Java SE 8 Programmer II: Java Class Design

Java Class Design tests your ability to build legal inheritance hierarchies, apply encapsulation, and predict polymorphic behavior. Questions often ask which override compiles, what a constructor call dispatches to, or how access modifiers constrain an override.

Core rules

• A subclass method may widen access but never narrow it; it may return a covariant subtype and throw fewer or narrower checked exceptions.
• private, static, and final methods are not overridden: private methods are new declarations, static methods are hidden, and final methods cannot be replaced.
• Constructors are not inherited and are not polymorphic. super(...) or this(...) must be the first statement; if neither is written, the compiler inserts super().
• An abstract class may have zero abstract methods but cannot be instantiated. A concrete subclass must implement all inherited abstract methods.
• Interface methods are implicitly public abstract (except default/static in Java 8); implementing methods must be public.
• Fields are hidden, not overridden; field access depends on the reference type, while instance method calls use the runtime type.

Worked example

class A { public Number get() throws IOException { return 1; } } class B extends A { @Override public Integer get() { return 2; } } This compiles: Integer is covariant with Number, access stays public, and B.get() throws no checked exception. Calling new B().get() through an A reference still invokes B.get().

Traps

• Reducing visibility in an override is a compile error.
• Adding a broader checked exception than the overridden method is illegal.
• static methods called through an instance resolve by reference type, not runtime type.
• A final class cannot be extended; a final method cannot be overridden.
• Calling an overridable method from a constructor runs before subclass fields are initialized.