Compare whether two lists are equal

1、 Use it directly equals() Compare

as everyone knows , When two lists have exactly the same elements and have exactly the same order , They are equal . therefore , If our business requires two list In the same order , have access to equals（） Method to check the equality ：

@Test
public void equalityCheckOfTwoLists() {
    
  
    List<Integer> list1 = Arrays.asList(1, 2, 3);
    List<Integer> list2 = Arrays.asList(1, 2, 3);
    List<Integer> list3 = Arrays.asList(2, 1, 3);
  
    assertTrue(list1.equals(list2));
    assertFalse(list1.equals(list3));
  
}

Even if list1 And projects list3 Contains the same elements {1,2,3}, But the order of elements is different , therefore list1 and list3 It's still not equal .

Be careful ：
Some businesses require that the order of elements need not be consistent What we need to do is to check whether the two lists contain the same elements , Regardless of their order in the list , So how to achieve it ？

2、 take list Compare after sorting

Processing logic mainly includes ：
（1） If two list All for null, Then return to true
（2） If one list Not empty , The other one points to null value and returns false
（3） Two list Of size（） Different , return false.

public <T extends Comparable<T>> boolean isEquals(List<T> list1, List<T> list2){
         
    if (list1 == null && list2 == null) {
    
        return true;
    }
    //Only one of them is null
    else if(list1 == null || list2 == null) {
    
        return false;
    }
    else if(list1.size() != list2.size()) {
    
        return false; 
    }
     
    //copying to avoid rearranging original lists
    list1 = new ArrayList<T>(list1); 
    list2 = new ArrayList<T>(list2);   
  
    Collections.sort(list1);
    Collections.sort(list2);      
     
    return list1.equals(list2);
}

Please note that ：
Here we create two copies of the list to ensure that the elements in the original list remain unchanged .

3、 Use Sets / contains（） Compare list

If the list has no repeating elements , We can use list Created in TreeSet, And then use equals（） Compare them ：

ublic <T extends Comparable<T>> boolean isEquals(List<T> list1, List<T> list2){
         
    if (list1 == null && list2 == null) {
    
        return true;
    }
    //Only one of them is null
    else if(list1 == null || list2 == null) {
    
        return false;
    }
    else if(list1.size() != list2.size()) {
    
        return false; 
    }
  
    Set<T> set1 = new TreeSet<>(list1);
    Set<T> set2 = new TreeSet<>(list2);
     
    return set1.equals(set2);
}

We can use it more simply contains（） Compare , No need to create Sets：

return list1.containsAll(list2) && list2.containsAll(list1);

But pay attention here If we list Duplicate element , Use contains（） perhaps Sets There will be problems in comparison . Look at the following case and you will know what happened .

List<Integer> list1 = Arrays.asList(1, 2, 3, 3);
List<Integer> list2 = Arrays.asList(3, 1, 2, 2);
  
// will return true, but actual value should be false
System.out.println(list1.isEquals(list2));

In the example above ,list1 Contains a 2 And two 3, and list2 Contains two 2 And a 3, We can see two list In fact, they are not equal , But the program will return incorrectly true. So when using this method, make sure list Elements are not repeated .

4、 Use Apache Commons Tool class

CollectionUtils It provides tools and methods for judging the equality of sets isEqualCollection, As long as we ensure that two sets are not empty, we can directly use this method to judge the equality of sets .

List<Integer> list1 = Arrays.asList(1, 2, 3, 3);
List<Integer> list2 = Arrays.asList(3, 1, 3, 2);
  
System.out.println(CollectionUtils.isEqualCollection(list1, list2)); //true

5、 Conclusion

Today we learned to check Java Whether the two lists in are equal 4 Methods , By default , When two lists have the same elements in the same order , We know they are equal . If we don't care much about the order of elements , Several other methods can be used to compare the equality of lists .