length and length in Java

Question

Why do we have the length of an array as an attribute  array length  and for String we have a method  str length     Is there some reason

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Whenever an array is created  its size is specified  So length can be considered as a construction attribute  For String  it essentially a char array  Length is a property of the char array  There is no need to put length as a field  because not everything needs this field  http   www programcreek com 2013 11 start-from-length-length-in-java

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length is a one-off property of Java  It s used to find the size of a single dimensional array    length   is a method  It s used to find the length of a String  It avoids duplicating the value

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Consider   int   myArray   new int 10   String myString    hello world    List lt int gt  myList   new ArrayList lt int gt      myArray length       Gives the length of the array myString length      Gives the length of the string myList size          Gives the length of the list   It s very likely that strings and arrays were designed at different times and hence ended up using different conventions  One justification is that since Strings use arrays internally  a method  length    was used to avoid duplication of the same information  Another is that using a method length   helps emphasize the immutability of strings  even though the size of an array is also unchangeable   Ultimately this is just an inconsistency that evolved that would definitely be fixed if the language were ever redesigned from the ground up  As far as I know no other languages  C   Python  Scala  etc   do the same thing  so this is likely just a slight flaw that ended up as part of the language   You ll get an error if you use the wrong one anyway

User · Answer

I just want to add some remarks to the great answer by Fredrik   The Java Language Specification in Section 4 3 1 states     An object is a class instance or an array    So array has indeed a very special role in Java  I do wonder why    One could argue that current implementation array is was important for a better performance  But than it is an internal structure  which should not be exposed       They could of course have masked the property as a method call and handled it in the compiler but I think it would have been even more confusing to have a method on something that isn t a real class    I agree with Fredrik  that a smart compiler optimazation would have been the better choice  This would also solve the problem  that even if you use a property for arrays  you have not solved the problem for strings and other  immutable  collection types  because  e g   string is based on a char array as you can see on the class definition of String   public final class String implements java io Serializable  Comparable lt String gt   CharSequence                  private final char value             And I do not agree with that it would be even more confusing  because array does inherit all methods from java lang Object    As an engineer I really do not like the answer  Because it has been always this way   and wished there would be a better answer  But in this case it seems to be    tl dr  In my opinion  it is a design flaw of Java and should not have implemented this way

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A bit simplified you can think of it as arrays being a special case and not ordinary classes  a bit like primitives  but not   String and all the collections are classes  hence the methods to get size  length or similar things    I guess the reason at the time of the design was performance  If they created it today they had probably come up with something like array-backed collection classes instead   If anyone is interested  here is a small snippet of code to illustrate the difference between the two in generated code  first the source   public class LengthTest     public static void main String   args        int   array    12 1 4       String string    Hoo       System out println array length       System out println string length             Cutting a way the not so important part of the byte code  running javap -c on the class results in the following for the two last lines   20  getstatic    3    Field java lang System out Ljava io PrintStream  23  aload 1 24  arraylength 25  invokevirtual    4    Method java io PrintStream println  I V 28  getstatic    3    Field java lang System out Ljava io PrintStream  31  aload 2 32  invokevirtual    5    Method java lang String length   I 35  invokevirtual    4    Method java io PrintStream println  I V   In the first case  20-25  the code just asks the JVM for the size of the array  in JNI this would have been a call to GetArrayLength    whereas in the String case  28-35  it needs to do a method call to get the length    In the mid 1990s  without good JITs and stuff  it would have killed performance totally to only have the java util Vector  or something similar  and not a language construct which didn t really behave like a class but was fast  They could of course have masked the property as a method call and handled it in the compiler but I think it would have been even more confusing to have a method on something that isn t a real class

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Let me first highlight three different ways for similar purpose   length -- arrays  int    double    String    -- to know the length of the arrays  length   -- String related Object  String  StringBuilder  etc  -- to know the length of the String  size   --  Collection Object  ArrayList  Set  etc  -- to know the size of the Collection   Now forget about length   consider just length and size     length is not a method  so it completely makes sense that it will not work on objects  It only works on arrays  size   its name describes it better and as it is a method  it will be used in the case of those objects who work with collection  collection frameworks  as I said up there   Now come to length    String is not a primitive array  so we can t use  length  and also not a Collection  so we cant use  size    that s why we also need a different one which is length    keep the differences and serve the purpose    As answer to Why  I find it useful  easy to remember and use and friendly

User · Answer

I was taught that for arrays  length is not retrieved through a method due to the following fear  programmers would just assign the length to a local variable before entering a loop  think a for loop where the conditional uses the array s length   The programmer would supposedly do so to trim down on function calls  and thereby improve performance   The problem is that the length might change during the loop  and the variable wouldn t

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In Java  an Array stores its length separately from the structure that actually holds the data  When you create an Array  you specify its length  and that becomes a defining attribute of the Array  No matter what you do to an Array of length N  change values  null things out  etc    it will always be an Array of length N   A String s length is incidental  it is not an attribute of the String  but a byproduct  Though  Java Strings are in fact immutable  if it were possible to change their contents  you could change their length  Knocking off the last character  if it were possible  would lower the length   I understand this is a fine distinction  and I may get voted down for it  but it s true  If I make an Array of length 4  that length of four is a defining characteristic of the Array  and is true regardless of what is held within  If I make a String that contains  dogs   that String is length 4 because it happens to contain four characters   I see this as justification for doing one with an attribute and the other with a method  In truth  it may just be an unintentional inconsistency  but it s always made sense to me  and this is always how I ve thought about it

[java] length and length() in Java

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