Excuse any minor syntax errors or whatnot, I'm experiencing this with a Jitsi module and not being super familiar with Java want to confirm what is going on and why and how it should be fixed.
public abstract class A { public A() { this.load(); } protected void load() { } } public class B extends A { private String testString = null; public B() { super(); } @Override protected void load() { testString = "test"; } }
The application is doing this when creating an instance of the class B using a load class by name method:
Is this expected Java behavior? What could cause this? It's a Java 1.6 application running on the 1.7 JDK.
Is this expected Java behavior?
Yes.
What could cause this?
Your invocation of non-final overridden method in non-final super class constructor.
Let's see what happens step-by-step:
B
.B()
calls super class constructor - A()
, to initialize the super class members.A()
now invokes a non-final method which is overridden in B
class, as a part of initialization.B
class, the method load()
invoked is of B
class.load()
initializes the B
class instance field - testString
.Object
class have been finished)B()
constructor starts executing further, initializing it's own member.B
overwrites the previous written value in testString
, and re-initializes it to null
.Moral: Never call a non-final public method of a non-final class in it's constructor.
This is a common problem-pattern with initialization-on-construction, and can frequently be found in infrastructure code & home-made DAOs.
The assignment to 'null' is unneeded & can be removed.
If that's not enough as a quick patch, then: Move all the post-construction init to a separate method, and wrap it all in a "static method" pseudo-constructor.
And if you're doing DAO stuff, it's really good to distinguish between "load" and "create", since these are completely different instantiations. Define separate "static constructor" methods & perhaps separate internal inits, for these.
abstract public class A { protected void initAfterCreate() {} } public class B { @Override protected void initAfterCreate() { this.testString = "test"; } // static constructors; // -- static public B createB() { B result = new B(); result.initAfterCreate(); } }
Demonstrating load/create separation for a DAO:
public class Order { protected int id; protected boolean dbExists; static public load (int id) { Order result = new Order( id, true); // populate from SQL query.. return result; } static public create() { // allocate a key. int id = KeyAlloc.allocate( "Order"); Order result = new Order( id, false); } // internal constructor; not for external access. // protected Order (int id, boolean dbExists) { this.id = id; this.dbExists = dbExists; } }
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