List.contains problem
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List.contains problem
by Jimmy200012
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Hi, need some help to understand why this won't work:
import com.sun.webui.jsf.model.Option; import java.util.*; public class ArrayListContains { /** Create a new instance of ArrayListContains */ public ArrayListContains() { } public static void main(String[] args) { // Now the real test; Option[] myOpt = new Option[]{ new Option("R2", "R2"), new Option("L22", "L2") }; Option testOpt = new Option("R2", "R2"); ArrayList<Option> optList = new ArrayList<Option>(Arrays.asList(myOpt)); if (optList.contains(testOpt)) { // YOU THOUGHT WOULD PRINT THIS, BUT NOT. // com.sun.webui.jsf.model.Option DID NOT IMPLEMENTS EQUALS? // WHERE CAN I FIND SOME DOC ABOUT THIS? System.out.println("Yes!!!!!!!!!!!!R2"); } else { System.out.println("why?????????????"); } } } |
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Re: List.contains problem
by Rick Fincher
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Hi Jimmy,
This does not work because testOpt and MyOpt[0] are two different objects, even though their labels and values are the same. If your code was the following it would work: Option testOpt = new Option("R2", "R2"); Option[] myOpt = new Option[]{ testOpt, new Option("L22", "L2") }; The equals(0 method compares pointers to the object. For Strings this works because Strings are immutable values and only one copy of a String containing the same characters is stored in memory. If you do code like this: String s1="hello"; String s2="hello"; Only one copy of the String "hello" is stored in memory with both s1 and s2 pointing to it. If you do this later: s2="goodby"; A new String "goodby" is created and s2 is set to point to it, unless "goodby" was already in memory, then s2 would be set to point to the pre-existing String "goodby". Because of that the String method equals() will work in situations like this. That's one of the reasons String methods are slow in Java. Every time you change it a new String is created and the old one is left for garbage collection. Manipulating StringBuffers is much faster and more memory efficient. Rick Jimmy200012 wrote: > Hi, need some help to understand why this won't work: > > import com.sun.webui.jsf.model.Option; > import java.util.*; > > public class ArrayListContains { > > /** Create a new instance of ArrayListContains */ > public ArrayListContains() { > } > > public static void main(String[] args) { > // Now the real test; > Option[] myOpt = new Option[]{ > new Option("R2", "R2"), > new Option("L22", "L2") > }; > > Option testOpt = new Option("R2", "R2"); > > ArrayList<Option> optList = new > ArrayList<Option>(Arrays.asList(myOpt)); > if (optList.contains(testOpt)) { > > // YOU THOUGHT WOULD PRINT THIS, BUT NOT. > // com.sun.webui.jsf.model.Option DID NOT IMPLEMENTS EQUALS? > // WHERE CAN I FIND SOME DOC ABOUT THIS? > System.out.println("Yes!!!!!!!!!!!!R2"); > } else { > System.out.println("why?????????????"); > } > } > } > --------------------------------------------------------------------- To unsubscribe, e-mail: users-unsubscribe@... For additional commands, e-mail: users-help@... |
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Re: List.contains problem
by Jimmy200012
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Rick,
Thank you so much for the quick reply. So, is there any quick way to check if a new Option is in the Option Array []? Do I have to loop through to find out? So what's the contians method for? Thanks again! Jimmy
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Re: List.contains problem
by Rick Fincher
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Hi Jimmy,
The contains() method just loops through the list and checks to see if any of the objects in the list are the same object as the one you pass in through the contains() method. By "the same" I mean actually the same, not just one that has the same entries for label and value, but physically the very same option object. If you are trying to see if an option with the same label and same value is already in there, you can get an iterator and loop through quickly, as in: ArrayList<Option> opt = new ArrayList<Option>(); Iterator<Option> optIt = opt.iterator(); boolean duplicateFound = false; while (optIt.hasNext()) { if ( (optIt.next().getValue().toString().equals (testOpt.value()).toString()) && (optIt.next().getLabel().equals(testOpt.label())) ) { duplicateFound = true; } } You can make that a method that returns a boolean to make it a little cleaner if you like. This is assuming that the toString() method for the value is meaningful. If the value is something else, ubstitute the appropriate comparison. The label is already a String so you don't have to use toString(). Rick Jimmy200012 wrote: > Rick, > > Thank you so much for the quick reply. So, is there any quick way to check > if a new Option is in the Option Array []? Do I have to loop through to find > out? So what's the contians method for? > > Thanks again! > Jimmy > > > Rick Fincher wrote: > >> Hi Jimmy, >> >> This does not work because testOpt and MyOpt[0] are two different >> objects, even though their labels and values are the same. If your code >> was the following it would work: >> >> Option testOpt = new Option("R2", "R2"); >> >> Option[] myOpt = new Option[]{ >> testOpt, >> new Option("L22", "L2") >> }; >> >> The equals(0 method compares pointers to the object. For Strings this >> works because Strings are immutable values and only one copy of a String >> containing the same characters is stored in memory. >> >> If you do code like this: >> >> String s1="hello"; >> String s2="hello"; >> >> Only one copy of the String "hello" is stored in memory with both s1 and >> s2 pointing to it. If you do this later: >> >> s2="goodby"; >> >> A new String "goodby" is created and s2 is set to point to it, unless >> "goodby" was already in memory, then s2 would be set to point to the >> pre-existing String "goodby". >> >> Because of that the String method equals() will work in situations like >> this. >> >> That's one of the reasons String methods are slow in Java. Every time you >> change it a new String is created and the old one is left for garbage >> collection. >> >> Manipulating StringBuffers is much faster and more memory efficient. >> >> Rick >> >> >> Jimmy200012 wrote: >> >>> Hi, need some help to understand why this won't work: >>> >>> import com.sun.webui.jsf.model.Option; >>> import java.util.*; >>> >>> public class ArrayListContains { >>> >>> /** Create a new instance of ArrayListContains */ >>> public ArrayListContains() { >>> } >>> >>> public static void main(String[] args) { >>> // Now the real test; >>> Option[] myOpt = new Option[]{ >>> new Option("R2", "R2"), >>> new Option("L22", "L2") >>> }; >>> >>> Option testOpt = new Option("R2", "R2"); >>> >>> ArrayList<Option> optList = new >>> ArrayList<Option>(Arrays.asList(myOpt)); >>> if (optList.contains(testOpt)) { >>> >>> // YOU THOUGHT WOULD PRINT THIS, BUT NOT. >>> // com.sun.webui.jsf.model.Option DID NOT IMPLEMENTS EQUALS? >>> // WHERE CAN I FIND SOME DOC ABOUT THIS? >>> System.out.println("Yes!!!!!!!!!!!!R2"); >>> } else { >>> System.out.println("why?????????????"); >>> } >>> } >>> } >>> >>> >> --------------------------------------------------------------------- >> To unsubscribe, e-mail: users-unsubscribe@... >> For additional commands, e-mail: users-help@... >> >> >> >> > > --------------------------------------------------------------------- To unsubscribe, e-mail: users-unsubscribe@... For additional commands, e-mail: users-help@... |
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Re: List.contains problem
by Jimmy200012
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Rick,
Ok. I see. I was hoping that as long as two Options's lable and value are same, they are treated as equal. But I guess, not the case. Looks like the contains method is not very useful at all. B/c most of time, you have to loop throught to find it out if there's another one already in the list. Thanks and have a good day. Jimmy
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Re: List.contains problem
by Rick Fincher
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Hi Jimmy,
The contains() method is useful if you have a central list of options that you use in multiple pages. You can use it to see if a particular option has been inserted in a particular list. If you store a list of options in the session bean, for example, you don't have to re-create all those options each time a page loads. Changes can be made to the central option in the session bean (text color, for example, or icons) and these will be reflected in any dropdown the option is used in on any page. You can use contains() to see which of your central list of options has been added to your dropDown and add or delete as necessary. Rick Jimmy200012 wrote: > Rick, > > Ok. I see. I was hoping that as long as two Options's lable and value are > same, they are treated as equal. But I guess, not the case. > > Looks like the contains method is not very useful at all. B/c most of time, > you have to loop throught to find it out if there's another one already in > the list. > > Thanks and have a good day. > Jimmy > > > Rick Fincher wrote: > >> Hi Jimmy, >> >> The contains() method just loops through the list and checks to see if >> any of the objects in the list are the same object as the one you pass >> in through the contains() method. >> >> By "the same" I mean actually the same, not just one that has the same >> entries for label and value, but physically the very same option object. >> >> If you are trying to see if an option with the same label and same value >> is already in there, you can get an iterator and loop through quickly, >> as in: >> >> ArrayList<Option> opt = new ArrayList<Option>(); >> >> Iterator<Option> optIt = opt.iterator(); >> boolean duplicateFound = false; >> >> while (optIt.hasNext()) { >> if ( (optIt.next().getValue().toString().equals >> (testOpt.value()).toString()) && >> (optIt.next().getLabel().equals(testOpt.label())) ) { >> duplicateFound = true; >> } >> } >> >> You can make that a method that returns a boolean to make it a little >> cleaner if you like. This is assuming that the toString() method for >> the value is meaningful. If the value is something else, ubstitute the >> appropriate comparison. >> >> The label is already a String so you don't have to use toString(). >> >> >> Rick >> >> Jimmy200012 wrote: >> >>> Rick, >>> >>> Thank you so much for the quick reply. So, is there any quick way to >>> check >>> if a new Option is in the Option Array []? Do I have to loop through to >>> find >>> out? So what's the contians method for? >>> >>> Thanks again! >>> Jimmy >>> >>> >>> Rick Fincher wrote: >>> >>> >>>> Hi Jimmy, >>>> >>>> This does not work because testOpt and MyOpt[0] are two different >>>> objects, even though their labels and values are the same. If your code >>>> was the following it would work: >>>> >>>> Option testOpt = new Option("R2", "R2"); >>>> >>>> Option[] myOpt = new Option[]{ >>>> testOpt, >>>> new Option("L22", "L2") >>>> }; >>>> >>>> The equals(0 method compares pointers to the object. For Strings this >>>> works because Strings are immutable values and only one copy of a String >>>> containing the same characters is stored in memory. >>>> >>>> If you do code like this: >>>> >>>> String s1="hello"; >>>> String s2="hello"; >>>> >>>> Only one copy of the String "hello" is stored in memory with both s1 and >>>> s2 pointing to it. If you do this later: >>>> >>>> s2="goodby"; >>>> >>>> A new String "goodby" is created and s2 is set to point to it, unless >>>> "goodby" was already in memory, then s2 would be set to point to the >>>> pre-existing String "goodby". >>>> >>>> Because of that the String method equals() will work in situations like >>>> this. >>>> >>>> That's one of the reasons String methods are slow in Java. Every time >>>> you >>>> change it a new String is created and the old one is left for garbage >>>> collection. >>>> >>>> Manipulating StringBuffers is much faster and more memory efficient. >>>> >>>> Rick >>>> >>>> >>>> Jimmy200012 wrote: >>>> >>>> >>>>> Hi, need some help to understand why this won't work: >>>>> >>>>> import com.sun.webui.jsf.model.Option; >>>>> import java.util.*; >>>>> >>>>> public class ArrayListContains { >>>>> >>>>> /** Create a new instance of ArrayListContains */ >>>>> public ArrayListContains() { >>>>> } >>>>> >>>>> public static void main(String[] args) { >>>>> // Now the real test; >>>>> Option[] myOpt = new Option[]{ >>>>> new Option("R2", "R2"), >>>>> new Option("L22", "L2") >>>>> }; >>>>> >>>>> Option testOpt = new Option("R2", "R2"); >>>>> >>>>> ArrayList<Option> optList = new >>>>> ArrayList<Option>(Arrays.asList(myOpt)); >>>>> if (optList.contains(testOpt)) { >>>>> >>>>> // YOU THOUGHT WOULD PRINT THIS, BUT NOT. >>>>> // com.sun.webui.jsf.model.Option DID NOT IMPLEMENTS >>>>> EQUALS? >>>>> // WHERE CAN I FIND SOME DOC ABOUT THIS? >>>>> System.out.println("Yes!!!!!!!!!!!!R2"); >>>>> } else { >>>>> System.out.println("why?????????????"); >>>>> } >>>>> } >>>>> } >>>>> >>>>> >>>>> >>>> --------------------------------------------------------------------- >>>> To unsubscribe, e-mail: users-unsubscribe@... >>>> For additional commands, e-mail: users-help@... >>>> >>>> >>>> >>>> >>>> >>> >>> >> --------------------------------------------------------------------- >> To unsubscribe, e-mail: users-unsubscribe@... >> For additional commands, e-mail: users-help@... >> >> >> >> > > --------------------------------------------------------------------- To unsubscribe, e-mail: users-unsubscribe@... For additional commands, e-mail: users-help@... |
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