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Easy Learn Java: Programming Articles, Examples and Tips - Page 420


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Q: Is null an Object?

Go to all tips in Java IAQ by Peter Norvig

Q: Is null an Object?

Answer: Absolutely not. By that, I mean (null instanceof Object) is false. Some other things you should know about null:
  1. You can't call a method on null: x.m() is an error when x is null and m is a non-static method. (When m is a static method it is fine, because it is the class of x that matters; the value is ignored.)
  2. There is only one null, not one for each class. Thus, ((String) null == (Hashtable) null), for example.
  3. It is ok to pass null as an argument to a method, as long as the method is expecting it. Some methods do; some do not. So, for example, System.out.println(null) is ok, but string.compareTo(null) is not. For methods you write, your javadoc comments should say whether null is ok, unless it is obvious.
  4. In JDK 1.1 to 1.1.5, passing null as the literal argument to a constructor of an anonymous inner class (e.g., new SomeClass(null) { ...} caused a compiler error. It's ok to pass an expression whose value is null, or to pass a coerced null, like new SomeClass((String) null) { ...}
  5. There are at least three different meanings that null is commonly used to express:
    • Uninitialized. A variable or slot that hasn't yet been assigned its real value.
    • Non-existant/not applicable. For example, terminal nodes in a binary tree might be represented by a regular node with null child pointers.
    • Empty. For example, you might use null to represent the empty tree. Note that this is subtly different from the previous case, although some people make the mistake of confusing the two cases. The difference is whether null is an acceptable tree node, or whether it is a signal to not treat the value as a tree node. Compare the following three implementations of binary tree nodes with an in-order print method:

// null means not applicable
// There is no empty tree.

class Node {
  Object data;
  Node left, right;

  void print() {
    if (left != null)
      left.print();
    System.out.println(data);
    if (right != null)
      right.print();
  }
}
// null means empty tree
// Note static, non-static methods

class Node {
  Object data;
  Node left, right;

  void static print(Node node) {
    if (node != null) node.print();
  }

  void print() {
    print(left);
    System.out.println(data);
    print(right);
  }
}
// Separate class for Empty
// null is never used

interface Node { void print(); }

class DataNode implements Node{
  Object data;
  Node left, right;

  void print() {
    left.print();
    System.out.println(data);
    right.print();
  }
}

class EmptyNode implements Node { 
  void print() { }
}


This tip is reprinted on JavaFAQ.nu by by courtesy of Peter Norvig I am thankful for his important contributions to my site - 21 Infrequently Answered Java Questions. Alexandre Patchine


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Posted by jalex on Wednesday, November 30, 2005 (00:00:00) (5342 reads)

Q: How big is an Object? Why is there no sizeof?

Go to all tips in Java IAQ by Peter Norvig

Q: How big is an Object? Why is there no sizeof?

Ansswer: C has a sizeof operator, and it needs to have one, because the user has to manage calls to malloc, and because the size of primitive types (like long) is not standardized. Java doesn't need a sizeof, but it would still have been a convenient aid. Since it's not there, you can do this:

static Runtime runtime = Runtime.getRuntime();
...
long start, end;
Object obj;
runtime.gc();
start = runtime.freememory();
obj = new Object(); // Or whatever you want to look at
end =  runtime.freememory();
System.out.println("That took " + (start-end) + " 
bytes.");

This method is not foolproof, because a garbage collection could occur in the middle of the code you are instrumenting, throwing off the byte count. Also, if you are using a just-in-time compiler, some bytes may come from generating code.

You might be surprised to find that an Object takes 16 bytes, or 4 words, in the Sun JDK VM. This breaks down as follows: There is a two-word header, where one word is a pointer to the object's class, and the other points to the instance variables. Even though Object has no instance variables, Java still allocates one word for the variables. Finally, there is a "handle", which is another pointer to the two-word header. Sun says that this extra level of indirection makes garbage collection simpler. (There have been high performance Lisp and Smalltalk garbage collectors that do not use the extra level for at least 15 years. I have heard but have not confirmed that the Microsoft JVM does not have the extra level of indirection.)


An empty new String() takes 40 bytes, or 10 words: 3 words of pointer overhead, 3 words for the instance variables (the start index, end index, and character array), and 4 words for the empty char array. Creating a substring of an existing string takes "only" 6 words, because the char array is shared. Putting an Integer key and Integer value into a Hashtable takes 64 bytes (in addition to the four bytes that were pre-allocated in the Hashtable array): I'll let you work out why.


This tip is reprinted on JavaFAQ.nu by by courtesy of Peter Norvig I am thankful for his important contributions to my site - 21 Infrequently Answered Java Questions. Alexandre Patchine


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Posted by jalex on Wednesday, November 30, 2005 (00:00:00) (2923 reads)

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