Java泛型到变量

Question

How can a java generics assign to variable and then pass it? I don't want to do this :

MyClass myClass = new MyClass();

if (someCondition) {
    myClass.<Foo>getDetails();
} else if (someCondition) {
    myClass.<Bar>getDetails();
} ... more conditional objects

How can I achieve this:

MyClass myClass = new MyClass();
JavaGeneric jg;
if (someCondition) {
   jg = Foo;
} else if (someCondition) {
   jg = Bar;
}

myClass.<jg>getDetails();

Is this possible? I've tried to search the documentation about java generics but there's no example like this or how to assign it on a variable, they only have examples of passing it on the method/class (T) .

Update:

The getDetails() is on the object :

public class MyClass {
   <T> void getDetails() {
      //call method that uses T...
   }
}

Answer 1

To achieve, what you are expecting, you need an interface and class should implement it.

interface JavaGeneric
{
   public String getDetails();
}

And, now implement it in classes.

class Foo implements JavaGeneric
{
     public String getDetails()
     {
           return "Foo";
     }
}

And,

class Bar implements JavaGeneric
{
     public String getDetails()
     {
           return "Bar";
     }
}

Now, create instance in your if-else and at the end call getDetails method

JavaGeneric jg;
if (someCondition) {
   jg = new Foo();
} else if (someCondition) {
   jg = new Bar();
}

jg.getDetails ();

==Update==

I'm not quite sure, what exactly you are trying to achieve. But, assuming you need to generalized the return type of getDetails method.

interface JavaGeneric<T>
{
   public T getDetails();
}

And, then while implementing

class Foo<T> implements JavaGeneric<T>
{
     public T getDetails()
     {
          // your code
     }
}

Answer 2

interface JavaGeneric {
public String getDetails(); //also you can have default methods implementation here.
}

class Foo implements JavaGeneric {
 public String getDetails(){
   return "Foo Details";
 }
}


class Bar implements JavaGeneric {
 public String getDetails(){
   return "Bar Details";
 }
}


// Somewhere in code

JavaGeneric jg;
if (someCondition) { //lets say this is false
   jg = Foo;
} else if (someCondition) { // lets say this is true
   jg = Bar;
}

jg.getDetails(); //we will get "Bar Details"

Answer 3

Just call

myClass.getDetails()

Due to type erasure there is no difference between

myClass.<T>getDetails()

And

myClass.<R>getDetails()

Answer 4

It seems that you are declaring getDetails() as generic only because you want to use the type variable T in the method body. The type variable T is not part of the method's signature or of its return type.

This is usually not needed. Depending on what your method actually does, maybe you can remove the <T> from the method declaration and replace T with ? everywhere in the message body.

Answer 5

If you want to pass a type from a variable to a method, use a Class<?> parameter:

public class MyClass {
   void getDetails(Class<?> cls) {
      //...
   }
}

Then you just need an object of type Class<?> :

MyClass myClass = new MyClass();
Class<?> jg;
if (someCondition) {
   jg = Foo.class;
} else if (someCondition) {
   jg = Bar.class;
}

myClass.getDetails(jg);

Generics only exist at compile time due to type erasure , so it's impossible to pass a generic type parameter from a run-time variable. It's not clear what you're actually doing with the type in getDetails , but if you can't make it work with a Class<?> object, you'll need to stick with the code in your first example and have a separate method call for each explicit type.