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I try to create a tuple class that allows a tuple-like structure in Java. The general type for two elements in tuple are X and Y respectively. I try to override a correct equals for this class.

Thing is, I know Object.equals falls into default that it still compares based on references like "==", so I am not so sure I can use that. I looked into Objects and there is an equals() in it. Does this one still compare on references, or it compares on contents?

Quickly imagined the return statement as something like:

return Objects.equals(compared.prev, this.prev) && Objects.equals(compared.next, this.next);

where prev and next are elements of tuple. Would this work?

jfcjohn
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4 Answers4

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The difference is the Objects.equals() considers two nulls to be "equal". The pseudo code is:

  1. if both parameters are null or the same object, return true
  2. if the first parameter is null return false
  3. return the result of passing the second parameter to the equals() method of the first parameter

This means it is "null safe" (non null safe implementation of the first parameter’s equals() method notwithstanding).

Bohemian
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    Point 2 is documented incorrectly. The code is : `return (a == b) || (a != null && a.equals(b));` so it actually is *if **first** parameter is `null` return `false`*. If the second one is `null`, your `equals` gets called with `null` parameter. Which might be of interest if you've implemented your `equals` wrong, depending on `Objects.equals` one null too far, just like I did. – Agent_L Jan 11 '18 at 15:31
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this is literal code from java source: as you can see, @Agent_L is right this is literal code from java source: as you can see, @Agent_L is right

Marek Kamiński
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The answer to your question "Does this one [Objects.equals] still compare on references, or it compares on contents?" - Objects.equals does some comparisons on the references but it expects the first argument's class to implement equals() in which the comparison of contents is done as well as on reference.

Your second question about the implementation of equals in your tupple-like class having prev and next as its tupple attributes the answer is: your suggested implementation would work only if prev and next are primitives or if their type implements equals properly. So if prev for example is of type Foo, then you can use Objects.equals to test the two Foo's only if class Foo implements equals as expected.

inor
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Objects.equals just calls it's first arguments .equals method. In java, if you want to be able to test for equality in instances of a class you made, then you have to override the equals method. instance.equals() only uses == if that instances type doesn't override the equals method.

Natecat
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