Java Arrays.equals() returns false for two dimensional arrays

Question

I was just curious to know - why does Arrays.equals(double[][], double[][]) return false? when in fact the arrays have the same number of elements and each element is the same?

For example I performed the following test.

double[][] a,  b; int size =5;  a=new double[size][size]; b=new double[size][size];  for( int i = 0; i < size; i++ )     for( int j = 0; j < size; j++ ) {         a[i][j]=1.0;         b[i][j]=1.0;     }  if(Arrays.equals(a, b))     System.out.println("Equal"); else     System.out.println("Not-equal"); 

Returns false and prints "Not-equal".

on the other hand, if I have something like this:

double[] a,  b; int size =5;  a=new double[size]; b=new double[size];  for( int i = 0; i < size; i++ ){     a[i]=1.0;     b[i]=1.0; }   if(Arrays.equals(a, b))     System.out.println("Equal"); else     System.out.println("Not-equal"); 

returns true and prints "Equal". Does the method only work with single dimensions? if so, is there something similar for multi-dimensional arrays in Java?

Answer 1

Use deepEquals(Object[], Object[]).

Returns true if the two specified arrays are deeply equal to one another.

Since an int[] is an instanceof Object, an int[][] is an instanceof Object[].

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As to why Arrays.equals doesn't "work" for two dimensional arrays, it can be explained step by step as follows:

For arrays, equals is defined in terms of object identity

System.out.println(     (new int[] {1,2}).equals(new int[] {1,2}) ); // prints "false" 

This is because arrays inherit their equals from their common superclass, Object.

Often we really want value equality for arrays, of course, which is why java.util.Arrays provides the static utility method equals(int[], int[]).

System.out.println(     java.util.Arrays.equals(         new int[] {1,2},         new int[] {1,2}     ) ); // prints "true" 

Array of arrays in Java

• An int[] is an instanceof Object
• An int[][] is an instanceof Object[]
• An int[][] is NOT an instanceof int[]

Java doesn't really have two dimensional arrays. It doesn't even really have multidimensional arrays. Java has array of arrays.

java.util.Arrays.equals is "shallow"

Now consider this snippet:

System.out.println(     java.util.Arrays.equals(         new int[][] {             { 1 },             { 2, 3 },         },         new int[][] {             { 1 },             { 2, 3 },         }     ) ); // prints "false" 

Here are the facts:

• Each argument is an Object[]
  • The element at index 0 is an int[] { 1 }
  • The element at index 1 is an int[] { 2, 3 }.
• There are two Object[] instances
• There are four int[] instances

It should be clear from the previous point that this invokes Arrays.equals(Object[], Object[]) overload. From the API:

Returns true if the two specified arrays of Objects are equal to one another. The two arrays are considered equal if both arrays contain the same number of elements, and all corresponding pairs of elements in the two arrays are equal. Two objects e1 and e2 are considered equal if (e1==null ? e2==null : e1.equals(e2)).

Now it should be clear why the above snippet prints "false"; it's because the elements of the Object[] arrays are not equal by the above definition (since an int[] has its equals defined by object identity).

java.util.Arrays.deepEquals is "deep"

In contrast, here's what Arrays.deepEquals(Object[], Object[]) does:

Returns true if the two specified arrays are deeply equal to one another. Unlike the equals(Object[],Object[]) method, this method is appropriate for use with nested arrays of arbitrary depth.

System.out.println(     java.util.Arrays.deepEquals(         new int[][] {             { 1 },             { 2, 3 },         },         new int[][] {             { 1 },             { 2, 3 },         }     ) ); // prints "true" 

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On Arrays.toString and Arrays.deepToString

It's worth noting the analogy between these two methods and what we've discussed so far with regards to nested arrays.

System.out.println(     java.util.Arrays.toString(         new int[][] {             { 1 },             { 2, 3 },         }     ) ); // prints "[[I@187aeca, [I@e48e1b]"  System.out.println(     java.util.Arrays.deepToString(         new int[][] {             { 1 },             { 2, 3 },         }     ) ); // prints "[[1], [2, 3]]" 

Again, the reasoning is similar: Arrays.toString(Object[]) treats each element as an Object, and just call its toString() method. Arrays inherit its toString() from their common superclass Object.

If you want java.util.Arrays to consider nested arrays, you need to use deepToString, just like you need to use deepEquals.