Because a subclass instance may be cast to and treated as an instance of the superclass, overriding methods should uphold the aspects of the
superclass contract that relate to the Liskov Substitution Principle. Specifically, if the parameters or return type of the superclass method are
marked with any of the following: @Nullable, @CheckForNull, @NotNull, @NonNull, and
@Nonnull, then subclass parameters are not allowed to tighten the contract, and return values are not allowed to loosen it.
Code examples
Noncompliant code example
public class Fruit {
private Season ripe;
private String color;
public void setRipe(@Nullable Season ripe) {
this.ripe = ripe;
}
public @NotNull Integer getProtein() {
return 12;
}
}
public class Raspberry extends Fruit {
public void setRipe(@NotNull Season ripe) { // Noncompliant: the ripe argument annotated as @Nullable in parent class
this.ripe = ripe;
}
public @Nullable Integer getProtein() { // Noncompliant: the return type annotated as @NotNull in parent class
return null;
}
}
Compliant solution
public class Fruit {
private Season ripe;
private String color;
public void setRipe(@Nullable Season ripe) {
this.ripe = ripe;
}
public @NotNull Integer getProtein() {
return 12;
}
}
public class Raspberry extends Fruit {
public void setRipe(@Nullable Season ripe) {
this.ripe = ripe;
}
public @NotNull Integer getProtein() {
return 12;
}
}
