{"id":16552,"date":"2022-10-20T00:47:51","date_gmt":"2022-10-19T19:17:51","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-why-this-declaration-is-correct-in-java\/"},"modified":"2022-10-20T00:47:51","modified_gmt":"2022-10-19T19:17:51","slug":"solved-why-this-declaration-is-correct-in-java","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-why-this-declaration-is-correct-in-java\/","title":{"rendered":"[Solved] Why this declaration is correct in Java"},"content":{"rendered":"

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The type of a variable and the type of the object the variable refers to do not need to be the same, so long as the type of the object is compatible<\/em> with the type of the variable. So what does it mean to be compatible?<\/p>\n

It means that the object’s type has an “is a” relationship with the variable’s type. ArrayList<String><\/code> “is a” List<String><\/code>, because ArrayList<\/code> implements the List<\/code> interface. That is, ArrayList<\/code> has all of the methods that List<\/code> defines, and declares (by saying implements List<\/code>) that it conforms to the definition of those methods.<\/p>\n

This is a fundamental concept in object-oriented programming called polymorphism<\/em><\/a>. That is, we can say that there is a thing called a List<\/code>, and we can write code that works with List<\/code>s, without that code knowing that the actual List<\/code>s it’s working with are ArrayList<\/code>s or LinkedList<\/code>s or any other concrete type<\/em>.<\/p>\n

In this case, it’s with an interface, but the same concept applies in object type hierarchies as well. If you have class Animal<\/code>, you might have a more specific subclass (Dog<\/code>). Anything that works with the Animal<\/code> type can work with Dog<\/code>s, because Dog<\/code> is an<\/em> Animal<\/code>.<\/p>\n<\/p><\/div>\n

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solved Why this declaration is correct in Java <\/p>\n","protected":false},"excerpt":{"rendered":"

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