ConstructorCallsOverride.java

/**
 * 
 * A quote from Effective Java 2nd Edition, 
 * Item 17: Design and document for inheritance, or else prohibit it:
 * 
 * <pre>
 * There are a few more restrictions that a class must obey to allow inheritance. 
 * Constructors must not invoke overridable methods, directly or indirectly. 
 * If you violate this rule, program failure will result. 
 * The superclass constructor runs before the subclass constructor, 
 * so the overriding method in the subclass will be invoked before the subclass constructor has run. 
 * If the overriding method depends on any initialization performed by the subclass constructor, 
 * the method will not behave as expected.
 * </pre>
 */
public class ConstructorCallsOverride {
    public static void main(String[] args) {
        abstract class Base {
            Base() { overrideMe(); }
            abstract void overrideMe(); 
        }
        class Child extends Base {
            final int x;
            Child(int x) { this.x = x; }
            @Override void overrideMe() {
                System.out.println(x);
            }
        }
        new Child(42); // prints "0"
    }
}