Question

Can overridden methods have different return types?

Answer 1

Java supports^* covariant return types when overriding methods. An overriding method may have a more specific return type than the method being overridden. That is, as long as the new method’s return type is assignable to the return type of the method it overrides, it’s allowed.

For example:

class ShapeBuilder {
    ...
    public Shape build() {
    ....
}

class CircleBuilder extends ShapeBuilder{
    ...
    @Override
    public Circle build() {
    ....
}

This is specified in section 8.4.5 of the Java Language Specification:

Return types may vary among methods that override each other if the return types are reference types. The notion of return-type-substitutability supports covariant returns, that is, the specialization of the return type to a subtype.

A method declaration d1 with return type R1 is return-type-substitutable for another method d2 with return type R2, if and only if the following conditions hold:

• If R1 is void then R2 is void.

• If R1 is a primitive type, then R2 is identical to R1.

• If R1 is a reference type then:

  • R1 is either a subtype of R2 or R1 can be converted to a subtype of R2 by unchecked conversion (§5.1.9), or

  • R1 = |R2|

("|R2|" refers to the erasure of the type R2, as defined in §4.6 of the JLS.)

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^{* Prior to Java 5, Java had invariant return types, which meant the return type of a method override needed to exactly match the method being overridden.}