Java可选类

Java在jdk8中引入了新的Optional类。它是一个公共的最终类, 用于处理Java应用程序中的NullPointerException。你必须导入java.util包才能使用此类。它提供了用于检查特定变量的值是否存在的方法。

Java可选的类方法

示例：不使用Optional的Java程序

在下面的示例中, 我们没有使用Optional类。该程序异常终止并抛出nullPointerException。

public class OptionalExample {
	public static void main(String[] args) {
		String[] str = new String[10];
		String lowercaseString = str[5].toLowerCase();
		System.out.print(lowercaseString);
	}
}

输出：

Exception in thread "main" java.lang.NullPointerException
	at lambdaExample.OptionalExample.main(OptionalExample.java:6)

为了避免异常终止, 我们使用Optional类。在下面的示例中, 我们使用Optional。因此, 我们的程序可以执行而不会崩溃。

Java可选示例：如果不存在Value

import java.util.Optional;
public class OptionalExample {
	public static void main(String[] args) {
		String[] str = new String[10];
		Optional<String> checkNull = Optional.ofNullable(str[5]);
	    if(checkNull.isPresent()){	// check for value is present or not
	    	String lowercaseString = str[5].toLowerCase();
			System.out.print(lowercaseString);
	    }else
	    	System.out.println("string value is not present");
	}
}

输出：

string value is not present

Java可选示例：如果存在值

import java.util.Optional;
public class OptionalExample {
	public static void main(String[] args) {
		String[] str = new String[10];		
		str[5] = "JAVA OPTIONAL CLASS EXAMPLE";// Setting value for 5th index
		Optional<String> checkNull = Optional.ofNullable(str[5]);
	    if(checkNull.isPresent()){	// It Checks, value is present or not
	    	String lowercaseString = str[5].toLowerCase();
			System.out.print(lowercaseString);
	    }else
	    	System.out.println("String value is not present");
	}
}

输出：

java optional class example

另一个Java可选示例

import java.util.Optional;
public class OptionalExample {
	public static void main(String[] args) {
		String[] str = new String[10];		
		str[5] = "JAVA OPTIONAL CLASS EXAMPLE";  // Setting value for 5th index
		Optional<String> checkNull = Optional.ofNullable(str[5]);
		checkNull.ifPresent(System.out::println);	// printing value by using method reference
		System.out.println(checkNull.get());	// printing value by using get method
		System.out.println(str[5].toLowerCase());	
	}
}

输出：

JAVA OPTIONAL CLASS EXAMPLE
JAVA OPTIONAL CLASS EXAMPLE
java optional class example

Java可选方法示例

import java.util.Optional;
public class OptionalExample {
	public static void main(String[] args) {
		String[] str = new String[10];		
		str[5] = "JAVA OPTIONAL CLASS EXAMPLE";  // Setting value for 5th index
		// It returns an empty instance of Optional class 
		Optional<String> empty = Optional.empty();
		System.out.println(empty);
		// It returns a non-empty Optional
		Optional<String> value = Optional.of(str[5]);
		// If value is present, it returns an Optional otherwise returns an empty Optional
		System.out.println("Filtered value: "+value.filter((s)->s.equals("Abc")));
		System.out.println("Filtered value: "+value.filter((s)->s.equals("JAVA OPTIONAL CLASS EXAMPLE")));
		// It returns value of an Optional. if value is not present, it throws an NoSuchElementException  
		System.out.println("Getting value: "+value.get());
		// It returns hashCode of the value
		System.out.println("Getting hashCode: "+value.hashCode());
		// It returns true if value is present, otherwise false
		System.out.println("Is value present: "+value.isPresent());
		// It returns non-empty Optional if value is present, otherwise returns an empty Optional
		System.out.println("Nullable Optional: "+Optional.ofNullable(str[5]));
		// It returns value if available, otherwise returns specified value, System.out.println("orElse: "+value.orElse("Value is not present"));
		System.out.println("orElse: "+empty.orElse("Value is not present"));
		value.ifPresent(System.out::println);	// printing value by using method reference	
	}
}

输出：

Optional.empty
Filtered value: Optional.empty
Filtered value: Optional[JAVA OPTIONAL CLASS EXAMPLE]
Getting value: JAVA OPTIONAL CLASS EXAMPLE
Getting hashCode: -619947648
Is value present: true
Nullable Optional: Optional[JAVA OPTIONAL CLASS EXAMPLE]
orElse: JAVA OPTIONAL CLASS EXAMPLE
orElse: Value is not present
JAVA OPTIONAL CLASS EXAMPLE