Q. The code in a finally clause will never fail to execute, right?
Well, hardly ever. But here's an example where the finally code will not execute, regardless of the value of the boolean choice:

try {
if (choice) {
while (true) ;
} else {
System.exit(1);
}
} finally {
code.to.cleanup();
}



Q. Within a method m in a class C, isn't this.getClass() always C?
No. It's possible that for some object x that is an instance of some subclass C1 of C either there is no C1.m() method, or some method on x called super.m(). In either case, this.getClass() is C1, not C within the body of C.m(). If C is final, then you're ok.

Q: I defined an equals method, but Hashtable ignores it. Why?
equals methods are surprisngly hard to get right. Here are the places to look first for a problem:
You defined the wrong equals method. For example, you wrote:
public class C {
public boolean equals(C that) { return id(this) == id(that); }
}
But in order for table.get(c) to work you need to make the equals method take an Object as the argument, not a C:
public class C {
public boolean equals(Object that) {
return (that instanceof C) && id(this) == id((C)that);
}
}
Why? The code for Hashtable.get looks something like this:
public class Hashtable {
public Object get(Object key) {
Object entry;
...
if (entry.equals(key)) ...
}
}
Now the method invoked by entry.equals(key) depends upon the actual run-time type of the object referenced by entry, and the declared, compile-time type of the variable key. So when you as a user call table.get(new C(...)), this looks in class C for the equals method with argument of type Object. If you happen to have defined an equals method with argument of type C, that's irrelevent. It ignores that method, and looks for a method with signature equals(Object), eventually finding Object.equals(Object). If you want to over-ride a method, you need to match argument types exactly. In some cases, you may want to have two methods, so that you don't pay the overhead of casting when you know you have an object of the right class:
public class C {
public boolean equals(Object that) {
return (this == that)
((that instanceof C) && this.equals((C)that));
}
public boolean equals(C that) {
return id(this) == id(that); // Or whatever is appropriate for class C
}
}
You didn't properly implement equals as an equality predicate: equals must be symmetric, transitive, and reflexive. Symmetric means a.equals(b) must have the same value as b.equals(a). (This is the one most people mess up.) Transitive means that if a.equals(b) and b.equals(c) then a.equals(c) must be true. Reflexive means that a.equals(a) must be true, and is the reason for the (this == that) test above (it's also often good practice to include this because of efficiency reasons: testing for == is faster than looking at all the slots of an object, and to partially break the recursion problem on objects that might have circular pointer chains).
You forgot the hashCode method. Anytime you define a equals method, you should also define a hashCode method. You must make sure that two equal objects have the same hashCode, and if you want better hashtable performance, you should try to make most non-equal objects have different hashCodes. Some classes cache the hash code in a private slot of an object, so that it need be computed only once. If that is the case then you will probably save time in equals if you include a line that says if (this.hashSlot != that.hashSlot) return false.
You didn't handle inheritance properly. First of all, consider if two objects of different class can be equal. Before you say "NO! Of course not!" consider a class Rectangle with width and height fields, and a Box class, which has the above two fields plus depth. Is a Box with depth == 0 equal to the equivalent Rectangle? You might want to say yes. If you are dealing with a non-final class, then it is possible that your class might be subclassed, and you will want to be a good citizen with respect to your subclass. In particular, you will want to allow an extender of your class C to use your C.equals method using super as follows:
public class C2 extends C {
int newField = 0;
public boolean equals(Object that) {
if (this == that) return true;
else if (!(that instanceof C2)) return false;
else return this.newField == ((C2)that).newField) && super.equals(that);
}
}
To allow this to work, you have to be careful about how you treat classes in your definition of C.equals. For example, check for that instanceof C rather than that.getClass() == C.class. See the previous IAQ question to learn why. Use this.getClass() == that.getClass() if you are sure that two objects must be of the same class to be considered equals.
You didn't handle circular references properly. Consider:
public class LinkedList {
Object contents;
LinkedList next = null;
public boolean equals(Object that) {
return (this == that)
((that instanceof LinkedList) && this.equals((LinkedList)that));
}
public boolean equals(LinkedList that) { // Buggy!
return Util.equals(this.contents, that.contents) &&
Util.equals(this.next, that.next);
}
}
Here I have assumed there is a Util class with:
public static boolean equals(Object x, Object y) {
return (x == y) (x != null && x.equals(y));
}
I wish this method were in Object; without it you always have to throw in tests against null. Anyway, the LinkedList.equals method will never return if asked to compare two LinkedLists with circular references in them (a pointer from one element of the linked list back to another element). See the description of the Common Lisp function list-length for an explanation of how to handle this problem in linear time with only two words of extra storge. (I don't give the answer here in case you want to try to figure it out for yourself first.)

Q: I tried to forward a method to super, but it occasionally doesn't work. Why?

This is the code in question, simplified for this example:
/** A version of Hashtable that lets you do
* table.put("dog", "canine");, and then have
* table.get("dogs") return "canine". **/
public class HashtableWithPlurals extends Hashtable {
/** Make the table map both key and key + "s" to value. **/
public Object put(Object key, Object value) {
super.put(key + "s", value);
return super.put(key, value);
}
}
You need to be careful when passing to super that you fully understand what the super method does. In this case, the contract for Hashtable.put is that it will record a mapping between the key and the value in the table. However, if the hashtable gets too full, then Hashtable.put will allocate a larger array for the table, copy all the old objects over, and then recursively re-call table.put(key, value). Now, because Java resolves methods based on the runtime type of the target, in our example this recursive call within the code for Hashtable will go to HashtableWithPlurals.put(key, value), and the net result is that occasionally (when the size of the table overflows at just the wrong time), you will get an entry for "dogss" as well as for "dogs" and "dog". Now, does it state anywhere in the documentation for put that doing this recursive call is a possibility? No. In cases like this, it sure helps to have source code access to the JDK.

Q: Why does my Properties object ignore the defaults when I do a get?

You shouldn't do a get on a Properties object; you should do a getProperty instead. Many people assume that the only difference is that getProperty has a declared return type of String, while get is declared to return an Object. But actually there is a bigger difference: getProperty looks at the defaults. get is inherited from Hashtable, and it ignores the default, thereby doing exactly what is documented in the Hashtable class, but probably not what you expect. Other methods that are inherited from Hashtable (like isEmpty and toString) will also ignore defaults. Example code:
Properties defaults = new Properties();
defaults.put("color", "black");
Properties props = new Properties(defaults);
System.out.println(props.get("color") + ", " +
props.getProperty(color));
// This prints "null, black"
Is this justified by the documentation? Maybe. The documentation in Hashtable talks about entries in the table, and the behavior of Properties is consistent if you assume that defauls are not entries in the table. If for some reason you thought defaults were entries (as you might be led to believe by the behavior of getProperty) then you will be confused.


Q:Inheritance seems error-prone. How can I guard against these errors?

The previous two questions show that a programmer neeeds to be very careful when extending a class, and sometimes just in using a class that extends another class. Problems like these two lead John Ousterhout to say "Implementation inheritance causes the same intertwining and brittleness that have been observed when goto statements are overused. As a result, OO systems often suffer from complexity and lack of reuse." (Scripting, IEEE Computer, March 1998) and Edsger Dijkstra to allegedly say "Object-oriented programming is an exceptionally bad idea which could only have originated in California." (from a collection of signature files). I don't think there's a general way to insure being safe, but there are a few things to be aware of:
Extending a class that you don't have source code for is always risky; the documentation may be incomplete in ways you can't foresee.
Calling super tends to make these unforeseen problems jump out.
You need to pay as much attention to the methods that you don't over-ride as the methods that you do. This is one of the big fallacies of Object-Oriented design using inheritance. It is true that inheritance lets you write less code. But you still have to think about the code you don't write.
You're especially looking for trouble if the subclass changes the contract of any of the methods, or of the class as a whole. It is difficult to tell when a contract is changed, since contracts are informal (there is a formal part in the type signature, but the rest appears only in comments). In the Properties example, it is not clear if a contract is being broken, because it is not clear if the defaults are to be considered "entries" in the table or not.

Q:What are some alternatives to inheritance?

Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
For the HashtableWithPlurals example, delegation would give you this (note: as of JDK 1.2, Dictionary is considered obsolete; use Map instead):
/** A version of Hashtable that lets you do
* table.put("dog", "canine");, and then have
* table.get("dogs") return "canine". **/
public class HashtableWithPlurals extends Dictionary {
Hashtable table = new Hashtable();
/** Make the table map both key and key + "s" to value. **/
public Object put(Object key, Object value) {
table.put(key + "s", value);
return table.put(key, value);
}
... // Need to implement other methods as well
}
The Properties example, if you wanted to enforce the interpretation that default values are entries, would be better done with delegation. Why was it done with inheritance, then? Because the Java implementation team was rushed, and took the course that required writing less code.


Q: Why are there no global variables in Java?Global variables are considered bad form for a variety of reasons:
Adding state variables breaks referential transparency (you no longer can understand a statement or expression on its own: you need to understand it in the context of the settings of the global variables).
State variables lessen the cohesion of a program: you need to know more to understand how something works. A major point of Object-Oriented programming is to break up global state into more easily understood collections of local state.
When you add one variable, you limit the use of your program to one instance. What you thought was global, someone else might think of as local: they may want to run two copies of your program at once. For these reasons, Java decided to ban global variables.


Q: I still miss global variables. What can I do instead?

That depends on what you want to do. In each case, you need to decide two things: how many copies of this so-called global variable do I need? And where would be a convenient place to put it? Here are some common solutions:
If you really want only one copy per each time a user invokes Java by starting up a Java virtual machine, then you probably want a static instance variable. For example, you have a MainWindow class in your application, and you want to count the number of windows that the user has opened, and initiate the "Really quit?" dialog when the user has closed the last one. For that, you want:
// One variable per class (per JVM)
public Class MainWindow {
static int numWindows = 0;
...
// when opening: MainWindow.numWindows++;
// when closing: MainWindow.numWindows--;
}
In many cases, you really want a class instance variable. For example, suppose you wrote a web browser and wanted to have the history list as a global variable. In Java, it would make more sense to have the history list be an instance variable in the Browser class. Then a user could run two copies of the browser at once, in the same JVM, without having them step on each other.
// One variable per instance
public class Browser {
HistoryList history = new HistoryList();
...
// Make entries in this.history
}
Now suppose that you have completed the design and most of the implementation of your browser, and you discover that, deep down in the details of, say, the Cookie class, inside the Http class, you want to display an error message. But you don't know where to display the message. You could easily add an instance variable to the Browser class to hold the display stream or frame, but you haven't passed the current instance of the browser down into the methods in the Cookie class. You don't want to change the signatures of many methods to pass the browser along. You can't use a static variable, because there might be multiple browsers running. However, if you can guarantee that there will be only one browser running per thread (even if each browser may have multiple threads) then there is a good solution: store a table of thread-to-browser mappings as a static variable in the Browser class, and look up the right browser (and hence display) to use via the current thread:
// One "variable" per thread
public class Browser {
static Hashtable browsers = new Hashtable();
public Browser() { // Constructor
browsers.put(Thread.currentThread(), this);
}
...
public void reportError(String message) {
Thread t = Thread.currentThread();
((Browser)Browser.browsers.get(t))
.show(message)
}
}
Finally, suppose you want the value of a global variable to persist between invocations of the JVM, or to be shared among multiple JVMs in a network of machines. Then you probably should use a database which you access through JDBC, or you should serialize data and write it to a file.


Q: Can I write sin(x) instead of Math.sin(x)?

Short answer: no. Get used to writing the class name to access static methods from outside the class. However, if you insist on a longer answer ...
If you only want a few methods, you can put in calls to them within your own class:
public static double sin(double x) { return Math.sin(x); }
public static double cos(double x) { return Math.cos(x); }
...
sin(x)
Static methods take a target (thing to the left of the dot) that is either a class name, or is an object whose value is ignored, but must be declared to be of the right class. So you could save three characters per call by doing:
// Can't instantiate Math, so it must be null.
Math m = null;
...
m.sin(x)
java.lang.Math is a final class, so you can't inherit from it, but if you have your own set of static methods that you would like to share among many of your own classes, then you can package them up and inherit them:
public abstract class MyStaticMethods {
public static double mysin(double x) { ... }
}
public class MyClass1 extends MyStaticMethods {
...
mysin(x)
}


Peter van der Linden, author of Just Java, recommends against both of the last two practices in his FAQ. I agree with him that Math m = null is a bad idea in most cases, but I'm not convinced that the MyStaticMethods demonstrates "very poor OOP style to use inheritance to obtain a trivial name abbreviation (rather than to express a type hierarchy)." First of all, trivial is in the eye of the beholder; the abbreviation may be substantial. (See an example of how I used this approach to what I thought was good effect.) Second, it is rather presumptuous to say that this is very bad OOP style. You could make a case that it is bad Java style, but in languages with multiple inheritance, this idiom would be more acceptable.
Another way of looking at it is that features of Java (and any language) necessarily involve trade-offs, and conflate many issues. I agree it is bad to use inheritance in such a way that you mislead the user into thinking that MyClass1 is inheriting behavior from MyStaticMethods, and it is bad to prohibit MyClass1 from extending whatever other class it really wants to extend. But in Java the class is also the unit of encapsulation, compilation (mostly), and name scope. The MyStaticMethod approach scores negative points on the type hierarchy front, but positive points on the name scope front. If you say that the type hierarchy view is more important, I won't argue with you. But I will argue if you think of a class as doing only one thing, rather than many things at once, and if you think of style guides as absolute rather than as trade-offs.


Q: Is null an Object?

Absolutely not. By that, I mean (null instanceof Object) is false. Some other things you should know about null:
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.)
There is only one null, not one for each class. Thus, ((String) null == (Hashtable) null), for example.
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.
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) { ...}
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() { }
}