How to use the Java comparator?

Question

If I have the following class:

public class Employee { private int empId; private String name; private int age; public Employee(int empId, String name, int age) { // set values on attributes } // getters & setters } 

How to use a comparator that compares by name, then by age, then by id?

Answer 1

You need to implement it so that it is ordered by preferred elements. That is, you need to compare by name, then if this comparison is equal, compare by age, etc. The following is an example:

 public class EmployeeComparator implements Comparator<Employee> { @Override public int compare(Employee e1, Employee e2) { int nameDiff = e1.getName().compareTo(e2.getName()); if(nameDiff != 0) { return nameDiff; } int ageDiff = e1.getAge() - e2.getAge(); if(ageDiff != 0) { return ageDiff; } int idDiff = e1.getEmpId() - e2.getEmpId(); return idDiff; } } 

Answer 2

Update

Came through this instant ago: How to compare objects over several fields One of the answers related to ComparatorChain , which will call several comparators in a sequence until a non-zero result is obtained from the comparator or all the comparators are involved. This will probably be your preferred solution.

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Perhaps this (untested) implementation of Comparator#compare() will do the trick.

 int compare(Employee e, Employee f) { int val = e.name.compareTo(f.name); if(val == 0) { val = e.age - f.age; if(val == 0) { val = e.empId - f.empId; } } return val; } 

Answer 3

You can also implement the Comparable interface in your class.

for example, something like this:

 public class Employee implements Comparable<Employee>{ private int empId; private String name; private int age; public Employee(int empId, String name, int age) { // set values on attributes } // getters & setters public int compareTo(Employee o) { int ret = this.name.compareTo(o.name); if(ret == 0) ret = this.age - o.age; if(ret == 0) ret = this.empId - o.empId; return ret; } } 

therefore, you do not need to do an extra class to compare your employees.

Answer 4

Implement it

 public class Employee { private int empId; private String name; private int age; /** * @param empId * @param name * @param age */ public Employee(int empId, String name, int age) { super(); this.empId = empId; this.name = name; this.age = age; } /** * */ public Employee() { super(); // TODO Auto-generated constructor stub } public int getEmpId() { return empId; } public void setEmpId(int empId) { this.empId = empId; } 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; } //Compare by name, age and then id public static Comparator<Employee> COMPARE_EMPLOYEE = new Comparator<Employee>() { public int compare(Employee one, Employee other) { //Compare Name if (one.getName().compareToIgnoreCase(other.getName()) == 0) { //Compare age if((one.getAge() - other.getAge()) == 0) { // Now check with id is useless // So directly return result of compare by id return one.getEmpId() - other.getEmpId(); } else { //If age Not equal return one.getAge() - other.getAge(); } } else { //If name not equal return one.getName().compareToIgnoreCase(other.getName()); } } }; } 

Using:

 List<Employee> contacts = new ArrayList<Employee>(); //Fill it. //Sort by address. Collections.sort(contacts, Employee.COMPARE_EMPLOYEE); 

Read Sort ArrayList's address list , this should help you, and you will get more ideas and different ways to use Comparator.

Answer 5

guava ComparisonChain :

  List<Employee> list = new ArrayList<Employee>(); //... Collections.sort(list, new Comparator<Employee>(){ @Override public int compare(Employee e1, Employee e2) { return ComparisonChain.start() .compare(e1.empId, e2.empId) .compare(e1.name, e2.name) .compare(e1.age, e2.age).result(); }}); 

Answer 6

Use this:

 public class Test { public static void main(String[] args) { Employee emp1 = new Employee(2, "Tom", 20); Employee emp2 = new Employee(1, "Tom", 20); Employee emp3 = new Employee(3, "Hank", 21); List<Employee> list = new ArrayList<>(); list.add(emp1); list.add(emp2); list.add(emp3); Collections.sort(list, new Employee().new MyComparator()); System.out.println(list); } } class Employee { private int empId; private String name; private int age; public Employee() {} public Employee(int empId, String name, int age) { this.empId = empId; this.name = name; this.age = age; } class MyComparator implements Comparator<Employee> { @Override public int compare(Employee e1, Employee e2) { if(e1.name.compareTo(e2.name) == 0) { if(((Integer)e1.age).compareTo(e2.age) == 0) { return ((Integer)e1.empId).compareTo(e2.empId); } else { return ((Integer)e1.age).compareTo(e2.age); } } return e1.name.compareTo(e2.name); } } @Override public String toString() { return "Employee [empId=" + empId + ", name=" + name + ", age=" + age + "]"; } } 

Answer 7

The Comparator interface defines two methods: compare() and equals() .

The compare() method compares two elements in order: int compare(Object obj1, Object obj2)

obj1 and obj2 are objects to compare. This method returns zero if the objects are equal. Returns a positive value if obj1 is greater than obj2. Otherwise, a negative value is returned.

By compare() , you can reorder the objects. For example, to sort in the reverse order, you can create a comparator that flips the comparison results.

The equals() method checks to see if the object is equal to the calling comparator: boolean equals(Object obj)

obj is an object that needs to be checked for equality. The method returns true if obj and the caller are Comparator objects and use the same order. Otherwise, false is returned.

Example:

 import java.util.*; class Dog implements Comparator<Dog>, Comparable<Dog> { private String name; private int age; Dog() { } Dog(String n, int a) { name = n; age = a; } public String getDogName() { return name; } public int getDogAge() { return age; } // Overriding the compareTo method public int compareTo(Dog d) { return (this.name).compareTo(d.name); } // Overriding the compare method to sort the age public int compare(Dog d, Dog d1) { return d.age - d1.age; } } public class Example { public static void main(String args[]) { // Takes a list o Dog objects List<Dog> list = new ArrayList<Dog>(); list.add(new Dog("Shaggy", 3)); list.add(new Dog("Lacy", 2)); list.add(new Dog("Roger", 10)); list.add(new Dog("Tommy", 4)); list.add(new Dog("Tammy", 1)); Collections.sort(list); // Sorts the array list for(Dog a: list) // printing the sorted list of names System.out.print(a.getDogName() + ", "); // Sorts the array list using comparator Collections.sort(list, new Dog()); System.out.println(" "); for(Dog a: list) // printing the sorted list of ages System.out.print(a.getDogName() +" : "+ a.getDogAge() + ", "); } } 

Check out other Java Comparator examples .