Covariant return types in Java
Before JDK 5.0, it was not possible to override a method by changing the return type. When we override a parent class method, the name, argument types and return type of the overriding method in child class has to be exactly same as that of parent class method. Overriding method was said to be invariant with respect to return type.
Covariant return types
Java 5.0 onwards it is possible to have different return type for a overriding method in child class, but child’s return type should be sub-type of parent’s return type. Overriding method becomes variant with respect to return type.
Co-variant return type is based on Liskov substitution principle.
Below is the simple example to understand the co-variant return type with method overriding.
// Java program to demonstrate that we can have different return types if return type in
// overridden method is sub-type
// Two classes used for return types.
class A {
A get() {
return this;
}
}
class B1 extends A {
B1 get() {
return this;
}
void message() {
System.out.println("welcome to covariant return type");
}
public static void main(String args[]) {
new B1().get().message();
}
}
Output:
welcome to covariant return type
Advantages:
- It helps to avoid confusing type casts present in the class hierarchy and thus making the code readable, usable and maintainable.
- We get a liberty to have more specific return types when overriding methods.
- Help in preventing run-time ClassCastExceptions on returns
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