When do instance variables get initialized and values assigned?

by

When the memory for the atribute 1 & 2 are allocated in the heap ?

The memory for the object as a whole is allocated when the new operator is invoked, before the java.lang.Object constructor is entered. Memory is allocated for individual Integer instances in init, but there is no point when memory is allocated for individual properties — only whole objects.

Curious to know why is attribute 2 is NULL ?

The init method is called in the super constructor, so attribute2 is assigned new Integer(200), and then the subclass constructor is invoked which applies property initializers in the order they appear in the source code. This line

private Integer attribute2 = null;

overwrites the value assigned by init() to null.

If you add a call to

 System.out.println("attribute 2 : " +attribute2);

right after your call to super(); then this will become apparent.

Are there any design flaws?

Calling sub-class methods before the base class has finished initializing is dangerous. The sub-class might rely on its base-class’s invariants to protect its own invariants, and if the base-class constructor has not completed, then its invariants may not hold.

This is also likely to confuse C++ programmers and the like who would expect a call to init from the base class to invoke the base class’s version since C++ rewrites the vtable pointer as constructors are entered.

See The Java Language Specification for all the gory details.

More Related Contents:

  1. How do I correctly implement this code segment without getting a logic error? [closed]
  2. The difference between Classes, Objects, and Instances
  3. How should I have explained the difference between an Interface and an Abstract class?
  4. What exactly is an instance in Java?
  5. Class is not abstract and does not override abstract method
  6. When do I have to use interfaces instead of abstract classes? [duplicate]
  7. Why does every object in Java implicitly extend java.lang.Object class?
  8. java StackOverflowError when local and instance objects creation
  9. Java generic method inheritance and override rules
  10. Using Gson and Abstract Classes
  11. Extends JFrame vs. creating it inside the program
  12. Java error: Implicit super constructor is undefined for default constructor
  13. What are classes, references, and objects?
  14. How to sort an array of objects in Java?
  15. Overriding vs Hiding Java – Confused
  16. Emulate annotation inheritance for interfaces and methods with AspectJ
  17. gson.toJson() throws StackOverflowError
  18. Superclass reference not able to call subclass method in Java
  19. Why is constructor of super class invoked when we declare the object of sub class? (Java)
  20. Java: Calling a super method which calls an overridden method
  21. JPA: Implementing Model Hierarchy – @MappedSuperclass vs. @Inheritance
  22. How can interfaces replace the need for multiple inheritance when have existing classes
  23. What is difference between references and objects in java? [duplicate]
  24. Java: Casting from List to List when B implements A?
  25. Why we should not use protected static in java
  26. Overriding a super class’s instance variables
  27. order of constructor calls in multilevel inheritance in java [duplicate]
  28. how to inherit Constructor from super class to sub class
  29. How can I access a private constructor of a class?
  30. Java: Use import or explicit package / class name?

Leave a Comment