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Your's Deeply - Why Arrays.deepEquals When We Have Arrays.equals

While everybody would naturally accept the following lines of code on grounds of reference equality and value equality and that String and wrappers override the equals method, it takes some effort at first to accept the behavior of Arrays.equals and Arrays.deepEquals

Reference and Value equality
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Object obj1=new Object();
Object obj2=new Object();
      
String hello1=new String("hello");
String hello2=new String("hello");
      
System.out.println(hello1.equals(hello2)); //returns true
System.out.println(hello1==hello2); //returns false
      
System.out.println(obj1.equals(obj2)); //returns false
System.out.println(obj1==obj2); //returns false

Boring Stuff :-(

First, the similarities

Both Arrays.equals and Arrays.deepEquals are similar in certain behaviors as

  1. If they are the same object (reference equality), they return true
  2. If either of the compared objects are null, then return false
  3. If the array lengths are not equal, then return false
  4. They care about order (position)

Next, the differences

Arrays.equals is really just skin deep

Opening up the souce, we could see that the lousy Array.equals just does this

Arrays.equals
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 for (int i=0; i<length; i++) {
     Object o1 = a[i];
     Object o2 = a2[i];

     if (!(o1==null ? o2==null : o1.equals(o2)))
         return false;
 }

So, it just loops through the given arrays, does a equals on each of the pairs. This means that if you are passing in a String/Wrapper array or any other arrays whose equals method is overridden, then they are equal. Non-equals-overridden classes (derived from Object) will return false.

Arrays.deepEquals looks really deep

From the source, we could understand that Arrays.deepEquals

  1. Loops through the input arrays, gets each pair
  2. Analyses the type of each pair
  3. Delegates the equal deciding logic to one of the overloaded Arrays.equals if they are one of the primitive arrays
  4. Delegates recursively to Arrays.deepEquals if it is an Object array
  5. Calls the respective object’s equals, for any other object
Arrays.deepEquals
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for (int i = 0; i < length; i++) {
            Object e1 = a1[i];
            Object e2 = a2[i];

            if (e1 == e2)
                continue;
            if (e1 == null)
                return false;

            // Figure out whether the two elements are equal
            boolean eq;
            if (e1 instanceof Object[] && e2 instanceof Object[])
                eq = deepEquals ((Object[]) e1, (Object[]) e2);
            else if (e1 instanceof byte[] && e2 instanceof byte[])
                eq = equals((byte[]) e1, (byte[]) e2);
           
           
            else
                eq = e1.equals(e2);

            if (!eq)
                return false;
        }
        return true;

Now, the Awesome stuff !!

Equals not overridden (nested and non-nested)

While doing an Arrays.equals for nested or non-nested ‘non-overridden equals’ objects, it is safe to assume that if Arrays.equals return false, then Arrays.deepEquals also return false.

Non-Nested

So, given two arrays

Non-overridden Non-nested
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private YourClass[] equalsNotOverriddenArrayNonNested1={new YourClass(), new YourClass()};
private YourClass[] equalsNotOverriddenArrayNonNested2={new YourClass(), new YourClass()};
  

where YourClass is simply

YourClass.java
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public class YourClass {
}

then the following assertions are true

Non-overridden Non-nested assertions
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assertFalse(Arrays.equals(equalsNotOverriddenArrayNonNested1, equalsNotOverriddenArrayNonNested2));
assertFalse(Arrays.deepEquals(equalsNotOverriddenArrayNonNested1, equalsNotOverriddenArrayNonNested2));

Nested

Also given two arrays,

Non-overridden Nested
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private Object[] equalsNotOverriddenArrayNested1={new YourClass(), new YourClass[]{new YourClass()}};
private Object[] equalsNotOverriddenArrayNested2={new YourClass(), new YourClass()};

the following assertions are true

Non-overridden Nested assertions
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 assertFalse(Arrays.equals(equalsNotOverriddenArrayNested1, equalsNotOverriddenArrayNested2));
  assertFalse(Arrays.deepEquals(equalsNotOverriddenArrayNested1, equalsNotOverriddenArrayNested2));
  

Equals overridden

Non-Nested

While doing an Arrays.equals for non-nested ‘overridden equals’ objects, it can be said that if Arrays.equals is true, then Arrays.deepEquals also return true.

Given two String arrays

Overridden Non-nested
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private Object[] equalsOverriddenArrayNonNested1={"101","201"};
private Object[] equalsOverriddenArrayNonNested2={"101","201"};

the following assertions are true

Overridden Non-nested assertions
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assertTrue(Arrays.equals(equalsOverriddenArrayNonNested1, equalsOverriddenArrayNonNested2));
assertTrue(Arrays.deepEquals(equalsOverriddenArrayNonNested1, equalsOverriddenArrayNonNested2));

since they are just one-to-one equals call on each pair.

Nested

Interesting scenario : While doing an Arrays.equals for nested ‘overridden equals’ objects, if Arrays.equals is false, then Arrays.deepEquals need not be false.

Consider two Object arrays which has two values - a String and a String array

Overridden Nested
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private Object[] equalsOverriddenArrayNested1={new String("hello"), new String[]{new String("hello")}};
private Object[] equalsOverriddenArrayNested2={new String("hello"), new String[]{new String("hello")}};

you can notice that the Arrays.equals return false while Arrays.deepEquals return true

Overridden Nested assertions
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assertFalse(Arrays.equals(equalsOverriddenArrayNested1, equalsOverriddenArrayNested2));
assertTrue(Arrays.deepEquals(equalsOverriddenArrayNested1, equalsOverriddenArrayNested2));

The result for Arrays.deepEquals is logical since each (from the source), the method loops through each pair of elements, checks whether it is an array type and calls deepEquals on each of the pair. If it is an non-array type, then it just calls the equals on the object.

However, the result of Arrays.equals is tricky but at the same time obvious. The Arrays.equals method blindly calls equals on each pair and since the second arguments String[] are of Object type (whose equals is not overridden), it checks for reference equality and fails !!

The entire testcase can be found below :

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