java subclass have method to return its class
Ok, maybe this is a stupid question. But i'm just wondering if this can be done in java.
abstract public class ParentClass<T> {
abstract public T getTest();
}
in the subclass
public class SubClass extends ParentClass<MyObject> {
public MyObject getTest开发者_JAVA百科() {
// I can return the object with class MyObject
return null;
}
}
My question is can I return the class type in the child method? I mean, is it can be done by adding some code in the ParentClass, so I can do this below?
For example
public class Sub1Class extends parentClass<Object1> {
public Object1 getTest() { }
// I want to have a method that return it's class in the superclass
public Sub1Class getItClassObject() { }
}
other example
public class Sub2Class extends parentClass<Object2> {
public Object2 getTest() { }
// I want to have a method that return it's class in the superclass
public Sub2Class getItClassObject() { }
}
one example again
public class Sub3Class extends parentClass<Object3> {
public Object3 getTest() { }
// I want to have a method that return it's class in the superclass
public Sub3Class getItClassObject() { }
}
if you see, method getItClassObject in Sub1Class, Sub2Class and Sub3Class will follow it's class. But I don't want to add same method for every subclass, just want to add some code (if feasible) in the ParentClasss, so in the subclass, I just can call getItClassObject directly without write all the code in every subclass.
Usually I add method in ParentClass like this.
abstract public class ParentClass<T> {
abstract public T getTest();
public Object getItClassObject() { }
}
so in the subclass I just instance the class, but I have to cast again :(
Sub1Class sub1Class = new Sub1Class();
Sub1Class after1Cast = (Sub1Class) sub1Class.getItClassObject();
Sub2Class sub2Class = new Sub2Class();
Sub2Class after2Cast = (Sub2Class) sub2Class.getItClassObject();
I think it cannot be done in java. But I don't know if there is a clue to solve this. Thanks
This is what you want I think. The following compiles:
abstract class A {
public abstract A getA();
}
class B extends A {
// Declared to return a B, but it still properly overrides A's method
@Override
public B getA() {
return new B();
}
}
class C extends A {
// Declared to return a B, but it still properly overrides A's method
@Override
public C getA() {
return new C();
}
}
As you can see, A declares that the getA()
method returns an A
. But, you can restrict the return type in subclasses as shown.
I'm not sure if I understand your intent correctly, but I think the built-in Object.getClass() method will do what you want. Given classes defined as:
public abstract class ParentClass<T> {
public abstract T getTest();
}
class SubClassString extends ParentClass<String> {
public String getTest() {
return "";
}
}
class SubClassInteger extends ParentClass<Integer> {
public Integer getTest() {
return Integer.valueOf(0);
}
}
getClass() will return the correct run-time class
public static void main(String[] args) {
SubClassString subString = new SubClassString();
// displays "class SubClassString"
System.out.println(subString.getClass());
SubClassInteger subInteger = new SubClassInteger();
// displays "class SubClassInteger"
System.out.println(subInteger.getClass());
ParentClass<?> parentInstance = new SubClassInteger();
// displays "class SubClassInteger"
System.out.println(parentInstance.getClass());
}
The only way I can think of is by telling the parent class what the subclass is when you extend it (just like you did with 'T'). Eg:
public abstract class ParentClass<T,U> {
abstract public T getTest();
abstract public U getItClassObject();
}
They you define your subclass like so:
public class Sub1Class extends ParentClass<Object1,Sub1Class> {
public Object1 getTest() { }
public Sub1Class getItClassObject() { }
}
Then you can do what you want without the typecast:
Sub1Class sub1Class = new Sub1Class();
Sub1Class after1Cast = sub1Class.getItClassObject();
If your objects have no-arg constructors (or some consistent form of constructor across all of them), you can use reflection to do it. Some pseudocode would be
public class MyClass {
public MyClass instantiateType() {
Class<?> actualClass = getClass();
return actualClass.newInstance();
}
}
This is using the runtime type of the class, so subclasses will return their type. This works only for a no-arg constructor though.
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