I am trying to write a method that takes in a list of numeric values – eg List<Integer>, List<Float>, List<Double> etc – and give me the average. These are the errors I get: Operator ‘+’ cannot be applied to ‘capture<? extends java.lang.Number>’, ‘capture<? extends java.lang.Number>’ Answer OptionalDouble average() Where, OptionalDouble is a container object which may or may not contain
Tag: generics
How can a interface Stream extend Base stream interface having type bounds just as the stream interface i.e. Stream ?
I was going through Streams Documentation and had observed that Stream is a typed interface that extends Base Stream with types as T and again Stream <T> How can a interface Stream can extend Base stream interface having type bounds just as the stream interface i.e. Stream <T>? Answer Why couldn’t it? This is a trick used in many places,
Instantiate generic classes with different constructor arguments
I have two similar classes, Foo and Bar And I’ve got two methods in another class that creates a Set of Foo (1st method) and Bar (2nd method) and are pretty much the same. The first one: And the second one: As you can see, both methods are pretty much the same so I thought I could use generics to
Using generics with custom object in method parameters
I have 2 classes as below :: And I have a method in a service class : Now I need to pass ItemA or ItemB based upon some conditions in the caller. The return type doesn’t change in either case. How I can do that using generics or wild cards. This is what I am trying to achieve : Is
Generic type-casting method returning an Optional object
I have a method: That allows me to invoke it like so: And this is great. Except, now, I want load(…) to return an Optional<???> so that the interaction would look something like: What changes I need to make to load(…) method to make it capable of returning an Optional of <T>? Answer You can do it by declaring something
An issue with Generic type parameters and Inheritance
Couldn’t concretize the title, sorry for that. For example, we have a Parent class and a Child class: And a class that can contain Parent’s child in its field called a: Let’s create an instance of that class and try to assign a value to a: So far, so good. But let’s say we want to move this assignation to
Passing generic types inner class
Here is following example: Is there a way just to pass the generic <String> one time? Actually the inner class should already know its type. Answer That is a limitation of the Java compiler. Although it looks obvious in this case that the type parameter must be String, inferring that is not as easy as it seems to be: The
Java generics input type vs returned type
I’m learning about generics and am slightly confused. I’m confused about the difference between the input type and returned type for a class using generics. We are pushing in an int value of 0 and 2. But the first print statement will print “Integer”. If the stack is declared with Integer as its generic type, why are we able to
Generic Method in Java with out parameters
I am trying to implement something similar to EventBus implemented in eshopOnContainers. Is it Possible to define a method like this in java & read the meta data about T and TH at runtime ? There can be multiple classes extending IntegrationEvent(e.g. PriceChangedEvent) & we should be able to Identify the exact class name at runtime. Answer You can pass
DownCasting in generics in java
I am stuck in generics downcasting. Because I learn type erasure. When code compiles all parameterized types are converted to the upper bound if the bound is not defined then it changes to object. Gen Class GenDemo Class String str = strob.getob(); is converted to String implictly. how JVM converted strob.getob() to String. From where JVM found the strob.getob() is