Comparator using an external sort comparator
When we need to sort the elements of the set, you can use java.util.Comparator create a comparator to sort. Comparator interface is also a function interface, we can use a lambda expression. The following example,
package com.common;
import java.util.*;
import java.util.stream.Collectors;
public class ComparatorTest { public static void main(String[] args) { Employee e1 = new Employee("John", 25, 3000, 9922001); Employee e2 = new Employee("Ace", 22, 2000, 5924001); Employee e3 = new Employee("Keith", 35, 4000, 3924401); List<Employee> employees = new ArrayList<>(); employees.add(e1); employees.add(e2); employees.add(e3); /** * @SuppressWarnings({"unchecked", "rawtypes"}) * default void sort(Comparator<? super E> c) { * Object[] a = this.toArray(); * Arrays.sort(a, (Comparator) c); * ListIterator<E> i = this.listIterator(); * for (Object e : a) { * i.next(); * i.set((E) e); * } * } * * sort 对象接收一个 Comparator 函数式接口，可以传入一个lambda表达式 */ employees.sort((o1, o2) -> o1.getName().compareTo(o2.getName())); Collections.sort(employees, (o1, o2) -> o1.getName().compareTo(o2.getName())); employees.forEach(System.out::println); } } /** * [Employee(name=John, age=25, salary=3000.0, mobile=9922001), * Employee(name=Ace, age=22, salary=2000.0, mobile=5924001), * Employee(name=Keith, age=35, salary=4000.0, mobile=3924401)] */ class Employee { String name; int age; double salary; long mobile; // constructors, getters & setters public Employee(String name, int age, double salary, long mobile) { this.name = name; this.age = age; this.salary = salary; this.mobile = mobile; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public double getSalary() { return salary; } public void setSalary(double salary) { this.salary = salary; } public long getMobile() { return mobile; } public void setMobile(long mobile) { this.mobile = mobile; } @Override public String toString() { final StringBuilder sb = new StringBuilder("Employee{"); sb.append("name='").append(name).append('\''); sb.append(", age=").append(age); sb.append(", salary=").append(salary); sb.append(", mobile=").append(mobile); sb.append('}'); return sb.toString(); } }
Sort using Comparator.comparing
comparing a method
Comparator class implements an internal view, there is a method of comparing, to achieve the following,
public static <T, U extends Comparable<? super U>> Comparator<T> comparing( Function<? super T, ? extends U> keyExtractor) { Objects.requireNonNull(keyExtractor); return (Comparator<T> & Serializable) (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2)); } The return value is (c1, c2) -> keyExtractor.apply (c1) .compareTo (keyExtractor.apply (c2)); a lambda expression, which is a Compator. So the example above can also be transformed into the following,
package com.common;
import java.util.*;
public class ComparatorTest { public static void main(String[] args) { Employee e1 = new Employee("John", 25, 3000, 9922001); Employee e2 = new Employee("Ace", 22, 2000, 5924001); Employee e3 = new Employee("Keith", 35, 4000, 3924401); List<Employee> employees = new ArrayList<>(); employees.add(e1); employees.add(e2); employees.add(e3); /** * @SuppressWarnings({"unchecked", "rawtypes"}) * default void sort(Comparator<? super E> c) { * Object[] a = this.toArray(); * Arrays.sort(a, (Comparator) c); * ListIterator<E> i = this.listIterator(); * for (Object e : a) { * i.next(); * i.set((E) e); * } * } * * sort 对象接收一个 Comparator 函数式接口，可以传入一个lambda表达式 */ employees.sort((o1, o2) -> o1.getName().compareTo(o2.getName())); Collections.sort(employees, (o1, o2) -> o1.getName().compareTo(o2.getName())); employees.forEach(System.out::println); /** * Comparator.comparing 方法的使用 * * comparing 方法接收一个 Function 函数式接口 ，通过一个 lambda 表达式传入 * */ employees.sort(Comparator.comparing(e -> e.getName())); /** * 该方法引用 Employee::getName 可以代替 lambda表达式 */ employees.sort(Comparator.comparing(Employee::getName)); } } /** * [Employee(name=John, age=25, salary=3000.0, mobile=9922001), * Employee(name=Ace, age=22, salary=2000.0, mobile=5924001), * Employee(name=Keith, age=35, salary=4000.0, mobile=3924401)] */ class Employee { String name; int age; double salary; long mobile; // constructors, getters & setters public Employee(String name, int age, double salary, long mobile) { this.name = name; this.age = age; this.salary = salary; this.mobile = mobile; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public double getSalary() { return salary; } public void setSalary(double salary) { this.salary = salary; } public long getMobile() { return mobile; } public void setMobile(long mobile) { this.mobile = mobile; } @Override public String toString() { final StringBuilder sb = new StringBuilder("Employee{"); sb.append("name='").append(name).append('\''); sb.append(", age=").append(age); sb.append(", salary=").append(salary); sb.append(", mobile=").append(mobile); sb.append('}'); return sb.toString(); } }
comparing Method Two
public static <T, U> Comparator<T> comparing( Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator) { Objects.requireNonNull(keyExtractor); Objects.requireNonNull(keyComparator); return (Comparator<T> & Serializable) (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1), keyExtractor.apply(c2)); }And a method for comparing different is that the process one more parameter keyComparator, keyComparator is to create a custom comparator.
Collections.sort(employees, Comparator.comparing(
Employee::getName, (s1, s2) -> {
return s2.compareTo(s1); }));
Use Comparator.reversed sort
Returns the collation contrary,
/**
* 相反的排序规则
*/
Collections.sort(employees, Comparator.comparing(Employee::getName).reversed()); employees.forEach(System.out::println); Output,
Employee{name='Keith', age=35, salary=4000.0, mobile=3924401}
Employee{name='John', age=25, salary=3000.0, mobile=9922001} Employee{name='Ace', age=22, salary=2000.0, mobile=5924001}
Use Comparator. NullsFirst sort
When there is null elements in a collection, you may be used for the comparator friendly null, null elements at the top of the collection
employees.add(null); //插入一个null元素
Collections.sort(employees, Comparator.nullsFirst(Comparator.comparing(Employee::getName)));
employees.forEach(System.out::println);
Collections.sort(employees, Comparator.nullsLast(Comparator.comparing(Employee::getName)));
employees.forEach(System.out::println);
Use Comparator . ThenComparing sort
First, using sort name, followed by sorting using ege, look at the effect of the use of
Collections.sort(employees, Comparator.comparing(Employee::getAge).thenComparing(Employee::getName)); employees.forEach(System.out::println);