Say I have a car that has a certain amount of gas in it and if it gets below:
public static final double TESTCLASS = 0.000001;
then it is empty.
How do I represent this with something like this:
public boolean isEmpty(){
}
I've tried doing just this (below), but I get an error with the "<"
public boolean isEmpty(){
gas < TESTCLASS;
}
You've missed out the return statement.
public boolean isEmpty(){
return (gas < TESTCLASS);
}
Putting the Boolean expression in parentheses makes sure it is evaluated as a Boolean - not sure if you really need it in this case though.
PS: It's a good idea to name your variables semantically - so TESTCLASS would be better as "gasLimit" or "emptyGas"
Related
I have pointcut the String .hashCode in an around adivce. I would like to change the target (String) to uppercase then proceed with the call to original hashCode. I'm not sure how to do that, the following code doesnt work properly.
#Pointcut("call(int hashCode(..)) && target(sourceString) && within(com.sample.package..*)")
public void hashCodePointcut(final String sourceString) {}
#Around("hashCodePointcut(sourceString)")
public Object around(final ProceedingJoinPoint joinPoint, String sourceString)
throws Throwable {
System.out.println("<<<<<<<<<<<<<<<<<Invoking hashCode on "+joinPoint.getSourceLocation().getFileName());
System.out.println("<<<<<<<<<<<<<<<<<Target String: "+ sourceString);
sourceString = sourceString.toUpperCase();
return joinPoint.proceed();
}
Let me preface this answer by saying that I advise strongly against using this. IMO, once you start mucking about with hash-methods after the fact and making the result less unique (by ignoring case in Strings, etc.) you are wading hip-deep in future problems and firmly on the dark side of coding. With that said strongly enough, here's how I'd do it:
#Pointcut("call(int java.lang.String.hashCode(..)) && target(sourceString) && within(com.sample.packages..*) && !within(your.package.AspectClass)")
public void hashCodePointcut(final String sourceString) {}
#Around("hashCodePointcut(sourceString)")
public Object around(final ProceedingJoinPoint joinPoint, String sourceString)
throws Throwable {
System.out.println("<<<<<<<<<<<<<<<<<Invoking hashCode on"+joinPoint.getSourceLocation().getFileName());
System.out.println("<<<<<<<<<<<<<<<<<Target String: "+ sourceString);
sourceString = sourceString.toUpperCase();
return sourceString.hashCode();
}
I have not tried this yet, but it should reroute all calls to the String.hashCode() method to a call of the same method on the .toUpperCase() of that String. It has to be done this way because you can not change the target of the joinPoint (which is why your advice probably does nothing as it is).
The addition of "!within(your.package.AspectClass)" to the pointcut prevents the same advice to be applied in an infinite loop on your calls within the aspect.
Let me know if this helps, or if there's still something going wrong (aside from the fact that you are mucking about with hashCode() ;) ).
The problem I encountered and am unable to solve goes something like this. I have two classes:
class1
{
private:
int identifier;
double value;
public:
setters,getters,etc...
}
class2
{
private:
vector<class1> objects;
vector<int> some_value;
vector<double> other_value;
...
}
The problem is I need to search through the vector of objects in an object of the second class by its identifier in the class1 object(from a member function of class2). I tried something like:
int getObj(const int &ident, double &returnedValue, double &returnedOther_value)
{
int p;
p = find(objects.begin()->getIdentifier(),objects.end()->getIdentifier(),ident);
..
.. and then i was hoping to find a way to return from the found iterator values of corresponding(non-const) member variables value and other_value from both classes, but the code so far does not compile, because I'm likely doing the search all wrong. Is there a way I could do this with the find(or any other algorithm) or should I stick to my previous working realization with no algorithms?
You need to use find_if with a custom predicate. Something like:
class HasIdentifier:public unary_function<class1, bool>
{
public:
HasIdentifier(int id) : m_id(id) { }
bool operator()(const class1& c)const
{
return (c.getIdentifier() == m_id);
}
private:
int m_id;
};
// Then, to find it:
vector<class1>::iterator itElem = find_if(objects.begin(), objects.end(), HasIdentifier(ident));
I haven't tested it, so maybe it needs some tweaking.
If you have C11, I guess you can use lambdas, but I don't have it, so I haven't had the chance to learn them.
UPDATE:
I've added an example in http://ideone.com/D1DWU
I have created two classes as interface / implementation classes, and wish to pass a particular example of one class to a method in the other. The definitions are as follows...
Class BigInt...
Option Explicit
Public Sub dothing(ByRef passed_object As MyInt)
End Sub
and an implementation BigImplementation...
Option Explicit
Implements BigInt
Public Sub BigInt_dothing(ByRef passed_obj As MyInt)
Dim i As Integer
i = passed_obj.getprop
End Sub
The class I am planning to pass is...
Option Explicit
Public Property Get getprop() As Integer
End Property
Public Property Let letprop(ByVal myval As Integer)
End Property
implemented as MyImplementation thus...
Option Explicit
Implements MyInt
Private myval As Integer
Public Property Get myint_getprop() As Integer
myint_getprop = myval
End Property
Public Property Let myint_letprop(ByVal passed_int As Integer)
myval = passed_int
End Property
I am then driving this with the following snippet of code:-
Private Sub Command_Click()
Dim myobj As MyInt
Set myobj = New MyImplementation
Dim mybigobj As BigInt
Set mybigobj = New BigImplementation
myobj.letprop = 1
Call mysub(myobj)
mybigobj.dothing (myobj) ' Line with problem
End Sub
Private Sub mysub(ByVal passed_obj As MyInt)
Dim i As Integer
i = passed_obj.getprop
End Sub
When the execution reaches the line marked, I get run-time error 438 - Object doesn't support property or method. The call to the ordinary function mysub works perfectly. Does anyone know what I am doing wrong and what I need to do to fix this?
Use either
mybigobj.dothing myobj
or
Call mybigobj.dothing(myobj)
Putting extra parentheses around a reference evaluates its default property and passes it's value as actual argument.
mybigobj.dothing requires its parameter to be a MyInt
Public Sub BigInt_dothing(ByRef passed_obj As MyInt)
Dim i As Integer
i = passed_obj.getprop
End Sub
You're passing a MyImplementation
Set myobj = New MyImplementation
So possiblly something like (my vb is rusty)
mybigobj.dothing (myobj.myint_getprop())
I've got an object, which I'll call MyObject. It's a class that controls a particular data row.
I've then got a collection class, called MyObjectCollection:
public class MyObjectCollection : List<MyObject> {}
Why can I not do the following:
List<MyObject> list = this.DoSomethingHere();
MyObjectCollection collection = (MyObjectCollection)list;
Thanks in advance.
Edit: The error is InvalidCastException
My guess is that DoSomethingHere doesn't return an instance of MyObjectCollection.
Let's get rid of all the generics etc here, as they're not relevant. Here's what I suspect you're trying to do:
public static object CreateAnObject()
{
return new object();
}
object o = CreateAnObject();
string s = (string) o;
That will fail (at execution time) and quite rightly so.
To bring it back to your code, unless DoSomethingHere actually returns a MyObjectCollection at execution time, the cast will fail.
Because a List<MyObject> is not a MyObjectCollection. The reverse is true: you could cast a MyObjectCollection to a List because MyObjectCollection inherits from List<MyObject> and thus, for all intents and purposes, IS A List<MyObject>.
The only thing you can do is to define a constructor on MyObjectCollection that takes an Ienumerable as a parameter and initalizes itself with the data in the other one, but that will make a new object containing the same data:
public class MyObjectCollection : List<MyObject>
{
public MyObjectCollection(IEnumerable<MyObject> items)
{
Addrange(items);
}
}
UPDATE:
As noted in the comment, you COULD have the cast succeed at runtime, provided that DoSomething actually returns an instance of MyObjectCollection. If it does, the object effectively is a MyObjectCollection, and the cast is completely legal.
I'd have to say, it is bad practice in my view to upcast something like that. If the function returns a List, you should not rely on a specific implementation of List. Either modify the return type of DoSomething, if you own that function, and return a MyObjectCollection, or deal with it as a list.
Without knowing what exactly is created inside DoSomething() we have to assume either:
You have a misunderstanding about the inheritence in .Net.
you have
A : B
B DoSomething()
{
return new B();
}
// then this is
B b = new B();
A a = (A)b;
Clearly b is a B but not an A. B might look much like A but it is not (if you traverse the parentage of b you won't find A anywhere)
This is true irrespective of the Generics involved (though that sometimes can cause situations where something that could work doesn't see the co-contra variance in c# 4.0)
or
A : B
B DoSomething()
{
return new A();
}
// then this is
B b = new A();
A a = (A)b;
Which in the absence of Generics will work.
You can't do it because (I guessing) the list instance returned from DoSomethingHere isn't derived from MyObjectCollection
You could create an implicit operator that would allow you to convert between your object and the list. You would need an constructor that takes a list and to property that returns the underlaying list.
public static implicit operator List<MyObject>(MyObjectCollection oCollection)
{
//Convert here
return MyObjectCollection.BaseList;
}
public static implicit operator MyObjectCollection(List<MyObject> oList)
{
//Convert here
return new MyObjectCollection(oList);
}
One advantage of lambda expressions is that you have to evaluate a function only when you need its result.
In the following (simple) example, the text function is only evaluated when a writer is present:
public static void PrintLine(Func<string> text, TextWriter writer)
{
if (writer != null)
{
writer.WriteLine(text());
}
}
Unfortunately, this makes using the code a little bit ugly. You cannot call it with a constant or variable like
PrintLine("Some text", Console.Out);
and have to call it this way:
PrintLine(() => "Some text", Console.Out);
The compiler is not able to "infer" a parameterless function from the passed constant. Are there any plans to improve this in future versions of C# or am I missing something?
UPDATE:
I just found a dirty hack myself:
public class F<T>
{
private readonly T value;
private readonly Func<T> func;
public F(T value) { this.value = value; }
public F(Func<T> func) {this.func = func; }
public static implicit operator F<T>(T value)
{
return new F<T>(value);
}
public static implicit operator F<T>(Func<T> func)
{
return new F<T>(func);
}
public T Eval()
{
return this.func != null ? this.func() : this.value;
}
}
Now i can just define the function as:
public static void PrintLine(F<string> text, TextWriter writer)
{
if (writer != null)
{
writer.WriteLine(text.Eval());
}
}
and call it both with a function or a value.
I doubt that C# will get this feature, but D has it. What you've outlined is a suitable way to implement lazy argument evaluation in C#, and probably compiles very similarly to lazy in D, and in more pure functional languages.
All things considered, the four extra characters, plus optional white space, are not an exceptionally large price to pay for clear overload resolution and expressiveness in what is becoming a multi-paradigm strong-typed language.
The compiler is very good at inferring types, it is not good at inferring intent. One of the tricky things about all the new syntactic sugar in C# 3 is that they can lead to confusion as to what exactly the compiler does with them.
Consider your example:
() => "SomeText"
The compiler sees this and understands that you intend to create an anonymous function that takes no parameters and returns a type of System.String. This is all inferred from the lambda expression you gave it. In reality your lambda gets compiled to this:
delegate {
return "SomeText";
};
and it is a delegate to this anonymous function that you are sending to PrintLine for execution.
It has always been important in the past but now with LINQ, lambdas, iterator blocks, automatically implemented properties, among other things it is of the utmost importance to use a tool like .NET Reflector to take a look at your code after it is compiled to see what really makes those features work.
Unfortunately, the ugly syntax is all you have in C#.
The "dirty hack" from the update does not work, because it does not delay the evaluation of string parameters: they get evaluated before being passed to operator F<T>(T value).
Compare PrintLine(() => string.Join(", ", names), myWriter) to PrintLine(string.Join(", ", names), myWriter) In the first case, the strings are joined only if they are printed; in the second case, the strings are joined no matter what: only the printing is conditional. In other words, the evaluation is not lazy at all.
Well those two statements are completely different. One is defining a function, while the other is a statement. Confusing the syntax would be much trickier.
() => "SomeText" //this is a function
"SomeText" //this is a string
You could use an overload:-
public static void PrintLine(string text, TextWriter writer)
{
PrintLine(() => text, writer);
}
You could write an extension method on String to glue it in. You should be able to write "Some text".PrintLine(Console.Out); and have it do the work for you.
Oddly enough, I did some playing with lazy evaluation of lambda expressions a few weeks back and blogged about it here.
To be honest I don't fully understand your problem, but your solutions seems a tad complicated to me.
I think a problem I solved using lambda call is similar, maybe you could use it as inspiration: I want to see if a key exists in a dictionary, if not, I would need to execute a (costly) load operation.
public static class DictionaryHelper
{
public static TValue GetValueOrLambdaDefault<TKey, TValue> (this IDictionary<TKey, TValue> dictionary, TKey key, Func<TValue> func)
{
if (dictionary.ContainsKey(key))
return dictionary[key];
else
return func.Invoke();
}
}
[TestClass]
public class DictionaryHelperTest
{
[TestMethod]
public void GetValueOrLambdaDefaultTest()
{
var dict = new Dictionary<int, string>();
try
{
var res1 = dict.GetValueOrLambdaDefault(1, () => LoadObject());
Assert.Fail("Exception should be thrown");
}
catch { /*Exception should be thrown*/ }
dict.Add(1, "");
try
{
var res1 = dict.GetValueOrLambdaDefault(1, () => LoadObject());
}
catch { Assert.Fail("Exception should not be thrown"); }
}
public static string LoadObject()
{
throw new Exception();
}
}