out.println("Apple iPhone 16s Screen Width = " + mobileWidth2);
    
    	}
    
    }

输出量

Apple iPhone 6s Screen Width = 750
    Apple iPhone 16s Screen Width = 0

## 7.使用Java 8可选 ##

现在使用干净整洁的方式使用Java 8 Optional结构开发相同的域模型。

*PS `ScreenResolution.java`没有变化。* *请参考以上部分。*

DisplayFeatures.java

package com.mkyong.with.optional;
    
    import java.util.Optional;
    
    public class DisplayFeatures {
    
    	private String size; // In inches
    	private Optional<ScreenResolution> resolution;
    	
    	public DisplayFeatures(String size, Optional<ScreenResolution> resolution){
    		this.size = size;
    		this.resolution = resolution;
    	}
    	
    	public String getSize() {
    		return size;
    	}
    	public Optional<ScreenResolution> getResolution() {
    		return resolution;
    	}
    	
    }

Mobile.java

package com.mkyong.with.optional;
    
    import java.util.Optional;
    
    public class Mobile {
    
    	private long id;
    	private String brand;
    	private String name;
    	private Optional<DisplayFeatures> displayFeatures;
    	// Like wise we can see MemoryFeatures, CameraFeatures etc.
    	// For simplicity, using only one Features
    	
    	public Mobile(long id, String brand, String name, Optional<DisplayFeatures> displayFeatures){
    		this.id = id;
    		this.brand = brand;
    		this.name = name;
    		this.displayFeatures = displayFeatures;
    	}
    
    	public long getId() {
    		return id;
    	}
    
    	public String getBrand() {
    		return brand;
    	}
    
    	public String getName() {
    		return name;
    	}
    
    	public Optional<DisplayFeatures> getDisplayFeatures() {
    		return displayFeatures;
    	}
    	
    }

在这里，我们可以观察到在没有null检查和样板代码的情况下如何清洁`getMobileScreenWidth()` API。 我们不担心运行时出现NullPointerExceptions。

MobileService.java

package com.mkyong.with.optional;
    
    import java.util.Optional;
    
    public class MobileService {
    
      public Integer getMobileScreenWidth(Optional<Mobile> mobile){
    	return mobile.flatMap(Mobile::getDisplayFeatures)
    		 .flatMap(DisplayFeatures::getResolution)
    		 .map(ScreenResolution::getWidth)
    		 .orElse(0);
    
      }
    
    }

现在开发一个测试组件

MobileTesterWithOptional.java

package com.mkyong.with.optional;
    
    import java.util.Optional;
    
    public class MobileTesterWithOptional {
    	
      public static void main(String[] args) {
    	ScreenResolution resolution = new ScreenResolution(750,1334);
    	DisplayFeatures dfeatures = new DisplayFeatures("4.7", Optional.of(resolution));
    	Mobile mobile = new Mobile(2015001, "Apple", "iPhone 6s", Optional.of(dfeatures));
    		
    	MobileService mService = new MobileService();
    		
    	int width = mService.getMobileScreenWidth(Optional.of(mobile));
    	System.out.println("Apple iPhone 6s Screen Width = " + width);
    
    	Mobile mobile2 = new Mobile(2015001, "Apple", "iPhone 6s", Optional.empty());		
    	int width2 = mService.getMobileScreenWidth(Optional.of(mobile2));
    	System.out.println("Apple iPhone 16s Screen Width = " + width2);
      }
    }

输出量

Apple iPhone 6s Screen Width = 750
    Apple iPhone 16s Screen Width = 0

## 8. Java Optional适合什么地方？ ##

如果我们观察到上述实时零售域用例，则应该知道Java Optional构造在以下位置很有用。

8.1方法参数

public void setResolution(Optional<ScreenResolution> resolution) {
    	this.resolution = resolution;
    }

8.2方法返回类型

public Optional<ScreenResolution> getResolution() {
    	return resolution;
    }

8.3构造函数参数

public DisplayFeatures(String size, Optional<ScreenResolution> resolution){
    	this.size = size;
    	this.resolution = resolution;
    }

8.4变量声明

private Optional<ScreenResolution> resolution;

8.5班级水平

public class B
    
    public class A<T extends Optional<B>> { }

## 下载源代码 ##

> 下载– [Java8Optional-example.zip][] （4 KB）

## 参考文献 ##

1.  [可选的JavaDoc][JavaDoc]

标签： [java8][] [可选][Link 1]

> 翻译自: [https://mkyong.com/java8/java-8-optional-in-depth/][https_mkyong.com_java8_java-8-optional-in-depth]

[Java8Optional-example.zip]: http://www.mkyong.com/wp-content/uploads/2017/02/Java8Optional.zip
[JavaDoc]: https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html
[java8]: https://mkyong.com/tag/java8/
[Link 1]: https://mkyong.com/tag/optional/
[https_mkyong.com_java8_java-8-optional-in-depth]: https://mkyong.com/java8/java-8-optional-in-depth/