A variation of Gili’s answer to check if the comparator satisfies the requirements described in the compare method’s javadoc — with a focus on completeness and readability, e.g. by naming the variables the same as in the javadoc. Note that this is O(n^3), only use it when debugging, maybe just on a subset of your elements, in order to be fast enough to finish at all.
public static <T> void verifyComparator(Comparator<T> comparator, Collection<T> elements) {
for (T x : elements) {
for (T y : elements) {
for (T z : elements) {
int x_y = comparator.compare(x, y);
int y_x = comparator.compare(y, x);
int y_z = comparator.compare(y, z);
int x_z = comparator.compare(x, z);
// javadoc: The implementor must ensure that sgn(compare(x, y)) == -sgn(compare(y, x))
if (Math.signum(x_y) == -Math.signum(y_x)) { // ok
} else {
System.err.println("not holding: sgn(compare(x, y)) == -sgn(compare(y, x))" //
+ " | x_y: " + x_y + ", y_x: " + y_x + ", x: " + x + ", y: " + y);
}
// javadoc: The implementor must also ensure that the relation is transitive:
// ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.
if (x_y > 0 && y_z > 0) {
if (x_z > 0) { // ok
} else {
System.err.println("not holding: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0" //
+ " | x_y: " + x_y + ", y_z: " + y_z + ", x_z: " + x_z + ", x: " + x + ", y: " + y + ", z: " + z);
}
}
// javadoc: Finally, the implementor must ensure that:
// compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z.
if (x_y == 0) {
if (Math.signum(x_z) == Math.signum(y_z)) { // ok
} else {
System.err.println("not holding: compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z" //
+ " | x_y: " + x_y + ", x_z: " + x_z + ", y_z: " + y_z + ", x: " + x + ", y: " + y + ", z: " + z);
}
}
}
}
}
}
The «Comparison method violates its general contract» error is a common issue faced by Java developers who are working with sorting algorithms. This error occurs when the sorting algorithm, such as the Collections.sort() method or the Arrays.sort() method, fails to adhere to the general contract that is established for comparing objects. The general contract of a comparison method, such as the compareTo() method, requires that it be transitive, reflexive, and consistent. When the comparison method fails to follow these rules, it can result in the «Comparison method violates its general contract» error being thrown at runtime.
Method 1: Overriding the compareTo() method
To fix the Java error «Comparison method violates its general contract» by overriding the compareTo() method, you need to follow these steps:
- Implement the Comparable interface in your class:
public class MyClass implements Comparable<MyClass> {
// class implementation
}- Override the compareTo() method:
@Override
public int compareTo(MyClass other) {
// comparison logic
}- Ensure that the compareTo() method returns consistent results:
@Override
public int compareTo(MyClass other) {
if (this.field < other.field) {
return -1;
} else if (this.field > other.field) {
return 1;
} else {
return 0;
}
}- Ensure that the compareTo() method is transitive:
@Override
public int compareTo(MyClass other) {
if (this.field < other.field) {
return -1;
} else if (this.field > other.field) {
return 1;
} else {
if (this.anotherField < other.anotherField) {
return -1;
} else if (this.anotherField > other.anotherField) {
return 1;
} else {
return 0;
}
}
}- Ensure that the compareTo() method is reflexive:
@Override
public int compareTo(MyClass other) {
if (this == other) {
return 0;
}
if (this.field < other.field) {
return -1;
} else if (this.field > other.field) {
return 1;
} else {
if (this.anotherField < other.anotherField) {
return -1;
} else if (this.anotherField > other.anotherField) {
return 1;
} else {
return 0;
}
}
}- Ensure that the compareTo() method is consistent with equals():
@Override
public boolean equals(Object obj) {
// implementation
}
@Override
public int compareTo(MyClass other) {
if (this == other) {
return 0;
}
if (!(other instanceof MyClass)) {
throw new ClassCastException("Cannot compare MyClass with " + other.getClass().getName());
}
MyClass otherMyClass = (MyClass) other;
if (this.field < otherMyClass.field) {
return -1;
} else if (this.field > otherMyClass.field) {
return 1;
} else {
if (this.anotherField < otherMyClass.anotherField) {
return -1;
} else if (this.anotherField > otherMyClass.anotherField) {
return 1;
} else {
return 0;
}
}
}By following these steps, you can ensure that your compareTo() method does not violate its general contract and fix the Java error «Comparison method violates its general contract».
Method 2: Implementing the Comparator interface
How to fix Java error: Comparison method violates its general contract with Implementing the Comparator interface
When sorting a collection of objects in Java, you may encounter the error message «Comparison method violates its general contract». This error occurs when the comparison method used to sort the objects does not follow the rules of the Java Comparator interface.
To fix this error, you can implement the Comparator interface and override the compare() method. Here is an example of how to implement the Comparator interface:
import java.util.Comparator;
public class PersonComparator implements Comparator<Person> {
@Override
public int compare(Person p1, Person p2) {
if (p1.getAge() > p2.getAge()) {
return 1;
} else if (p1.getAge() < p2.getAge()) {
return -1;
} else {
return 0;
}
}
}In this example, we have created a PersonComparator class that implements the Comparator interface. The compare() method compares two Person objects based on their age.
To use this comparator to sort a collection of Person objects, you can call the Collections.sort() method and pass in the collection and the comparator:
List<Person> people = new ArrayList<>();
people.add(new Person("John", 25));
people.add(new Person("Jane", 30));
people.add(new Person("Bob", 20));
PersonComparator comparator = new PersonComparator();
Collections.sort(people, comparator);In this example, we have created a List of Person objects and added some people to it. We then create an instance of our PersonComparator and pass it to the Collections.sort() method to sort the list based on age.
By implementing the Comparator interface and following its rules, you can avoid the «Comparison method violates its general contract» error and have full control over how your objects are sorted.
Method 3: Using a stable sorting algorithm
How to fix Java error: Comparison method violates its general contract using a stable sorting algorithm
When encountering the error «Comparison method violates its general contract», it means that the comparison method used in the sorting algorithm does not follow the general contract defined in the Java documentation. One way to fix this error is to use a stable sorting algorithm, such as merge sort or insertion sort, which preserves the order of equal elements.
Here is an example implementation of merge sort that can be used to fix the error:
public static void mergeSort(Object[] arr, Comparator comparator) {
if (arr.length > 1) {
int mid = arr.length / 2;
Object[] left = Arrays.copyOfRange(arr, 0, mid);
Object[] right = Arrays.copyOfRange(arr, mid, arr.length);
mergeSort(left, comparator);
mergeSort(right, comparator);
merge(arr, left, right, comparator);
}
}
public static void merge(Object[] arr, Object[] left, Object[] right, Comparator comparator) {
int i = 0;
int j = 0;
int k = 0;
while (i < left.length && j < right.length) {
if (comparator.compare(left[i], right[j]) <= 0) {
arr[k++] = left[i++];
} else {
arr[k++] = right[j++];
}
}
while (i < left.length) {
arr[k++] = left[i++];
}
while (j < right.length) {
arr[k++] = right[j++];
}
}In this implementation, the mergeSort method recursively splits the array into halves until it contains only one element, then merges the subarrays while preserving the order of equal elements using the merge method.
To use this implementation to fix the error, simply replace the sorting algorithm in your code with mergeSort and pass in a comparator that follows the general contract. For example:
Comparator<MyObject> comparator = new Comparator<MyObject>() {
public int compare(MyObject o1, MyObject o2) {
// compare based on some field
return o1.getField().compareTo(o2.getField());
}
};
mergeSort(myArray, comparator);In this example, the comparator compares MyObject instances based on a field, which follows the general contract.
Comparison method violates its general contract, error can be very painful to debug. I recently encountered this issue in one of the production servers and it was only sometimes the error was thrown. Here I would discuss some cases that can lead to such an error.
Stacktrace
Exception in thread "main" java.lang.IllegalArgumentException: Comparison method violates its general contract! at java.util.TimSort.mergeLo(TimSort.java:777) at java.util.TimSort.mergeAt(TimSort.java:514) at java.util.TimSort.mergeCollapse(TimSort.java:441) at java.util.TimSort.sort(TimSort.java:245) at java.util.Arrays.sort(Arrays.java:1438) at java.util.Arrays$ArrayList.sort(Arrays.java:3895) at java.util.Collections.sort(Collections.java:175)
Possible Coding Mistakes
Example 1:
In the below example the last statement in compare, return 0 is never executed and the compare method returns 1 even if the integers are equal.
static void sort(Integer... ints){
List<Integer> list = Arrays.asList(ints);
Collections.sort(list, new Comparator<Integer>() {
@Override
public int compare(Integer o1, Integer o2) {
if(o1 < o2){
return -1;
} else if (o1 <= o2){
return 1;
}
return 0;
}
});
System.out.println(list);
}
Solution :
The solution to the above example is quite simple. Change the <= to <
Example 2:
In the below example two integers being equal is completely ignored.
static void sort(Integer... ints){
List<Integer> list = Arrays.asList(ints);
Collections.sort(list, new Comparator<Integer>() {
@Override
public int compare(Integer o1, Integer o2) {
if(o1 < o2){
return -1;
} else {
return 1;
}
}
});
System.out.println(list);
}
Solution
The solution to this issue is also simple. Add the else block for the equals also.
Example 3:
We can also have a case when a comparison is being done and something is changed by another thread at the same time. If we have a comparison on size of a list but that list is being updated at the same time by another thread. This case is quite hard to debug as it may fail only in very rare cases. In the below example I am updating the list while running the comparison on the same time so it also throws the contract violation error.
static void sort(){
final List<List<Integer>> list = new ArrayList<>();
for(int i= 0 ; i < 5000;i++){
list.add(new ArrayList<>());
}
Executors.newSingleThreadExecutor().execute(new Runnable() {
@Override
public void run() {
for(int i= 0 ; i < 5000;i++){
list.get(0).add(2);
list.get(21).add(2);
list.get(300).add(2);
}
}
});
Collections.sort(list, new Comparator<List<Integer>>() {
@Override
public int compare(List<Integer> o1, List<Integer> o2) {
return Integer.compare(o1.size(), o2.size());
}
});
System.out.println(list);
}
Solution
The solution to this example is not no simple. You have to make sure that the list size is not changed while the comparison is being done. It anyways doesn’t make sense to do the sorting on size because the size is changing. So even if the comparison went through without any errors, the result would not be correct.
Complete Example
Here I have created a test array that throws the error.
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class Test {
public static void main(String[] args) {
sort(2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3);
sort(3, 2, 3, 2, 1, 31);
sort(3, 2, 2, 2, 2, 3, 2, 3, 2, 2, 3, 2, 3, 3, 2, 2, 2, 2, 2, 2, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1);
sort(1, 1, 1, 1, 1, 2, 1, 1, 1);
sort(1,3);
}
static void sort(Integer... ints) {
List<Integer> list = Arrays.asList(ints);
Collections.sort(list, new Comparator<Integer>() {
@Override
public int compare(Integer o1, Integer o2) {
if (o1 < o2) {
return -1;
} else {
return 1;
}
}
});
System.out.println(list);
}
}
Output :
In the output we see that the above method doesn’t throw the exception always.So for some cases it does pass
[2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3] [1, 2, 2, 3, 3, 31] Exception in thread "main" java.lang.IllegalArgumentException: Comparison method violates its general contract! at java.util.TimSort.mergeLo(TimSort.java:777) at java.util.TimSort.mergeAt(TimSort.java:514) at java.util.TimSort.mergeCollapse(TimSort.java:441) at java.util.TimSort.sort(TimSort.java:245) at java.util.Arrays.sort(Arrays.java:1438) at java.util.Arrays$ArrayList.sort(Arrays.java:3895) at java.util.Collections.sort(Collections.java:175)
После перехода с Java 6 на 7 одна из используемых приложением библиотек стала валиться с:
java.lang.IllegalArgumentException: Comparison method violates its general contract!
Причина кроется в том, что в семерке переработана имплементация алгоритма сортировки, используемая в java.util.Arrays.sort и java.util.Collections.sort, и в случае некорректной реализации интерфейса Comparable она может генерировать данное исключение. Под некорректной понимается реализация, неконсистентная с реализацией equals() (например, если для некоторых эквивалентных объектов она возвращает значение, отличное от 0). Найти эту информацию можно в заметках о совместимости Java 7.
Выход — переписать реализацию Comparable (в моем случае — не вариант, т.к. библиотека сторонняя и закрытая) либо воспользоваться системным свойством «java.util.Arrays.useLegacyMergeSort», позволяющим переключиться на старую имплементацию сортировки (сказать по правде, не пробовал, т.к. просто вернулся на JDK 6).
The comparison method violates its general contract mistake usually affects your Java program when the comparators have specific transitivity bugs or inconsistencies. In addition, our experts confirmed the same error due to the JDK version, which can affect several commands if the program is older and the functions are new.
As a result, this guide includes the best debugging methods for the comparison method violates the general contract mistake in your document. Moreover, less experienced developers will learn how to recreate the syntax and pinpoint the exact culprit, an essential skill when debugging this aggravating error, so stick around for more.
Contents
- What Causes the Comparison Method To Violate Its General Contract?
- – Writing Basic Comparators and Commands
- – Sorting Out Multiple Long-running Tasks
- Comparison Method Violates the General Contract: Quick Solution
- – Pinpointing Transitivity Bugs in Your Comparators
- Conclusion
What Causes the Comparison Method To Violate Its General Contract?
The comparison method tends to violate and halt the general contract when developers have a Java document with comparators that include specific transitivity bugs and inconsistencies. In addition, the system may launch an identical error message due to an incorrect JDK version that does not support some commands.
This might discourage less skillful programmers and developers because the solution is only sometimes obvious. Fortunately, the complete exception message is quite descriptive because it pinpoints the command that fails so that users can quickly remove the mistake. However, understanding and implementing the solutions in your program requires attention to detail because a single value or symbol can ruin a function. As a result, our experts wrote several chapters that recreate the invalid code and block the program.
Although it is a less typical cause, our experts confirmed an identical inconsistency when sorting our 2D arrays with specific contests. The contest values have a 2D array of simple integers, but the system needs to render them correctly. As a result, the computer throws an error that blocks your program and halts further code alterations. But first, let us discover the script that recreates this mistake.
– Writing Basic Comparators and Commands
This mistake nearly always affects comparators with specific transitivity bugs, which can annoy professional developers. In addition, the bug can affect documents with simple commands, although confusing their functions and values is rare. So, we will show you an example that creates the comparators and overrides the program. This example also includes several dependencies that appear full-proof, but the program does not recognize their values.
The following example features the comparator:
public int compareTo (Object o) {
if(this == o){
return 0;
}
CollectionItem item = (CollectionItem) o;
Card card1 = CardCache.getInstance().getCard(cardId);
Card card2 = CardCache.getInstance().getCard(item.getCardId());
if (card1.getSet() < card2.getSet()) {
return -1;
} else {
if (card1.getSet() == card2.getSet()) {
if (card1.getRarity() < card2.getRarity()) {
return 1;
} else {
if (card1.getId() == card2.getId()) {
if (cardType > item.getCardType()) {
return 1;
} else {
if (cardType == item.getCardType()) {
return 0;
}
return -1;
}
}
return -1;
}
}
return 1;
}
}
Unfortunately, this is different from where our example ends because we must provide the complete exception. The exception message pinpoints the Java utilities that confuse your system, as shown below:
at java.util.ComparableTimSort.mergeHi (ComparableTimSort.java: 865)
at java.util.ComparableTimSort.mergeAt (ComparableTimSort.java: 473)
at java.util.ComparableTimSort.mergeForceCollapse (ComparableTimSort.java: 382)
at java.util.ComparableTimSort.sort (ComparableTimSort.java: 181)
at java.util.ComparableTimSort.sort (ComparableTimSort.java: 156)
at java.util.Arrays.sort (Arrays.java: 412)
at java.util.Collections.sort (Collections.java: 135)
…
As you can tell, the comparator needs adequate utilities, as confirmed by the exact Java locations. Unfortunately, this error can have other culprits.
– Sorting Out Multiple Long-running Tasks
Unfortunately, Confluence throws an identical bug when developers attempt to sort multiple long-running tasks in a single document. Again, the syntax includes several comparators that need to be clarified for your system, although their values are simple.
The following example launches a mistake:
// Sorting by name is not as potent as sorting by start time (unavailable) but better than something
// else we can think of this iteration. Changing this order later cannot raise any blockers
// in production.
Message o1Name = o1.getName();
if (o1Name == null || o1Name.getKey() == null)
return 1; // push o1 to bottom
Message o2Name = o2.getName();
if (o2Name == null || o2Name.getKey() == null)
return -1; // push o2 to bottom
return o1Name.getKey().compareTo (o2Name.getKey());
});
Although it is not apparent, the return values in this example block the comparators. We suggest reading the following chapter because they include the most sophisticated debugging methods for this error.
Comparison Method Violates the General Contract: Quick Solution
The only and most effective solution of the comparison method violates the general contract error is finding out the transitivity bugs along with their causes in your comparators before running any kinds of commands in your program. For this purpose, you can use verify transitivity commands.
Readers can learn more about this in the next example:
return 1;
This example shows the return command has a value of 1, which is different from the dependencies in the same document.
For instance, the return value in the following order is different:
//…
}
return -1;
The negative value in this example confuses the system because both dependencies must be equal. As a result, our experts provide several examples with exact values, and their functions are neat.
The following example includes several dependencies with equal values:
return 1;
}
if (card1.getSet() < card2.getSet()) {
return -1;
};
if (card1.getRarity() < card2.getRarity()) {
return 1;
}
if (card1.getRarity() > card2.getRarity()) {
return -1;
}
if (card1.getId() > card2.getId()) {
return 1;
}
if (card1.getId() < card2.getId()) {
return -1;
}
return cardType – item.getCardType(); //watch out for overflow!
As you can tell, locating transitivity bugs is critical when fixing the mistake. As a result, our experts would like to present a code that debugs your script by pinpointing the error, so keep reading for more.
– Pinpointing Transitivity Bugs in Your Comparators
This solution includes the complete code that debugs the error. So, the following code locates the bug in the comparators using the verify transitivity commands, as shown below:
{
public static <T> void verifyTransitivity(Comparator<T> comparator, Collection<T> elements)
{
for (T first: elements)
{
for (T second: elements)
{
int result1 = comparator.compare(first, second);
int result2 = comparator.compare(second, first);
if (result1 != -result2)
{
// Uncomment the following line to step through the failed case
//comparator.compare(first, second);
throw new AssertionError(“compare(” + first + “, ” + second + “) == ” + result1 +
” but swapping the parameters returns ” + result2);
}
}
}
for (T first: elements)
{
for (T second: elements)
{
int firstGreaterThanSecond = comparator.compare(first, second);
if (firstGreaterThanSecond <= 0)
continue;
for (T third: elements)
{
int secondGreaterThanThird = comparator.compare(second, third);
if (secondGreaterThanThird <= 0)
continue;
int firstGreaterThanThird = comparator.compare(first, third);
if (firstGreaterThanThird <= 0)
{
// Uncomment the following line to step through the failed case
//comparator.compare(first, third);
throw new AssertionError(“compare(” + first + “, ” + second + “) > 0, ” +
“compare(” + second + “, ” + third + “) > 0, but compare(” + first + “, ” + third + “) == ” +
firstGreaterThanThird);
}
}
}
}
}
private Comparators()
{
}
}
So, developers must invoke the Comparators.verifyTransitivity (myComparator, myCollection) command in front of the code that fails. Let us summarize this guide’s critical information in the final chapter.
Conclusion
The comparison method tends to violate and halt the general contract when developers have a Java document with comparators that include specific transitivity bugs and inconsistencies. Therefore, we wrote this debugging guide based on practical knowledge that contains the following critical points:
- Our experts confirmed an identical inconsistency when sorting our 2D arrays with specific contests
- The complete exception message includes indicators that pinpoint the bugs that affect your Java program
- Recreating the mistake with basic commands helps developers and programmers understand the main culprits
- This guide contains two full-proof solutions that require changing the dependency values
- The final solution pinpoints the transitivity bugs and removes them with a simple command
Nothing will help you better understand how to fix this mistake from your computer besides examples that make the whole process simpler. So gear up, open your Java program, find the incorrect comparators, and implement and write code like a pro.
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