java8-Lambda中比较器Comparator的使用

秒速五厘米 2022-06-11 02:42 293阅读 0赞

> 此教程旨在让读者学会使用lambda并结合Comparator进行集合排序操作

*  典型的比较器示例

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

*  等价的Lambda的方式

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

# 不使用Lambda的排序 #

假如我们要通过Developer 对象的年龄进行排序，通常情况下我们使用Collections.sort，new个匿名Comparator 类，类似下面这种：

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("ricky", 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;
    
        }
    
    }

输出结果：

Before Sort
    Developer [name=ricky, 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=ricky, salary=70000, age=33]
    Developer [name=iris, salary=170000, age=55]

当比较规则发生变化时，你需要再次new个匿名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());
            }
        });

这样也可以，不过你会不会觉得这样有点怪，因为其实不同的只有一行代码而已，但是却需要重复写很多代码？

# 通过lambda进行排序 #

在java8中，List接口直接提供了排序方法， 所以你不需要使用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 示例

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("ricky", 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;
    
        }
    
    }

输出结果:

# 更多的Lambda 例子 #

*  根据年龄
    
        //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());
 *  根据名字
    
        //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()));
 *  根据薪水
    
        //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()))
 *  倒序
    
     *  正常排序
        
            Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
                listDevs.sort(salaryComparator);
     *  倒序
    
        Comparator<Developer> salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
            listDevs.sort(salaryComparator.reversed());

# 翻译 #

ricky 20170728

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