[java] Collections.sort with multiple fields

I have a list of "Report" objects with three fields (All String type)-

ReportKey
StudentNumber
School

I have a sort code goes like-

Collections.sort(reportList, new Comparator<Report>() {

@Override
public int compare(final Report record1, final Report record2) {
      return (record1.getReportKey() + record1.getStudentNumber() + record1.getSchool())                      
        .compareTo(record2.getReportKey() + record2.getStudentNumber() + record2.getSchool());
      }

});

For some reason, I don't have the sorted order. One advised to put spaces in between fields, but why?

Do you see anything wrong with the code?

This is an old question so I don't see a Java 8 equivalent. Here is an example for this specific case.

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

/**
 * Compares multiple parts of the Report object.
 */
public class SimpleJava8ComparatorClass {

    public static void main(String[] args) {
        List<Report> reportList = new ArrayList<>();
        reportList.add(new Report("reportKey2", "studentNumber2", "school1"));
        reportList.add(new Report("reportKey4", "studentNumber4", "school6"));
        reportList.add(new Report("reportKey1", "studentNumber1", "school1"));
        reportList.add(new Report("reportKey3", "studentNumber2", "school4"));
        reportList.add(new Report("reportKey2", "studentNumber2", "school3"));

        System.out.println("pre-sorting");
        System.out.println(reportList);
        System.out.println();

        Collections.sort(reportList, Comparator.comparing(Report::getReportKey)
            .thenComparing(Report::getStudentNumber)
            .thenComparing(Report::getSchool));

        System.out.println("post-sorting");
        System.out.println(reportList);
    }

    private static class Report {

        private String reportKey;
        private String studentNumber;
        private String school;

        public Report(String reportKey, String studentNumber, String school) {
            this.reportKey = reportKey;
            this.studentNumber = studentNumber;
            this.school = school;
        }

        public String getReportKey() {
            return reportKey;
        }

        public void setReportKey(String reportKey) {
            this.reportKey = reportKey;
        }

        public String getStudentNumber() {
            return studentNumber;
        }

        public void setStudentNumber(String studentNumber) {
            this.studentNumber = studentNumber;
        }

        public String getSchool() {
            return school;
        }

        public void setSchool(String school) {
            this.school = school;
        }

        @Override
        public String toString() {
            return "Report{" +
                   "reportKey='" + reportKey + '\'' +
                   ", studentNumber='" + studentNumber + '\'' +
                   ", school='" + school + '\'' +
                   '}';
        }
    }
}

Use Comparator interface with methods introduced in JDK1.8: comparing and thenComparing, or more concrete methods: comparingXXX and thenComparingXXX.

For example, if we wanna sort a list of persons by their id firstly, then age, then name:

            Comparator<Person> comparator = Comparator.comparingLong(Person::getId)
                    .thenComparingInt(Person::getAge)
                    .thenComparing(Person::getName);
            personList.sort(comparator);

If you want to sort based on ReportKey first then Student Number then School, you need to compare each String instead of concatenating them. Your method might work if you pad the strings with spaces so that each ReportKey is the same length and so on, but it is not really worth the effort. Instead just change the compare method to compare the ReportKeys, if compareTo returns 0 then try StudentNumber, then School.

Sorting with multiple fields in Java8

package com.java8.chapter1;

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import static java.util.Comparator.*;



 public class Example1 {

    public static void main(String[] args) {
        List<Employee> empList = getEmpList();


        // Before Java 8 
        empList.sort(new Comparator<Employee>() {

            @Override
            public int compare(Employee o1, Employee o2) {
                int res = o1.getDesignation().compareTo(o2.getDesignation());
                if (res == 0) {
                    return o1.getSalary() > o2.getSalary() ? 1 : o1.getSalary() < o2.getSalary() ? -1 : 0;
                } else {
                    return res;
                }

            }
        });
        for (Employee emp : empList) {
            System.out.println(emp);
        }
        System.out.println("---------------------------------------------------------------------------");

        // In Java 8

        empList.sort(comparing(Employee::getDesignation).thenComparing(Employee::getSalary));
        empList.stream().forEach(System.out::println);

    }
    private static List<Employee> getEmpList() {
        return Arrays.asList(new Employee("Lakshman A", "Consultent", 450000),
                new Employee("Chaitra S", "Developer", 250000), new Employee("Manoj PVN", "Developer", 250000),
                new Employee("Ramesh R", "Developer", 280000), new Employee("Suresh S", "Developer", 270000),
                new Employee("Jaishree", "Opearations HR", 350000));
    }
}

class Employee {
    private String fullName;
    private String designation;
    private double salary;

    public Employee(String fullName, String designation, double salary) {
        super();
        this.fullName = fullName;
        this.designation = designation;
        this.salary = salary;
    }

    public String getFullName() {
        return fullName;
    }

    public String getDesignation() {
        return designation;
    }

    public double getSalary() {
        return salary;
    }

    @Override
    public String toString() {
        return "Employee [fullName=" + fullName + ", designation=" + designation + ", salary=" + salary + "]";
    }

}

I'd make a comparator using Guava's ComparisonChain:

public class ReportComparator implements Comparator<Report> {
  public int compare(Report r1, Report r2) {
    return ComparisonChain.start()
        .compare(r1.getReportKey(), r2.getReportKey())
        .compare(r1.getStudentNumber(), r2.getStudentNumber())
        .compare(r1.getSchool(), r2.getSchool())
        .result();
  }
}

I suggest to use Java 8 Lambda approach:

List<Report> reportList = new ArrayList<Report>();
reportList.sort(Comparator.comparing(Report::getRecord1).thenComparing(Report::getRecord2));

A lot of answers above have fields compared in single comparator method which is not actually working. There are some answers though with different comparators implemented for each field, I am posting this because this example would be much more clearer and simple to understand I am believing.

class Student{
    Integer bornYear;
    Integer bornMonth;
    Integer bornDay;
    public Student(int bornYear, int bornMonth, int bornDay) {

        this.bornYear = bornYear;
        this.bornMonth = bornMonth;
        this.bornDay = bornDay;
    }
    public Student(int bornYear, int bornMonth) {

        this.bornYear = bornYear;
        this.bornMonth = bornMonth;

    }
    public Student(int bornYear) {

        this.bornYear = bornYear;

    }
    public Integer getBornYear() {
        return bornYear;
    }
    public void setBornYear(int bornYear) {
        this.bornYear = bornYear;
    }
    public Integer getBornMonth() {
        return bornMonth;
    }
    public void setBornMonth(int bornMonth) {
        this.bornMonth = bornMonth;
    }
    public Integer getBornDay() {
        return bornDay;
    }
    public void setBornDay(int bornDay) {
        this.bornDay = bornDay;
    }
    @Override
    public String toString() {
        return "Student [bornYear=" + bornYear + ", bornMonth=" + bornMonth + ", bornDay=" + bornDay + "]";
    }


}
class TestClass
{       

    // Comparator problem in JAVA for sorting objects based on multiple fields 
    public static void main(String[] args)
    {
        int N,c;// Number of threads

        Student s1=new Student(2018,12);
        Student s2=new Student(2018,12);
        Student s3=new Student(2018,11);
        Student s4=new Student(2017,6);
        Student s5=new Student(2017,4);
        Student s6=new Student(2016,8);
        Student s7=new Student(2018);
        Student s8=new Student(2017,8);
        Student s9=new Student(2017,2);
        Student s10=new Student(2017,9);

        List<Student> studentList=new ArrayList<>();
        studentList.add(s1);
        studentList.add(s2);
        studentList.add(s3);
        studentList.add(s4);
        studentList.add(s5);
        studentList.add(s6);
        studentList.add(s7);
        studentList.add(s8);
        studentList.add(s9);
        studentList.add(s10);

        Comparator<Student> byMonth=new Comparator<Student>() {
            @Override
            public int compare(Student st1,Student st2) {
                if(st1.getBornMonth()!=null && st2.getBornMonth()!=null) {
                    return st2.getBornMonth()-st1.getBornMonth();
                }
                else if(st1.getBornMonth()!=null) {
                    return 1;
                }
                else {
                    return -1;
                }
        }};

        Collections.sort(studentList, new Comparator<Student>() {
            @Override
            public int compare(Student st1,Student st2) {
                return st2.getBornYear()-st1.getBornYear();
        }}.thenComparing(byMonth));

        System.out.println("The sorted students list in descending is"+Arrays.deepToString(studentList.toArray()));



    }

}

OUTPUT

The sorted students list in descending is[Student [bornYear=2018, bornMonth=null, bornDay=null], Student [bornYear=2018, bornMonth=12, bornDay=null], Student [bornYear=2018, bornMonth=12, bornDay=null], Student [bornYear=2018, bornMonth=11, bornDay=null], Student [bornYear=2017, bornMonth=9, bornDay=null], Student [bornYear=2017, bornMonth=8, bornDay=null], Student [bornYear=2017, bornMonth=6, bornDay=null], Student [bornYear=2017, bornMonth=4, bornDay=null], Student [bornYear=2017, bornMonth=2, bornDay=null], Student [bornYear=2016, bornMonth=8, bornDay=null]]

(originally from Ways to sort lists of objects in Java based on multiple fields)

Original working code in this gist

Using Java 8 lambda's (added April 10, 2019)

Java 8 solves this nicely by lambda's (though Guava and Apache Commons might still offer more flexibility):

Collections.sort(reportList, Comparator.comparing(Report::getReportKey)
            .thenComparing(Report::getStudentNumber)
            .thenComparing(Report::getSchool));

Thanks to @gaoagong's answer below.

Note that one advantage here is that the getters are evaluated lazily (eg. getSchool() is only evaluated if relevant).

Messy and convoluted: Sorting by hand

Collections.sort(pizzas, new Comparator<Pizza>() {  
    @Override  
    public int compare(Pizza p1, Pizza p2) {  
        int sizeCmp = p1.size.compareTo(p2.size);  
        if (sizeCmp != 0) {  
            return sizeCmp;  
        }  
        int nrOfToppingsCmp = p1.nrOfToppings.compareTo(p2.nrOfToppings);  
        if (nrOfToppingsCmp != 0) {  
            return nrOfToppingsCmp;  
        }  
        return p1.name.compareTo(p2.name);  
    }  
});  

This requires a lot of typing, maintenance and is error prone. The only advantage is that getters are only invoked when relevant.

The reflective way: Sorting with BeanComparator

ComparatorChain chain = new ComparatorChain(Arrays.asList(
   new BeanComparator("size"), 
   new BeanComparator("nrOfToppings"), 
   new BeanComparator("name")));

Collections.sort(pizzas, chain);  

Obviously this is more concise, but even more error prone as you lose your direct reference to the fields by using Strings instead (no typesafety, auto-refactorings). Now if a field is renamed, the compiler won’t even report a problem. Moreover, because this solution uses reflection, the sorting is much slower.

Getting there: Sorting with Google Guava’s ComparisonChain

Collections.sort(pizzas, new Comparator<Pizza>() {  
    @Override  
    public int compare(Pizza p1, Pizza p2) {  
        return ComparisonChain.start().compare(p1.size, p2.size).compare(p1.nrOfToppings, p2.nrOfToppings).compare(p1.name, p2.name).result();  
        // or in case the fields can be null:  
        /* 
        return ComparisonChain.start() 
           .compare(p1.size, p2.size, Ordering.natural().nullsLast()) 
           .compare(p1.nrOfToppings, p2.nrOfToppings, Ordering.natural().nullsLast()) 
           .compare(p1.name, p2.name, Ordering.natural().nullsLast()) 
           .result(); 
        */  
    }  
});  

This is much better, but requires some boiler plate code for the most common use case: null-values should be valued less by default. For null-fields, you have to provide an extra directive to Guava what to do in that case. This is a flexible mechanism if you want to do something specific, but often you want the default case (ie. 1, a, b, z, null).

And as noted in the comments below, these getters are all evaluated immediately for each comparison.

Sorting with Apache Commons CompareToBuilder

Collections.sort(pizzas, new Comparator<Pizza>() {  
    @Override  
    public int compare(Pizza p1, Pizza p2) {  
        return new CompareToBuilder().append(p1.size, p2.size).append(p1.nrOfToppings, p2.nrOfToppings).append(p1.name, p2.name).toComparison();  
    }  
});  

Like Guava’s ComparisonChain, this library class sorts easily on multiple fields, but also defines default behavior for null values (ie. 1, a, b, z, null). However, you can’t specify anything else either, unless you provide your own Comparator.

Again, as noted in the comments below, these getters are all evaluated immediately for each comparison.

Thus

Ultimately it comes down to flavor and the need for flexibility (Guava’s ComparisonChain) vs. concise code (Apache’s CompareToBuilder).

Bonus method

I found a nice solution that combines multiple comparators in order of priority on CodeReview in a MultiComparator:

class MultiComparator<T> implements Comparator<T> {
    private final List<Comparator<T>> comparators;

    public MultiComparator(List<Comparator<? super T>> comparators) {
        this.comparators = comparators;
    }

    public MultiComparator(Comparator<? super T>... comparators) {
        this(Arrays.asList(comparators));
    }

    public int compare(T o1, T o2) {
        for (Comparator<T> c : comparators) {
            int result = c.compare(o1, o2);
            if (result != 0) {
                return result;
            }
        }
        return 0;
    }

    public static <T> void sort(List<T> list, Comparator<? super T>... comparators) {
        Collections.sort(list, new MultiComparator<T>(comparators));
    }
}

Ofcourse Apache Commons Collections has a util for this already:

ComparatorUtils.chainedComparator(comparatorCollection)

Collections.sort(list, ComparatorUtils.chainedComparator(comparators));

If the StudentNumber is numeric it will not be sorted numeric but alphanumeric. Do not expect

"2" < "11"

it will be:

"11" < "2"

If you want to sort by report key, then student number, then school, you should do something like this:

public class ReportComparator implements Comparator<Report>
{
    public int compare(Report r1, Report r2)
    {
        int result = r1.getReportKey().compareTo(r2.getReportKey());
        if (result != 0)
        {
            return result;
        }
        result = r1.getStudentNumber().compareTo(r2.getStudentNumber());
        if (result != 0)
        {
            return result;
        }
        return r1.getSchool().compareTo(r2.getSchool());
    }
}

This assumes none of the values can be null, of course - it gets more complicated if you need to allow for null values for the report, report key, student number or school.

While you could get the string concatenation version to work using spaces, it would still fail in strange cases if you had odd data which itself included spaces etc. The above code is the logical code you want... compare by report key first, then only bother with the student number if the report keys are the same, etc.

I had the same issue and I needed an algorithm using a config file. In This way you can use multiple fields define by a configuration file (simulate just by a List<String) config)

  public static void test() {  
    // Associate your configName with your Comparator 
    Map<String, Comparator<DocumentDto>> map = new HashMap<>();
    map.put("id", new IdSort());
    map.put("createUser", new DocumentUserSort());
    map.put("documentType", new DocumentTypeSort());
    /**
      In your config.yml file, you'll have something like
      sortlist: 
        - documentType
        - createUser
        - id
    */
    List<String> config = new ArrayList<>();
    config.add("documentType");
    config.add("createUser");
    config.add("id");
    

    List<Comparator<DocumentDto>> sorts = new ArrayList<>();

    for (String comparator : config) {
        sorts.add(map.get(comparator));
    }

    // Begin creation of the list 
    DocumentDto d1 = new DocumentDto();
    d1.setDocumentType(new DocumentTypeDto());
    d1.getDocumentType().setCode("A");
    d1.setId(1);
    d1.setCreateUser("Djory");

    DocumentDto d2 = new DocumentDto();
    d2.setDocumentType(new DocumentTypeDto());
    d2.getDocumentType().setCode("A");
    d2.setId(2);
    d2.setCreateUser("Alex");

    DocumentDto d3 = new DocumentDto();
    d3.setDocumentType(new DocumentTypeDto());
    d3.getDocumentType().setCode("A");
    d3.setId(3);
    d3.setCreateUser("Djory");

    DocumentDto d4 = new DocumentDto();
    d4.setDocumentType(new DocumentTypeDto());
    d4.getDocumentType().setCode("A");
    d4.setId(4);
    d4.setCreateUser("Alex");

    DocumentDto d5 = new DocumentDto();
    d5.setDocumentType(new DocumentTypeDto());
    d5.getDocumentType().setCode("D");
    d5.setId(5);
    d5.setCreateUser("Djory");

    DocumentDto d6 = new DocumentDto();
    d6.setDocumentType(new DocumentTypeDto());
    d6.getDocumentType().setCode("B");
    d6.setId(6);
    d6.setCreateUser("Alex");

    DocumentDto d7 = new DocumentDto();
    d7.setDocumentType(new DocumentTypeDto());
    d7.getDocumentType().setCode("B");
    d7.setId(7);
    d7.setCreateUser("Alex");

    List<DocumentDto> documents = new ArrayList<>();

    documents.add(d1);
    documents.add(d2);
    documents.add(d3);
    documents.add(d4);
    documents.add(d5);
    documents.add(d6);
    documents.add(d7);
    // End creation of the list


    // The Sort 
    Stream<DocumentDto> docStream = documents.stream();

    // we need to reverse this list in order to sort by documentType first because stream are pull-based, last sorted() will have the priority
    Collections.reverse(sorts);

    for(Comparator<DocumentDto> entitySort : sorts){
        docStream = docStream.sorted(entitySort);
    }
    documents = docStream.collect(Collectors.toList());
    // documents has been sorted has you configured
    // in case of equality second sort will be used.

    System.out.println(documents);

  }

Comparator objects are really simple.

public class IdSort implements Comparator<DocumentDto> {

    @Override
    public int compare(DocumentDto o1, DocumentDto o2) {
        return o1.getId().compareTo(o2.getId());
    }
}
public class DocumentUserSort implements Comparator<DocumentDto> {

    @Override
    public int compare(DocumentDto o1, DocumentDto o2) {
        return o1.getCreateUser().compareTo(o2.getCreateUser());
    }
}
public class DocumentTypeSort implements Comparator<DocumentDto> {

    @Override
    public int compare(DocumentDto o1, DocumentDto o2) {
        return o1.getDocumentType().getCode().compareTo(o2.getDocumentType().getCode());
    }
}

Conclusion : this method isn't has efficient but you can create generic sort using a file configuration in this way.

Here is a full example comparing 2 fields in an object, one String and one int, also using Collator to sort.

public class Test {

    public static void main(String[] args) {

        Collator myCollator;
        myCollator = Collator.getInstance(Locale.US);

        List<Item> items = new ArrayList<Item>();

        items.add(new Item("costrels", 1039737, ""));
        items.add(new Item("Costs", 1570019, ""));
        items.add(new Item("costs", 310831, ""));
        items.add(new Item("costs", 310832, ""));

        Collections.sort(items, new Comparator<Item>() {
            @Override
            public int compare(final Item record1, final Item record2) {
                int c;
                //c = record1.item1.compareTo(record2.item1); //optional comparison without Collator                
                c = myCollator.compare(record1.item1, record2.item1);
                if (c == 0) 
                {
                    return record1.item2 < record2.item2 ? -1
                            :  record1.item2 > record2.item2 ? 1
                            : 0;
                }
                return c;
            }
        });     

        for (Item item : items)
        {
            System.out.println(item.item1);
            System.out.println(item.item2);
        }       

    }

    public static class Item
    {
        public String item1;
        public int item2;
        public String item3;

        public Item(String item1, int item2, String item3)
        {
            this.item1 = item1;
            this.item2 = item2;
            this.item3 = item3;
        }       
    }

}

Output:

costrels 1039737

costs 310831

costs 310832

Costs 1570019