1. Overview
In Java, when we want to compare two byte arrays, we can get an unexpected result if we perform the comparison improperly.
So, in this quick tutorial, we’ll learn the proper way to compare two arrays in value.
2. Introduction to the Problem
An example can explain the problem quickly. Let’s say we have a string:
final static String INPUT = "I am a magic string.";
Now, we get two byte arrays from the string above via the String.getBytes() method:
final static byte[] ARRAY1 = INPUT.getBytes();
final static byte[] ARRAY2 = INPUT.getBytes();
Apparently, if we compare ARRAY1 and ARRAY2, we expect the two arrays to be equal in value since they are created from the same input string.
So next, let’s see what common mistakes we may make and figure out the proper way to do the comparison.
For simplicity, we’ll use unit test assertions to verify if each comparison approach returns the expected result.
3. The == Operator and the equals() Method Aren’t Good Choices
In Java, == is called the “equal to” operator. So first, let’s try to compare the two arrays using the == operator:
assertFalse(ARRAY1 == ARRAY2);
The simple test above passes when we run it. That is to say, ARRAY1 == ARRAY2 returns false, even if the elements in two arrays are the same in value — which is not our desired result. This is because the == operator compares the memory addresses of the two arrays rather than their contents. This means that two arrays can have the same contents, but they are different objects. So, the == operator will return false even if the two arrays are equivalent.
We may have learned the difference between the == operator and the equals() method in Java: == does the reference equality check, but the equals() method performs the value equality check.
As our target is to compare two arrays’ values, let’s create another simple test to compare ARRAY1 and ARRAY2 using the equals() method:
assertFalse(ARRAY1.equals(ARRAY2));
If we run this test, it passes, too. It means the equals() method doesn’t give us the expected result, either. Next, let’s figure out why equals() doesn’t do the job right.
When we call ARRAY1.equals(ARRAY2), the Object class’s equals() method is actually called. So, let’s have a look at the implementation of the Object class’s equals() method:
public boolean equals(Object obj) {
return (this == obj);
}
As we can see, Object‘s equals() method internally compares two objects using the == operator. When we compare arrays, == and equals() are the same. Therefore, they both perform reference equality checks.
4. Using the Arrays.equals() Method
Now, we understand neither the == operator nor the equals() method is the right way to check the value equality of two arrays. But, value comparison of two arrays is a pretty common operation in Java programming. Therefore, the Java standard library has provided the Arrays.equals() method to do the job:
assertTrue(Arrays.equals(ARRAY1, ARRAY2));
The test passes if we give it a run. So, Arrays.equals() returns the expected result of comparing our two byte arrays.
Moreover, Arrays.equals() works for other types of arrays. We should use the Arrays.equals() method for value equality checks of all array types.
Finally, let’s see another value comparison example of two String arrays:
String[] strArray1 = new String[] { "Java", "is", "great" };
String[] strArray2 = new String[] { "Java", "is", "great" };
assertFalse(strArray1 == strArray2);
assertFalse(strArray1.equals(strArray2));
assertTrue(Arrays.equals(strArray1, strArray2));
In the code above, strArray1 and strArray2’s contents are the same. The test result shows that == and equals() report false, but using the Arrays.equals() method gives the expected result.
5. Conclusion
In this article, we’ve discussed the common pitfalls when we compare the contents of two arrays. Also, we’ve explored the correct way to compare two byte arrays’ values.
Both the == operator and the equals() method perform reference equality checks for arrays. If we need to compare two arrays’ values, the Arrays.equals(array1, array2) method is the right way to go.
As usual, all code snippets presented here are available over on GitHub.
