Exact difference between CharSequence and String in java

Question

I read this previous post  Can any one say what the exact difference between CharSequence and String is  other than the fact that String implements CharSequence and that String is a sequence of character  For example   CharSequence obj    hello   String str    hello   System out println  output is       obj          str     What happens when  hello  is assigned to obj and again to str

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CharSequence is a contract  interface   and String is an implementation of this contract   public final class String extends Object      implements Serializable  Comparable lt String gt   CharSequence   The documentation for CharSequence is      A CharSequence is a readable sequence of char values  This interface   provides uniform  read-only access to many different kinds of char   sequences  A char value represents a character in the Basic   Multilingual Plane  BMP  or a surrogate  Refer to Unicode Character   Representation for details

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tl dr One is an interface  CharSequence  while other is a concrete implementation of that interface  String   CharSequence animal    quot cat quot       String  object presented as the interface  CharSequence    Just like ArrayList is a List  and HashMap is a Map  so too String is a CharSequence  As an interface  normally the CharSequence would be more commonly seen than String  but some twisted history resulted in the interface being defined years after the implementation  So in older APIs we often see String while in newer APIs we tend to see CharSequence used to define arguments and return types  Details Nowadays we know that generally an API framework should focus on exporting interfaces primarily and concrete classes secondarily  But we did not always know this lesson so well  The String class came first in Java  Only later did they place a front-facing interface  CharSequence  Twisted History A little history might help with understanding  In its early days  Java was rushed to market a bit ahead of its time  due to the Internet Web mania animating the industry  Some libraries were not as well thought-through as they should have been  String handling was one of those areas  Also  Java was one of the earliest production-oriented non-academic Object-Oriented Programming  OOP  environments  The only successful real-world rubber-meets-the-road implementations of OOP before that was some limited versions of SmallTalk  then Objective-C with NeXTSTEP OpenStep  So  many practical lessons were yet to be learned  Java started with the String class and StringBuffer class  But those two classes were unrelated  not tied to each other by inheritance nor interface  Later  the Java team recognized that there should have been a unifying tie between string-related implementations to make them interchangeable  In Java 4 the team added the CharSequence interface and retroactively implemented that interface on String and String Buffer  as well as adding another implementation CharBuffer  Later in Java 5 they added StringBuilder  basically a unsynchronized and therefore somewhat faster version of StringBuffer  So these string-oriented classes are a bit of a mess  and a little confusing to learn about  Many libraries and interfaces were built to take and return String objects  Nowadays such libraries should generally be built to expect CharSequence  But  a  String seems to still dominate the mindspace  and  b  there may be some subtle technical issues when mixing the various CharSequence implementations  With the 20 20 vision of hindsight we can see that all this string stuff could have been better handled  but here we are  Ideally Java would have started with an interface and or superclass that would be used in many places where we now use String  just as we use the Collection or List interfaces in place of the ArrayList or LinkedList implementations  Interface Versus Class The key difference about CharSequence is that it is an interface  not an implementation  That means you cannot directly instantiate a CharSequence  Rather you instantiate one of the classes that implements that interface  For example  here we have x that looks like a CharSequence but underneath is actually a StringBuilder object  CharSequence x   new StringBuilder   quot dog quot         Looks like a  CharSequence  but is actually a  StringBuilder  instance   This becomes less obvious when using a String literal  Keep in mind that when you see source code with just quote marks around characters  the compiler is translating that into a String object  CharSequence y    quot cat quot       Looks like a  CharSequence  but is actually a  String  instance   Literal versus constructor There are some subtle differences between  quot cat quot  and new String  quot cat quot   as discussed in this other Question  but are irrelevant here  Class Diagram This class diagram may help to guide you  I noted the version of Java in which they appeared to demonstrate how much change has churned through these classes and interfaces   Text Blocks Other than adding more Unicode characters including a multitude of emoji  in recent years not much has changed in Java for working with text  Until text blocks  Text blocks are a new way of better handling the tedium of string literals with multiple lines or character-escaping  This would make writing embedded code strings such as HTML  XML  SQL  or JSON much more convenient  To quote JEP 378   A text block is a multi-line string literal that avoids the need for most escape sequences  automatically formats the string in a predictable way  and gives the developer control over the format when desired   The text blocks feature does not introduce a new data type  Text blocks are merely a new syntax for writing a String literal  A text block produces a String object  just like the conventional literal syntax  A text block produces a String object  which is also a CharSequence object  as discussed above  SQL example To quote JSR 378 again    Using  quot one-dimensional quot  string literals  String query    quot SELECT   quot EMP ID  quot     quot LAST NAME  quot  FROM   quot EMPLOYEE TB  quot  n quot                    quot WHERE   quot CITY  quot     INDIANAPOLIS  n quot                    quot ORDER BY   quot EMP ID  quot     quot LAST NAME  quot   n quot    Using a  quot two-dimensional quot  block of text String query    quot  quot  quot                 SELECT  quot EMP ID quot    quot LAST NAME quot  FROM  quot EMPLOYEE TB quot                 WHERE  quot CITY quot     INDIANAPOLIS                 ORDER BY  quot EMP ID quot    quot LAST NAME quot                   quot  quot  quot    Text blocks are found in Java 15 and later  per JEP 378  Text Blocks  First previewed in Java 13  under JEP 355  Text Blocks  Preview   Then previewed again in Java 14 under JEP 368  Text Blocks  Second Preview   This effort was preceded by JEP 326  Raw String Literals  Preview   The concepts were reworked to produce the Text Blocks feature instead

User · Answer

From the Java API of CharSequence      A CharSequence is a readable sequence of characters  This interface provides uniform  read-only access to many different kinds of character sequences    This interface is then used by String  CharBuffer and StringBuffer to keep consistency for all method names

User · Answer

In charSequence you don t have very useful methods which are available for String  If you don t want to look in the documentation  type     obj  and    str    and see what methods your compilator offers you  That s the basic difference for me

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Consider UTF-8  In UTF-8 Unicode code points are built from one or more bytes  A class encapsulating a UTF-8 byte array can implement the CharSequence interface but is most decidedly not a String  Certainly you can t pass a UTF-8 byte array where a String is expected but you certainly can pass a UTF-8 wrapper class that implements CharSequence when the contract is relaxed to allow a CharSequence  On my project  I am developing a class called CBTF8Field  Compressed Binary Transfer Format - Eight Bit  to provide data compression for xml and am looking to use the CharSequence interface to implement conversions from CBTF8 byte arrays to from character arrays  UTF-16  and byte arrays  UTF-8    The reason I came here was to get a complete understanding of the subsequence contract

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I know it a kind of obvious  but CharSequence is an interface whereas String is a concrete class     java lang String is an implementation of this interface

User · Answer

General differences  There are several classes which implement the CharSequence interface besides String  Among these are   StringBuilder for variable-length character sequences which can be modified CharBuffer for fixed-length low-level character sequences which can be modified   Any method which accepts a CharSequence can operate on all of these equally well  Any method which only accepts a String will require conversion  So using CharSequence as an argument type in all the places where you don t care about the internals is prudent  However you should use String as a return type if you actually return a String  because that avoids possible conversions of returned values if the calling method actually does require a String   Also note that maps should use String as key type  not CharSequence  as map keys must not change  In other words  sometimes the immutable nature of String is essential   Specific code snippet  As for the code you pasted  simply compile that  and have a look at the JVM bytecode using javap -v  There you will notice that both obj and str are references to the same constant object  As a String is immutable  this kind of sharing is all right   The   operator of String is compiled as invocations of various StringBuilder append calls  So it is equivalent to  System out println     new StringBuilder       append  output is         append  Object obj     append         append str     toString       I must confess I m a bit surprised that my compiler javac 1 6 0 33 compiles the   obj using StringBuilder append Object  instead of StringBuilder append CharSequence   The former probably involves a call to the toString   method of the object  whereas the latter should be possible in a more efficient way  On the other hand  String toString   simply returns the String itself  so there is little penalty there  So  StringBuilder append String  might be more efficient by about one method invocation

User · Answer

other than the fact that String implements CharSequence and that String is a sequence of character    Several things happen in your code   CharSequence obj    hello     That creates a String literal   hello   which is a String object  Being a String  which implements CharSequence  it is also a CharSequence   you can read this post about coding to interface for example    The next line   String str    hello     is a little more complex  String literals in Java are held in a pool  interned  so the  hello  on this line is the same object  identity  as the  hello  on the first line  Therefore  this line only assigns the same String literal to str   At this point  both obj and str are references to the String literal  hello  and are therefore equals     and they are both a String and a CharSequence   I suggest you test this code  showing in action what I just wrote   public static void main String   args        CharSequence obj    hello       String str    hello       System out println  Type of obj      obj getClass   getSimpleName         System out println  Type of str      str getClass   getSimpleName         System out println  Value of obj      obj       System out println  Value of str      str       System out println  Is obj a String       obj instanceof String        System out println  Is obj a CharSequence       obj instanceof CharSequence        System out println  Is str a String       str instanceof String        System out println  Is str a CharSequence       str instanceof CharSequence        System out println  Is   hello   a String        hello  instanceof String        System out println  Is   hello   a CharSequence        hello  instanceof CharSequence        System out println  str equals obj       str equals obj        System out println   str    obj        str    obj

[java] Exact difference between CharSequence and String in java

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