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?
Do you see anything wrong with the code?
Yes. Why are you adding the three fields together before you compare them?
I would probably do something like this: (assuming the fields are in the order you wish to sort them in)
@Override public int compare(final Report record1, final Report record2) {
int c;
c = record1.getReportKey().compareTo(record2.getReportKey());
if (c == 0)
c = record1.getStudentNumber().compareTo(record2.getStudentNumber());
if (c == 0)
c = record1.getSchool().compareTo(record2.getSchool());
return c;
}
(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));
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();
}
}
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 + '\'' +
'}';
}
}
}