Equals VS Arrays.Equals in Java

equals vs Arrays.equals in Java

array1.equals(array2) is the same as array1 == array2, i.e. is it the same array. As @alf points out it's not what most people expect.

Arrays.equals(array1, array2) compares the contents of the arrays.

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Similarly array.toString() may not be very useful and you need to use Arrays.toString(array).

Java, what's the difference between == and equals

Yes, == with objects is a reference comparison* (checks if the operands are references to the same object), while equals is whatever the class involved defines it to mean (within the requirements documented for equals). Some classes define equals as being the same as ==, including Java's arrays. (Because they don't override the default implementation from Object.equals, which uses ==.)

If you want to compare Java arrays based on the equality of their contents, you use Arrays.equals instead.

Your experiment would have worked if you used a class that defined equals in a useful way, you were just unlucky picking arrays. It's a bit tricky to find a class in the JVM to use for this experiment, because so many either don't implement equals (like arrays) or could be confusing because there are several immutable classes which may reuse instances (although not if you explicitly use new; but I don't want to go down a path of having you use new with something you probably shouldn't, like String; more on that here). I'm going to give up on picking a good example and use the slightly old class SimpleDateFormat:

DateFormat a = new SimpleDateFormat("yyyy-MM-dd");
DateFormat b = new SimpleDateFormat("yyyy-MM-dd");
System.out.println(a == b ? "== Same" : "== Different");
System.out.println(a.equals(b) ? "equals Same" : "equals Different");

That outputs

== Different
equals Same

because SimpleDateFormat defines equals to check that the other object is also a SimpleDateFormat with the same formatting.

Live example

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Re your comment on the question:

I have someone the answer points but I only get the == part, if .equals is checking the content, how come the code didnt print "same" for the second if

Because equals doesn't, necessarily, check content. It only does if a class overrides the default Object.equals (which just uses ==) and implements a content check. Arrays don't. (One could argue that they should have, but they don't.) Other classes, like SimpleDateFormat and String and Integer and HashMap do.

In essence: == is always a reference comparison. equals may or may not be a contents comparison, depending on what class you're using it on.

So for instance, say we have this class:

class Example1
{
    private int content;

    Example1(int content) {
        this.content = content;
    }

    public static void main (String[] args) throws java.lang.Exception
    {
        Example1 a = new Example1(42);
        Example1 b = new Example1(42);
        System.out.println(a == b ? "== Same" : "== Different");
        System.out.println(a.equals(b) ? "equals Same" : "equals Different");
    }
}

Since that class doesn't override equals, you'll get the same answer ("Different") for both == and equals. Live Example.

But if we override equals to define what it would mean for two instances to be equal (in this case: because they have the same content):

class Example2
{
    private int content;

    Example2(int content) {
        this.content = content;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null || !obj.getClass().equals(this.getClass())) {
            return false;
        }
        Example2 other = (Example2)obj;
        return this.content == other.content;
    }

    @Override
    public int hashCode() {
        return this.content;
    }

    public static void main (String[] args) throws java.lang.Exception
    {
        Example2 a = new Example2(42);
        Example2 b = new Example2(42);
        System.out.println(a == b ? "== Same" : "== Different");
        System.out.println(a.equals(b) ? "equals Same" : "equals Different");
    }
}

Now equals says two instances with the same content are the same, because the class defines what that means. Live Example. (Also note that when overriding equals, you must override hashCode, which is why I've done so above.)

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* More generally, == tests if the values of its operands are the same. The value in the case of reference types is an object reference, and two object reference values are only the same when they refer to the same object are are only different when they refer to different objects (or one of them is null; not referring to an object at all).

Java why is this array equals printing false?

You are comparing the 2 address.
Maybe you can use the java.util.Arrays.equals(int[] a, int[] a2) method that returns "true" if the two specified arrays of ints are equal to one another.
Two arrays are equal if they contain the same elements in the same order.Two array references are considered equal if both are null.

Example :

int[] somea = {2};
int[] somea2 = {2};
System.out.println(Arrays.equals(somea , somea2 ));

You can read more from here: https://www.tutorialspoint.com/java/util/arrays_equals_int.htm

Why aren't these two arrays equal?

Isn't the result supposed to be true?

No. You're calling equals on two different array references. Arrays don't override equals, therefore you get reference equality. The references aren't equal, therefore it returns false...

To compare the values in the arrays, use Arrays.equals(char[], char[]).

System.out.println(Arrays.equals(d, e));

How to compare arrays without using .equals and with a false boolean

Instead of immediately returning when they are equal, check for inequalities and if there's one, then return false. If the for loop finishes without returning, then you can safely return true.

if (arrayOne.length == arrayTwo.length) {

    for (int i = 0; i < arrayOne.length; i ++) {

        if (arrayOne [ i ] != arrayTwo [ i ] ) {

            return false; 
        }

    }

    return true;

}
return false;

What is the runtime of equals() in java.util.Arrays?

As title stated, what's the runtime of default equals() in java.util.Arrays?

The default equals could mean Object.equals. The Arrays.equals() is usually what you really want.

For example if it's comparing two int[], does it loop through every element in the array, so O(n)?

yes, thats what the source suggests.

And for all default equals() in java, can we assume that the runtime is always O(n)?

For some collections this is true, but for Tree collections it can be O(n log n). The worst case for HashMap is O(N^2) For non-collections n has no meaning.

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