Java 8 Lambda：比较器示例

以你之姓@ 2023-02-15 05:15 50阅读 0赞

![java-lambda-expression][]![java-lambda-expression][java-lambda-expression 1]

在此示例中，我们将向您展示如何使用Java 8 Lambda表达式编写`Comparator`对List进行排序。

1.经典`Comparator`示例。

Comparator<Developer> byName = new Comparator<Developer>() {
    		@Override
    		public int compare(Developer o1, Developer o2) {
    			return o1.getName().compareTo(o2.getName());
    		}
    	};

2. Lambda表达式等效。

Comparator<Developer> byName = 
    		(Developer o1, Developer o2)->o1.getName().compareTo(o2.getName());

## 1.排序时不使用Lambda ##

使用年龄比较`Developer`对象的示例。 通常，您使用`Collections.sort`并传递一个匿名`Comparator`类，如下所示：

TestSorting.java

package com.mkyong.java8;
    
    import java.math.BigDecimal;
    import java.util.ArrayList;
    import java.util.Collections;
    import java.util.Comparator;
    import java.util.List;
    
    public class TestSorting {
    
    	public static void main(String[] args) {
    
    		List<Developer> listDevs = getDevelopers();
    
    		System.out.println("Before Sort");
    		for (Developer developer : listDevs) {
    			System.out.println(developer);
    		}
    		
    		//sort by age
    		Collections.sort(listDevs, new Comparator<Developer>() {
    			@Override
    			public int compare(Developer o1, Developer o2) {
    				return o1.getAge() - o2.getAge();
    			}
    		});
    	
    		System.out.println("After Sort");
    		for (Developer developer : listDevs) {
    			System.out.println(developer);
    		}
    		
    	}
    
    	private static List<Developer> getDevelopers() {
    
    		List<Developer> result = new ArrayList<Developer>();
    
    		result.add(new Developer("mkyong", new BigDecimal("70000"), 33));
    		result.add(new Developer("alvin", new BigDecimal("80000"), 20));
    		result.add(new Developer("jason", new BigDecimal("100000"), 10));
    		result.add(new Developer("iris", new BigDecimal("170000"), 55));
    		
    		return result;
    
    	}
    	
    }

输出量

Before Sort
    Developer [name=mkyong, salary=70000, age=33]
    Developer [name=alvin, salary=80000, age=20]
    Developer [name=jason, salary=100000, age=10]
    Developer [name=iris, salary=170000, age=55]
    
    After Sort
    Developer [name=jason, salary=100000, age=10]
    Developer [name=alvin, salary=80000, age=20]
    Developer [name=mkyong, salary=70000, age=33]
    Developer [name=iris, salary=170000, age=55]

更改排序要求后，您只需传入另一个新的匿名`Comparator`类：

//sort by age
    	Collections.sort(listDevs, new Comparator<Developer>() {
    		@Override
    		public int compare(Developer o1, Developer o2) {
    			return o1.getAge() - o2.getAge();
    		}
    	});
    	
    	//sort by name	
    	Collections.sort(listDevs, new Comparator<Developer>() {
    		@Override
    		public int compare(Developer o1, Developer o2) {
    			return o1.getName().compareTo(o2.getName());
    		}
    	});
    				
    	//sort by salary
    	Collections.sort(listDevs, new Comparator<Developer>() {
    		@Override
    		public int compare(Developer o1, Developer o2) {
    			return o1.getSalary().compareTo(o2.getSalary());
    		}
    	});

它可以工作，但是，您是否仅仅因为要更改一行代码而觉得创建类有点奇怪？

## 2.用Lambda排序 ##

在Java 8中， `List`接口直接支持`sort`方法，无需再使用`Collections.sort` 。

//List.sort() since Java 8
    	listDevs.sort(new Comparator<Developer>() {
    		@Override
    		public int compare(Developer o1, Developer o2) {
    			return o2.getAge() - o1.getAge();
    		}
    	});

Lambda表达式示例：

TestSorting.java

package com.mkyong.java8;
    
    import java.math.BigDecimal;
    import java.util.ArrayList;
    import java.util.List;
    
    public class TestSorting {
    
    	public static void main(String[] args) {
    
    		List<Developer> listDevs = getDevelopers();
    		
    		System.out.println("Before Sort");
    		for (Developer developer : listDevs) {
    			System.out.println(developer);
    		}
    		
    		System.out.println("After Sort");
    		
    		//lambda here!
    		listDevs.sort((Developer o1, Developer o2)->o1.getAge()-o2.getAge());
    	
    		//java 8 only, lambda also, to print the List
    		listDevs.forEach((developer)->System.out.println(developer));
    	}
    
    	private static List<Developer> getDevelopers() {
    
    		List<Developer> result = new ArrayList<Developer>();
    
    		result.add(new Developer("mkyong", new BigDecimal("70000"), 33));
    		result.add(new Developer("alvin", new BigDecimal("80000"), 20));
    		result.add(new Developer("jason", new BigDecimal("100000"), 10));
    		result.add(new Developer("iris", new BigDecimal("170000"), 55));
    		
    		return result;
    
    	}
    	
    }

输出量

## 3.更多Lambda示例 ##

3.1按年龄排序

//sort by age
    	Collections.sort(listDevs, new Comparator<Developer>() {
    		@Override
    		public int compare(Developer o1, Developer o2) {
    			return o1.getAge() - o2.getAge();
    		}
    	});
    	
    	//lambda
    	listDevs.sort((Developer o1, Developer o2)->o1.getAge()-o2.getAge());
    	
    	//lambda, valid, parameter type is optional
    	listDevs.sort((o1, o2)->o1.getAge()-o2.getAge());

3.2按名称排序

//sort by name
    	Collections.sort(listDevs, new Comparator<Developer>() {
    		@Override
    		public int compare(Developer o1, Developer o2) {
    			return o1.getName().compareTo(o2.getName());
    		}
    	});
    		
    	//lambda
    	listDevs.sort((Developer o1, Developer o2)->o1.getName().compareTo(o2.getName()));		
    	
    	//lambda
    	listDevs.sort((o1, o2)->o1.getName().compareTo(o2.getName()));

3.3按薪水排序

//sort by salary
    	Collections.sort(listDevs, new Comparator<Developer>() {
    		@Override
    		public int compare(Developer o1, Developer o2) {
    			return o1.getSalary().compareTo(o2.getSalary());
    		}
    	});				
    
    	//lambda
    	listDevs.sort((Developer o1, Developer o2)->o1.getSalary().compareTo(o2.getSalary()));
    	
    	//lambda
    	listDevs.sort((o1, o2)->o1.getSalary().compareTo(o2.getSalary()));

3.4反向排序。

3.4.1 Lambda表达式使用其薪水对列表进行排序。

Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
    	listDevs.sort(salaryComparator);

输出量

Developer [name=mkyong, salary=70000, age=33]
    Developer [name=alvin, salary=80000, age=20]
    Developer [name=jason, salary=100000, age=10]
    Developer [name=iris, salary=170000, age=55]

3.4.2 Lambda表达式使用其薪水，相反顺序对列表进行排序。

Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
    	listDevs.sort(salaryComparator.reversed());

输出量

Developer [name=iris, salary=170000, age=55]
    Developer [name=jason, salary=100000, age=10]
    Developer [name=alvin, salary=80000, age=20]
    Developer [name=mkyong, salary=70000, age=33]

## 参考文献 ##

1.  [开始使用Java Lambda表达式][Java Lambda]
2.  [Oracle：Lambda表达式][Oracle_Lambda]
3.  [Oracle：比较器][Oracle]

标签： [比较器][Link 1] [java8][] [lambda][] [排序][Link 2]

> 翻译自: [https://mkyong.com/java8/java-8-lambda-comparator-example/][https_mkyong.com_java8_java-8-lambda-comparator-example]

[java-lambda-expression]: http://imgconvert.csdnimg.cn/aHR0cDovL3d3dy5ta3lvbmcuY29tL3dwLWNvbnRlbnQvdXBsb2Fkcy8yMDE1LzA4L2phdmEtbGFtYmRhLWV4cHJlc3Npb24ucG5n?x-oss-process=image/format,png
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[Java Lambda]: http://www.developer.com/java/start-using-java-lambda-expressions.html
[Oracle_Lambda]: https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html
[Oracle]: https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html
[Link 1]: https://mkyong.com/tag/comparator/
[java8]: https://mkyong.com/tag/java8/
[lambda]: https://mkyong.com/tag/lambda/
[Link 2]: https://mkyong.com/tag/sorting/
[https_mkyong.com_java8_java-8-lambda-comparator-example]: https://mkyong.com/java8/java-8-lambda-comparator-example/