The equals method implements an equivalence relation on non-null object references: The equals contract is little more but a formalization of what we saw above. Conversely, if we leave anything out, we no longer have a meaningful equality. Yes, any way we can make up that compares things and has the three properties above, could be how we determine whether those things are equal. Because any relation that has the three properties above can be called an equality. No wait, don’t leave! That’s already all we need. That was an exercise in futility, right? Not so! We just worked through some basic algebraic properties of equivalence relations. Again, this is obvious in our laptop example. This one is more interesting: If we have three things and the first and second are equal and the second and third are equal, then the first and third are also equal.Clearly if my laptop is equal to yours, yours is equal to mine. There is another, which is not much more inspiring: If one thing is equal to another, the other is also equal to the first.One property is so trivial that it is hardly worth mentioning: Each thing is equal to itself.Let’s say we compare laptops and consider them equal if they have the same hardware specifications. It might help to think about it as we encounter it in our daily lives. We will look at the formal definition in a moment but let’s first discuss some properties of equality. (This is also the point where hashCode comes into play.)Īny implementation of equals must adhere to a specific contract or the class’s equality is ill-defined and all kinds of unexpected things happen. The variable contains is true because, while the instances of "b" are not identical, they are equal. asList ( "a", "b", "c" ) boolean contains = list. Many data structures, most notably Java’s own collection framework, use equals to check whether they contain an element.įor example: List list = Arrays. For strings, for example, it compares the character sequence and for dates it makes sure that both point to the same day. The implementation in Object checks identity (note that identical variables are equal as well), but many classes override it with something more suitable. The equals method is defined in Object and since all classes inherit from it, all have that method. This is checked with equals.īut what does “the same value” mean? It is, in fact, the implementation of equals that determines “sameness”. If two variables reference the same value, they are equal.
This is checked with =.Ī variable’s Equality is defined by the value it references. If two variables hold the same reference they are identical. equals (other ) Ī variable’s Identity (also called Reference Equality) is defined by the reference it holds.
In Java terms, they are equal, which is checked with equals: String some = "some string" String other = "some string" boolean equal = some.
(We’ll ignore String interning in this article if this bugs you, assume every string literal were wrapped in a new String(.).)īut they do have some relationship as they both “have the same value”. Now, some and other point to different instances and are no longer identical, so identical is false. What about this one? String some = "some string" String other = "some string" boolean identical = some = other In Java we say some and other are identical and, accordingly, identical returns a Boolean value that is true. Here we have only one String instance and some and other both reference it. What about these two? String some = "some string" String other = some boolean identical = some = other We have two strings and they are obviously different. Have a look at this piece of code: String some = "some string" String other = "other string"
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