We Keep Coding

Java class: limit instance variable to one of several possible values, depending on other instance variables

I am sorry for the vague question. I am not sure what I'm looking for here.
I have a Java class, let's call it Bar. In that class is an instance variable, let's call it foo. foo is a String.
foo cannot just have any value. There is a long list of strings, and foo must be one of them.
Then, for each of those strings in the list I would like the possibility to set some extra conditions as to whether that specific foo can belong in that specific type of Bar (depending on other instance variables in that same Bar).
What approach should I take here? Obviously, I could put the list of strings in a static class somewhere and upon calling setFoo(String s) check whether s is in that list. But that would not allow me to check for extra conditions - or I would need to put all that logic for every value of foo in the same method, which would get ugly quickly.
Is the solution to make several hundred classes for every possible value of foo and insert in each the respective (often trivial) logic to determine what types of Bar it fits? That doesn't sound right either.
What approach should I take here?
Here's a more concrete example, to make it more clear what I am looking for. Say there is a Furniture class, with a variable material, which can be lots of things, anything from mahogany to plywood. But there is another variable, upholstery, and you can make furniture containing cotton of plywood but not oak; satin furniture of oak but not walnut; other types of fabric go well with any material; et cetera.

I wouldn't suggest creating multiple classes/templates for such a big use case. This is very opinion based but I'll take a shot at answering as best as I can.
In such a case where your options can be numerous and you want to keep a maintainable code base, the best solution is to separate the values and the logic. I recommend that you store your foo values in a database. At the same time, keep your client code as clean and small as possible. So that it doesn't need to filter through the data to figure out which data is valid. You want to minimize dependency to data in your code. Think of it this way: tomorrow you might need to add a new material to your material list. Do you want to modify all your code for that? Or do you want to just add it to your database and everything magically works? Obviously the latter is a better option. Here is an example on how to design such a system. Of course, this can vary based on your use case or variables but it is a good guideline. The basic rule of thumb is: your code should have as little dependency to data as possible.
Let's say you want to create a Bar which has to have a certain foo. In this case, I would create a database for BARS which contains all the possible Bars. Example:
ID NAME FOO
1 Door 1,4,10
I will also create a database FOOS which contains the details of each foo. For example:
ID NAME PROPERTY1 PROPERTY2 ...
1 Oak Brown Soft
When you create a Bar:
Bar door = new Bar(Bar.DOOR);
in the constructor you would go to the BARS table and query the foos. Then you would query the FOOS table and load all the material and assign them to the field inside your new object.
This way whenever you create a Bar the material can be changed and loaded from DB without changing any code. You can add as many types of Bar as you can and change material properties as you goo. Your client code however doesn't change much.
You might ask why do we create a database for FOOS and refer to it's ids in the BARS table? This way, you can modify the properties of each foo as much as you want. Also you can share foos between Bars and vice versa but you only need to change the db once. cross referencing becomes a breeze. I hope this example explains the idea clearly.

You say:
Is the solution to make several hundred classes for every possible
value of foo and insert in each the respective (often trivial) logic
to determine what types of Bar it fits? That doesn't sound right
either.
Why not have separate classes for each type of Foo? Unless you need to define new types of Foo without changing the code you can model them as plain Java classes. You can go with enums as well but it does not really give you any advantage since you still need to update the enum when adding a new type of Foo.
In any case here is type safe approach that guarantees compile time checking of your rules:
public static interface Material{}
public static interface Upholstery{}
public static class Oak implements Material{}
public static class Plywood implements Material{}
public static class Cotton implements Upholstery{}
public static class Satin implements Upholstery{}
public static class Furniture<M extends Material, U extends Upholstery>{
private M matrerial = null;
private U upholstery = null;
public Furniture(M matrerial, U upholstery){
this.matrerial = matrerial;
this.upholstery = upholstery;
}
public M getMatrerial() {
return matrerial;
}
public U getUpholstery() {
return upholstery;
}
}
public static Furniture<Plywood, Cotton> cottonFurnitureWithPlywood(Plywood plywood, Cotton cotton){
return new Furniture<>(plywood, cotton);
}
public static Furniture<Oak, Satin> satinFurnitureWithOak(Oak oak, Satin satin){
return new Furniture<>(oak, satin);
}

It depends on what you really want to achieve. Creating objects and passing them around will not magically solve your domain-specific problems.
If you cannot think of any real behavior to add to your objects (except the validation), then it might make more sense to just store your data and read them into memory whenever you want. Even treat rules as data.
Here is an example:
public class Furniture {
String name;
Material material;
Upholstery upholstery;
//getters, setters, other behavior
public Furniture(String name, Material m, Upholstery u) {
//Read rule files from memory or disk and do all the checks
//Do not instantiate if validation does not pass
this.name = name;
material = m;
upholstery = u;
}
}
To specify rules, you will then create three plain text files (e.g. using csv format). File 1 will contain valid values for material, file 2 will contain valid values for upholstery, and file 3 will have a matrix format like the following:
upholstery\material plywood mahogany oak
cotton 1 0 1
satin 0 1 0
to check if a material goes with an upholstery or not, just check the corresponding row and column.
Alternatively, if you have lots of data, you can opt for a database system along with an ORM. Rule tables then can be join tables and come with extra nice features a DBMS may provide (like easy checking for duplicate values). The validation table could look something like:
MaterialID UpholsteryID Compatability_Score
plywood cotton 1
oak satin 0
The advantage of using this approach is that you quickly get a working application and you can decide what to do as you add new behavior to your application. And even if it gets way more complex in the future (new rules, new data types, etc) you can use something like the repository pattern to keep your data and business logic decoupled.
Notes about Enums:
Although the solution suggested by #Igwe Kalu solves the specific case described in the question, it is not scalable. What if you want to find what material goes with a given upholstery (the reverse case)? You will need to create another enum which does not add anything meaningful to the program, or add complex logic to your application.

This is a more detailed description of the idea I threw out there in the comment:
Keep Furniture a POJO, i.e., just hold the data, no behavior or rules implemented in it.
Implement the rules in separate classes, something along the lines of:
interface FurnitureRule {
void validate(Furniture furniture) throws FurnitureRuleException;
}
class ValidMaterialRule implements FurnitureRule {
// this you can load in whatever way suitable in your architecture -
// from enums, DB, an XML file, a JSON file, or inject via Spring, etc.
private Set<String> validMaterialNames;
#Overload
void validate(Furniture furniture) throws FurnitureRuleException {
if (!validMaterialNames.contains(furniture.getMaterial()))
throws new FurnitureRuleException("Invalid material " + furniture.getMaterial());
}
}
class UpholsteryRule implements FurnitureRule {
// Again however suitable to implement/config this
private Map<String, Set<String>> validMaterialsPerUpholstery;
#Overload
void validate(Furniture furniture) throws FurnitureRuleException {
Set<String> validMaterialNames = validMaterialsPerUpholstery.get(furniture.getUpholstery();
if (validMaterialNames != null && !validMaterialNames.contains(furniture.getMaterial()))
throws new FurnitureRuleException("Invalid material " + furniture.getMaterial() + " for upholstery " + furniture.getUpholstery());
}
}
// and more complex rules if you need to
Then have some service along the lines of FurnitureManager. It's the "gatekeeper" for all Furniture creation/updates:
class FurnitureManager {
// configure these via e.g. Spring.
private List<FurnitureRule> rules;
public void updateFurniture(Furniture furniture) throws FurnitureRuleException {
rules.forEach(rule -> rule.validate(furniture))
// proceed to persist `furniture` in the database or whatever else you do with a valid piece of furniture.
}
}

material should be of type Enum.
public enum Material {
MAHOGANY,
TEAK,
OAK,
...
}
Furthermore you can have a validator for Furniture that contains the logic which types of Furniture make sense, and then call that validator in every method that can change the material or upholstery variable (typically only your setters).
public class Furniture {
private Material material;
private Upholstery upholstery; //Could also be String depending on your needs of course
public void setMaterial(Material material) {
if (FurnitureValidator.isValidCombination(material, this.upholstery)) {
this.material = material;
}
}
...
private static class FurnitureValidator {
private static boolean isValidCombination(Material material, Upholstery upholstery) {
switch(material) {
case MAHOGANY: return upholstery != Upholstery.COTTON;
break;
//and so on
}
}
}
}

We often are oblivious of the power inherent in enum types. The Java™ Tutorials clearly states "you should use enum types any time you need to represent a fixed set of constants."
How do you simply make the best of enum in resolving the challenge you presented? - Here goes:
public enum Material {
MAHOGANY( "satin", "velvet" ),
PLYWOOD( "leather" ),
// possibly many other materials and their matching fabrics...
OAK( "some other fabric - 0" ),
WALNUT( "some other fabric - 0", "some other fabric - 1" );
private final String[] listOfSuitingFabrics;
Material( String... fabrics ) {
this.listOfSuitingFabrics = fabrics;
}
String[] getListOfSuitingFabrics() {
return Arrays.copyOf( listOfSuitingFabrics );
}
public String toString() {
return name().substring( 0, 1 ) + name().substring( 1 );
}
}
Let's test it:
public class TestMaterial {
for ( Material material : Material.values() ) {
System.out.println( material.toString() + " go well with " + material.getListOfSuitingFabrics() );
}
}

Probably the approach I'd use (because it involves the least amount of code and it's reasonably fast) is to "flatten" the hierarchical logic into a one-dimensional Set of allowed value combinations. Then when setting one of the fields, validate that the proposed new combination is valid. I'd probably just use a Set of concatenated Strings for simplicity. For the example you give above, something like this:
class Furniture {
private String wood;
private String upholstery;
/**
* Set of all acceptable values, with each combination as a String.
* Example value: "plywood:cotton"
*/
private static final Set<String> allowed = new HashSet<>();
/**
* Load allowed values in initializer.
*
* TODO: load allowed values from DB or config file
* instead of hard-wiring.
*/
static {
allowed.add("plywood:cotton");
...
}
public void setWood(String wood) {
if (!allowed.contains(wood + ":" + this.upholstery)) {
throw new IllegalArgumentException("bad combination of materials!");
}
this.wood = wood;
}
public void setUpholstery(String upholstery) {
if (!allowed.contains(this.wood + ":" + upholstery)) {
throw new IllegalArgumentException("bad combination of materials!");
}
this.upholstery = upholstery;
}
public void setMaterials(String wood, String upholstery) {
if (!allowed.contains(wood + ":" + upholstery)) {
throw new IllegalArgumentException("bad combination of materials!");
}
this.wood = wood;
this.upholstery = upholstery;
}
// getters
...
}
The disadvantage of this approach compared to other answers is that there is no compile-time type checking. For example, if you try to set the wood to plywoo instead of plywood you won’t know about your error until runtime. In practice this disadvantage is negligible since presumably the options will be chosen by a user through a UI (or through some other means), so you won’t know what they are until runtime anyway. Plus the big advantage is that the code will never have to be changed so long as you’re willing to maintain a list of allowed combinations externally. As someone with 30 years of development experience, take my word for it that this approach is far more maintainable.
With the above code, you'll need to use setMaterials before using setWood or setUpholstery, since the other field will still be null and therefore not an allowed combination. You can initialize the class's fields with default materials to avoid this if you want.

Hashtable returning null but object key is present

EDIT: FML! MY implementation of hashcode had a lowercase c. -.-
I've been trying to learn TDD and have been following the 'By Example' book by Kent Beck; it's very good!
However, I can't seem progress because a value is returning null when I access a hashtable. I've run a debug session and the object with the value is clearly there yet the result is null.
The code to build and access is:
public void addRate(String from, String to, int rate){
this.rates.put(new Pair(from, to), new Integer(rate));
}
from and to are "GBP" and "USD". Also verified by debug.
Test case calling the above:
#Test
public void testreduceMoneyDifferentCurrency(){
Bank bank = new Bank();
bank.addRate("GBP", "USD", 2);
Money result = bank.reduce(Money.gbpound(2), "USD");
assertEquals(Money.dollar(1), result);
}
The reduce method in bank calls the method rate:
public Money reduce(Bank bank, String to){
int rate = bank.rate(this.currency, to);
return new Money(this.amount / rate, to);
}
Which is where the issue is:
public int rate(String from, String to){
if (from.equals(to)) return 1;
Integer rate = (Integer) this.rates.get(new Pair(from, to));
return rate.intValue();
}
The first line copes with USD -> USD conversions etc.
The Pair object is 2 strings built to be used as a key.
I've not used has tables a great deal but I can't see what the issue is, I know for certain that the values are in the hashtable but 'rate' is always returning a null value.
I can't see the wood for the trees. :) Could someone point me in the right direction please?

I think the problem is in the Pair method.
When you do this:
this.rates.get(new Pair(from, to));
you are creating a new instance of Pair, which is not the same as the one you've put into the map in the addRate method.
If you want the code to work correctly, you either have to use the same instance of Pair class or correctly implement equals and hashCode method on Pair class.
Here's a bit deeper insight into the inner working on HashMap and what you have to do to make it work: https://stackoverflow.com/a/6493946/2266098

Java keeps the reference of objects. So when you are trying to do this
this.rates.get(new Pair(from, to));
you are basically creating a new instance of Pair which does not exists as a key in your HashMap.

Translating a string-representation of a function's parameter list to actual parameters, for a reflective call

UPDATE: After getting an unexpected-in-a-good-way answer, I've added some context to the bottom of this question, stating exactly how I'll be using these string-function-calls.
I need to translate a string such as
my.package.ClassName#functionName(1, "a string value", true)
into a reflective call to that function. Getting the package, class, and function name is not a problem. I have started rolling my own solution for parsing the parameter list, and determining the type of each and returning an appropriate object.
(I'm limiting the universe of types to the eight primitives, plus string. null would be considered a string, and commas and double-quotes must be strictly escaped with some simple marker, such as __DBL_QT__, to avoid complications with unescaping and splitting on the comma.)
I am not asking how to do this via string-parsing, as I understand how. It's just a lot of work and I'm hoping there's a solution already out there. Unfortunately it's such generic terminology, I'm getting nowhere with searching.
I understand asking for an external existing library is off topic for SO. I'm just hoping to get some feedback before it's shutdown, or even a suggestion on better search terms. Or perhaps, there is a completely different approach that might be suggested...
Thank you.
Context:
Each function call is found within a function's JavaDoc block, and represents a piece of example code--either its source code or its System.out output--which will be displayed in that spot.
The parameters are for customizing its display, such as
indentation,
eliminating irrelevant parts (like the license-block), and
for JavaDoc-linking the most important functions.
This customization is mostly for the source-code presentation, but may also be applied to its output.
(The first parameter is always an Appendable, which will do the actual outputting.)
The user needs to be be able to call any function, which in many cases will be a private-static function located directly below the JavaDoc-ed function itself.
The application I'm writing will read in the source-code file (the one containing the JavaDoc blocks, in which these string-function-calls exist), and create a duplicate of the *.java file, which will subsequently processed by javadoc.
So for every piece of example code, there will be likely two, and possibly more of these string-function-calls. There may be more, because I may want to show different slices of the same example, in different contexts--perhaps the whole example in the overall class JavaDoc block, and snippets from it in the relevant functions in that class.
I have already written the process that parses the source code (the source code containing the JavaDoc blocks, which is separate from the one that reads the example-code), and re-outputs its source-code blindly with insert example-code here and insert example-code-output here markers.
I'm now at the point where I have this string-function-call in an InsertExampleCode object, in a string-field. Now I need to do as described at the top of this question. Figure out which function they want to invoke, and do so.

Change the # to a dot (.), write a class definition around it so that you have a valid Java source file, include tools.jar in your classpath and invoke com.sun.tools.javac.Main.
Create your own instance of a ClassLoader to load the compiled class, and run it (make it implement a useful interface, such as java.util.concurrent.Callable so that you can get the result of the invocation easily)
That should do the trick.

The class I created for this, called com.github.aliteralmind.codelet.simplesig.SimpleMethodSignature, is a significant piece of Codelet, used to translate the "customizer" portion of each taglet, which is a function that customizes the taglet's output.
(Installation instructions. The only jars that must be in your classpath are codelet and xbnjava.)
Example string signatures, in taglets:
{#.codelet.and.out com.github.aliteralmind.codelet.examples.adder.AdderDemo%eliminateCommentBlocksAndPackageDecl()}
The customizer portion is everything following the percent sign (%). This customizer contains only the function name and empty parameters. This implies that the function must exist in one of a few, strictly-specified, set of classes.
{#.codelet.and.out com.github.aliteralmind.codelet.examples.adder.AdderDemo%lineRange(1, false, "Adder adder", 2, false, "println(adder.getSum())", "^ ")}
This specifies parameters as well, which are, by design, "simple"--either non-null strings, or a primitive type.
{#.codelet.and.out com.github.aliteralmind.codelet.examples.adder.AdderDemo%com.github.aliteralmind.codelet.examples.LineRangeWithLinksCompact#adderDemo_lineSnippetWithLinks()}
Specifies the explicit package and class in which the function exists.
Because of the nature of these taglets and how the string-signatures are implemented, I decided to stick with direct string parsing instead of dynamic compilation.
Two example uses of SimpleMethodSignature:
In this first example, the full signature (the package, class, and function name, including all its parameters) are specified in the string.
import com.github.aliteralmind.codelet.simplesig.SimpleMethodSignature;
import com.github.xbn.lang.reflect.InvokeMethodWithRtx;
import java.lang.reflect.Method;
public class SimpleMethodSigNoDefaults {
public static final void main(String[] ignored) {
String strSig = "com.github.aliteralmind.codelet.examples.simplesig." +
"SimpleMethodSigNoDefaults#getStringForBoolInt(false, 3)";
SimpleMethodSignature simpleSig = null;
try {
simpleSig = SimpleMethodSignature.newFromStringAndDefaults(
String.class, strSig, null, null,
null); //debug (on=System.out, off=null)
} catch(ClassNotFoundException cnfx) {
throw new RuntimeException(cnfx);
}
Method m = null;
try {
m = simpleSig.getMethod();
} catch(NoSuchMethodException nsmx) {
throw new RuntimeException(nsmx);
}
m.setAccessible(true);
Object returnValue = new InvokeMethodWithRtx(m).sstatic().
parameters(simpleSig.getParamValueObjectList().toArray()).invokeGetReturnValue();
System.out.println(returnValue);
}
public static final String getStringForBoolInt(Boolean b, Integer i) {
return "b=" + b + ", i=" + i;
}
}
Output:
b=false, i=3
This second example demonstrates a string signature in which the (package and) class name are not specified. The potential classes, one in which the function must exist, are provided directly.
import com.github.aliteralmind.codelet.simplesig.SimpleMethodSignature;
import com.github.xbn.lang.reflect.InvokeMethodWithRtx;
import java.lang.reflect.Method;
public class SimpleMethodSigWithClassDefaults {
public static final void main(String[] ignored) {
String strSig = "getStringForBoolInt(false, 3)";
SimpleMethodSignature simpleSig = null;
try {
simpleSig = SimpleMethodSignature.newFromStringAndDefaults(
String.class, strSig, null,
new Class[]{Object.class, SimpleMethodSigWithClassDefaults.class, SimpleMethodSignature.class},
null); //debug (on=System.out, off=null)
} catch(ClassNotFoundException cnfx) {
throw new RuntimeException(cnfx);
}
Method m = null;
try {
m = simpleSig.getMethod();
} catch(NoSuchMethodException nsmx) {
throw new RuntimeException(nsmx);
}
m.setAccessible(true);
Object returnValue = new InvokeMethodWithRtx(m).sstatic().
parameters(simpleSig.getParamValueObjectList().toArray()).invokeGetReturnValue();
System.out.println(returnValue);
}
public static final String getStringForBoolInt(Boolean b, Integer i) {
return "b=" + b + ", i=" + i;
}
}
Output:
b=false, i=3

How to write a recursive function using static variables

I am trying to write a code to get the set of points (x,y) that are accessible to a monkey starting from (0,0) such that each point satisfies |x| + |y| < _limitSum. I have written the below code and have used static HashSet of members of Coordinate type (not shown here) and have written a recursive method AccessPositiveQuadrantCoordinates. But the problem is the members of the HashSet passed across the recursive calls is not reflecting the Coordinate members added in previous calls. Can anybody help me on how to pass Object references to make this possible? Is there some other way that this problem can be solved?
public class MonkeyCoordinates {
public static HashSet<Coordinate> _accessibleCoordinates = null;
private int _limitSum;
public MonkeyCoordinates(int limitSum) {
_limitSum = limitSum;
if (_accessibleCoordinates == null)
_accessibleCoordinates = new HashSet<Coordinate>();
}
public int GetAccessibleCoordinateCount() {
_accessibleCoordinates.clear();
Coordinate start = new Coordinate(0,0);
AccessPositiveQuadrantCoordinates(start);
return (_accessibleCoordinates.size() * 4);
}
private void AccessPositiveQuadrantCoordinates(Coordinate current) {
if (current.getCoordinateSum() > _limitSum) { return; }
System.out.println("debug: The set _accessibleCoordinates is ");
for (Coordinate c : _accessibleCoordinates) {
System.out.println("debug:" + c.getXValue() + " " + c.getYValue());
}
if (!_accessibleCoordinates.contains(current)) { _accessibleCoordinates.add(current); }
AccessPositiveQuadrantCoordinates(current.Move(Coordinate.Direction.East));
AccessPositiveQuadrantCoordinates(current.Move(Coordinate.Direction.North));
}
I will give points to all acceptable answers.
Thanks ahead,
Somnath

But the problem is the members of the HashSet passed across the recursive calls is not reflecting the Coordinate members added in previous calls.
I think that's very unlikely. I think it's more likely that your Coordinate class doesn't override equals and hashCode appropriately, which is why the set can't "find" the values.
As an aside, using static variables like this seems like a very bad idea to me - why don't you create the set in GetAccessibleCoordinateCount() and pass the reference to AccessPositiveQuadrantCoordinates, which can in turn keep passing it down in the recursive calls?
(As another aside, I would strongly suggest that you start following Java naming conventions...)

i don't see any problem with making the field _accessibleCoordinates non-static
and you should know that HashSet does not guarantee the same iteration order everytime, you could better use a LinkedList for that purpose...
and about pass by reference, i found this post very useful
java - pass by value - SO link
From what you are doing you would be updating _accessibleCoordinates in every recursive call correctly.

Array parameter passing

I'm in a beginner's java class. This Lab is for me to make a class "Wallet" that manipulates an array that represents a Wallet. Wallet contains the "contents[]" array to store integers represing paper currency. The variable "count" holds the number of banknotes in a wallet. After writing methods (that match provided method calls in a serpate Driver class) to initialize the Wallet and add currency/update "count", I need to transfer the array of one instantiated Wallet to another. I don't know how that would work because the one Wallet class has only been messing with a wallet called "myWallet" and now I need to take a new Wallet called "yourWallet" and fill it with "myWallet"'s array values.
//I should note that using the Java API library is not allowed in for this course
My Wallet class looks like this so far:
public class Wallet
{
// max possible # of banknotes in a wallet
private static final int MAX = 10;
private int contents[];
private int count; // count # of banknotes stored in contents[]
public Wallet()
{
contents = new int[MAX];
count = 0;
}
/** Adds a banknote to the end of a wallet. */
public void addBanknote(int banknoteType)
{
contents[count] = banknoteType;
count = count + 1;
}
/**
* Transfers the contents of one wallet to the end of another. Empties the donor wallet.
*/
public void transfer(Wallet donor)
{
//my code belongs here
}
...
The Driver code looks like this:
public class Driver
{
public static void main(String args[])
{
Wallet myWallet = new Wallet();
myWallet.addBanknote(5);
myWallet.addBanknote(50);
myWallet.addBanknote(10);
myWallet.addBanknote(5);
System.out.println("myWallet contains: " + myWallet.toString());
// transfer all the banknotes from myWallet to yourWallet
Wallet yourWallet = new Wallet();
yourWallet.addBanknote(1);
yourWallet.transfer(myWallet);
System.out.println("\nnow myWallet contains: "
+ myWallet.toString());
System.out.println("yourWallet contains: "
+ yourWallet.toString());
I want to use addBanknote() to help with this, but I don't know how to tell the transfer() method to transfer all of myWallet into yourWallet.
I had the idea to do somethign like this in transfer():
yourWallet.addBanknote(myWallet.contents[i]);
with a traversal to increase i for myWallet contents. It seems horribly wrong, but I'm at a complete loss as to write this method.
If my problem is so unclear that nobody can help, I would be more than happy to receive advice on how to ask a better question or on how to search with correct terms.
Thanks for any help you can provide.

I don't want to spoil your homework as you seem to be going the right way, but I do have some comments which you may either take or not :)
First, I would probably put the bank note types in some enumeration. But as that sounds a bit to advanced, consider
public class Wallet {
public static final int ONE_DOLLAR_BILL = 1;
public static final int FIVE_DOLLAR_BILL = 5;
...
// looks a bit more readable to me
myWallet.addBanknote(ONE_DOLLAR_BILL);
Transferring all the banknotes from the donor to yourself should not be so much of a problem
(a for loop would do) but I think you're in a world of hurt if you are trying to implement a
removeBanknote(int banknoteType);
as you are using count not only as a length but also as an index variable. By this I mean that you assume contents[0] ... contents[count-1] hold valid banknotes. And how do you remove one without too much work?
Warning: a bit more advanced
In your case I would probably opt to have a banknoteType of 0 indicating an empty banknote slot in your wallet, and implement _addBanknote(int banknoteType) as:
public void addBanknote(int banknoteType) {
for (int i=0; i < contents.length; i++) {
if (contents[i] == 0) {
contents[i] = banknoteType;
count++;
return; // OK inserted banknote at the first empty slot
}
}
throw new RuntimeException("Wallet is full");
}
This may be a bit overwhelming at this point. But it would allow you to implement:
public void removeBanknote(int banknoteType) {
for (int i=0; i < contents.length; i++) {
if (contents[i] == banknoteType) {
contents[i] = 0; // better: NO_BANKNOTE = 0
count--;
return;
}
}
throw new RuntimeException("This wallet does not contain a banknote of type " + banknoteType);
}
Please note that in both methods I return when I successfully removed or added the banknote. Only when I could not find a free slot, or the requested banknote, I finish the for loop and end up throwing an exception and thereby stopping the program.

I think the question is fine and I think you're on the right path. The way you're calling Wallet#addBanknote(int) is correct. What you have said is the right thing:
public void transfer(Wallet donor)
{
// Traverse the donor's wallet
// Add the bank note from the donor to this wallet
// What do you think also needs to happen to make sure
// the donor is actually giving their bank note?
}
Just another thing, what would happen in your Wallet#addBanknote(int) method if you have more contents than the MAX?

You can create either a constructor that takes another wallet, or a function (as already mentioned) and use System.arraycopy to copy the array in one fell swoop. System.arraycopy is fast, and its definitely overkill for something small like this, but its good tool to have in your toolkit.
The other alternative mentioned, copy the elements from one array to the other element by element in a loop will work fine too.

The myWallet inside the transfer method is named 'donor', and with that, it doesn't look horribly wrong:
addBanknote (donor.contents [i]);
You just need a loop around it, and to remove the yourWallet. which is the name of an instance of that class. That instance is inside the Class/method this, but needn't be specified, because there is no other addBanknote-Method in scope, which could be meant. (Thanks to mangoDrunk).