You get the length
of an array, but you ask about the size()
of an ArrayList, and you can inquire into the capacity()
of a StringBuffer. Aren't they really all the same sort of thing? Which does Java use in which circumstance?
Java uses the word length
when you're asking about something that can be determined by telling you the distance from the start to the end of something - thus an array, with fixed size elements arranged sequentially in memory, is going to be a length.
Java uses size
where you're looking at a measure of the number of elements in something which is not in successive memory locations, or may not be, or where the elements themselves differ in size so you can't find out how many of them there are by measuring the distance from start to end.
And Java uses capacity
where you're being told how much something can hold, and not how much it's holding at the moment.
Do you add brackets after the length / capacity / size word? If you're looking at a variable within the object, you don't, but if you're calling a method you do. Which is it in which circumstances? I have no magic answer - you just need to learn / know.
myarray.length - array length - [example]
MyArrayList.size() - number of elements in an Array List - [example]
MyString.length() - String length - [example]
MyStringBuffer.length() - String Buffer length - [example]
MyFile.length() - Number of bytes in a file - [example]
MyVector.size() - Number of elements in a Vector [example]
and [example in Servlet]
MyAppletDimension.size() - returns an object with the width and height of an applet. This is deprecated
and you should now use getsize ...
MyAppletDimension.getsize() - returns an object with the width and height of an applet - [example]
MyStringBuffer.capacity() - the number of characters that a StringBuffer can currently hold
(written 2010-02-24, updated 2010-03-03)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articlesJ707 - Java - Strings 
String handling - from first steps to practical examples - (2010-11-13) 
Trying out our Java examples on our web site - (2008-02-27) 
An answer to a student asking 'Help' - (2007-11-27)J714 - Java - Fundamental classes 
Java - factory method, encapsulation, hashmap example - (2014-11-27) 
Sorting - naturally, or into a different order - (2010-08-14) 
for and foreach in Java - (2010-04-22) 
Sorting Collections of Objects in Java - (2009-09-25) 
Viv.java uses unchecked or unsafe operations - explanation and cure - (2009-09-24) 
Java Collection Objects in the java.util package - (2009-08-05) 
Java - Generics - (2008-11-27) 
Java, sorting, ArrayList example, generics - (2008-01-11) 
Java sorting - comparable v comparator - (2007-02-02) 
Do languages change? - (2004-09-08)J802 - Java - Abstract Windowing Toolkit 
Computer Graphics is fun - even if Java Applets are Old Hat - (2010-11-13) 
Java - from applets to servlets. JSPs and more - (2010-02-27) 
Client side (Applet) and Server side (Servlet) Graphics in Java - (2009-06-17) 
Java - Client side applet applications as well as server side - (2007-08-29)
Some other Articles
Java Server Pages - putting it all togetherReading and writing cookies in Java Servlets and JSPsCalculation within objects - early, last minute, or cached?Getting a phone line changed ...Length, size or capacity in Java?Java arrays - are they true arrays or not?Removing duplicates from a MySQL tableCompile but do not run PHP - syntax check onlyOptimising and caching your MySQL enquiriesCounting rows in joined MySQL tables