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What do you think is more efficient: HashMap.containsKey(key) or HashMap.keySet().stream().anyMatch(predicate) ?
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Map is an interface, it does not make sense to speak about efficiency or performance without a specific implementation.
But let's take HashMap as one of the common implementations.
HashMap.containsKey is amortized O(1).
Map.keySet().stream().anyMatch(predicate) is O(N) as you iterate over keys. And we don't even mention all the objects created by this statement.
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What is Java stream API alternative to LambdaJ indexing? Let's say I have code like this
List<Product> products = ...
Map<Month, Product> productsOnMonths = Lambda.index(products, Lambda.on(Product.class).getMonth());
Where I know that every product has unique month attribute.
products.stream().collect(Collectors.toMap(Product::getMonth, s -> s));
The difference here is that Collectors.toMap can take a third argument that says how to merge two entries when they are the same; I don't think lambdaj offers that
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Why do we have int type in Java? Couldn't the only numeric type be double (and maybe float)? You can keep any integer number in a variable of type double.
For performance reasons and memory saving. Plus , the fact that the numbers are precise with no decimal values is very handy for a lot of cases.
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I have tried all sorts of keyword related but I keep getting it wrong.
The only word in Java that precisely meets your vague description is null. If so, it isn't a keyword, it's a literal.
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If I have an initialized array in Java and I want to do a method to one element in that array I would do
array[n].method();
but how do I do the same thing with an arraylist?
arraylist[n].method() gives me an error.
With Java use get method with ArrayList:
arraylist.get(n).method();
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I have a List<A> of data, and wanna store it to the other list(List<B>).
any suggestion to make it possible?
You can use the Collections.copy(ListA, ListB) method, or even, if you're working with ArrayLists: ListB = ListA.clone()