Thread: covariant return type

public class MyMainClass {
public static void main (String[] args) {
MySuperClass msup=new MySuperClass();
MySubClass msub=new MySubClass();
//msup.a=10;
msub.a=5;
//System.out.println(msup.a +" "+ msub.a);
WithMethod wm=new WithMethod();
MySuperClass msc=new MySuperClass();
msc=wm.theMethod(msub);
System.out.println("Returned value : " + msc.a);
}
}

class MySuperClass {
int a;
}

class MySubClass {
int a;
}

class WithMethod {
public MySuperClass theMethod(MySubClass mscl) {
mscl.a+=9;
return mscl;
}
}


Fails with the error :

C:\Documents and Settings\Administrator\Desktop\Java Programs>javac MyMainClass.
java
MyMainClass.java:26: incompatible types
found : MySubClass
required: MySuperClass
return mscl;
^
1 error

why is covariant return not working ???

incompatible types
found : MySubClass
required: MySuperClass

The compiler's error message explains the problem.
Do you understand about data types? The compiler will NOT allow you to use a variable of the wrong type.
What type does the method's definition say that the method returns?
What type is the mscl variable?
Are they the same or compatible? The compiler does not think they are.