CharSequence VS String in Java

Question

Programming in Android  most of the text values are expected in CharSequence   Why is that  What is the benefit  and what are the main impacts of using CharSequence over String   What are the main differences  and what issues are expected  while using them  and converting from one to another

User · Answer

This is almost certainly performance reasons. For example, imagine a parser that goes through a 500k ByteBuffer containing strings.

There are 3 approaches to returning the string content:

Build a String[] at parse time, one character at a time. This will take a noticeable amount of time. We can use == instead of .equals to compare cached references.
Build an int[] with offsets at parse time, then dynamically build String when a get() happens. Each String will be a new object, so no caching returned values and using ==
Build a CharSequence[] at parse time. Since no new data is stored (other than offsets into the byte buffer), the parsing is much lower that #1. At get time, we don't need to build a String, so get performance is equal to #1 (much better than #2), as we're only returning a reference to an existing object.

In addition to the processing gains you get using CharSequence, you also reduce the memory footprint by not duplicating data. For example, if you have a buffer containing 3 paragraphs of text, and want to return either all 3 or a single paragraph, you need 4 Strings to represent this. Using CharSequence you only need 1 buffer with the data, and 4 instances of a CharSequence implementation that tracks the start and length.

User · Answer

CharSequence   interface String   concrete implementation   CharSequence is an interface   Several classes implement this interface    String is one such class  a concrete implementation of CharSequence     You said      converting from one to another   There is no converting from String     Every String object is a CharSequence  Every CharSequence can produce a String  Call CharSequence  toString  If the CharSequence happens to be a String  then the method returns a reference to its own object    In other words  every String is a CharSequence  but not every CharSequence is a String   Programming to an interface     Programming in Android  most of the text values are expected in CharSequence       Why is that  What is the benefit  and what are the main impacts of using CharSequence over String    Generally  programming to an interface is better than programming to concrete classes  This yields flexibility  so we can switch between concrete implementations of a particular interface without breaking other code    When developing an API to be used by various programmers in various situations  write your code to give and take the most general interfaces possible  This gives the calling programmer the freedom to use various implementations of that interface  whichever implementation is best for their particular context   For example  look at the Java Collections Framework  If your API gives or takes an ordered collection of objects  declare your methods as using List rather than ArrayList  LinkedList  or any other 3rd-party implementation of List    When writing a quick-and-dirty little method to be used only by your code in one specific place  as opposed to writing an API to be used in multiple places  you need not bother with using the more general interface rather than a specific concrete class  But even then  it does to hurt to use the most general interface you can       What are the main differences  and what issues are expected  while using them      With a String you know you have a single piece of text  entirely in memory  and is immutable   With a CharSequence  you do not know what the particular features of the concrete implementation might be     The CharSequence object might represent an enormous chunk of text  and therefore has memory implications  Or may be many chunks of text tracked separately that will need to be stitched together when you call toString  and therefore has performance issues  The implementation may even be retrieving text from a remote service  and therefore has latency implications       and converting from one to another    You generally won t be converting back and forth  A String is a CharSequence  If your method declares that it takes a CharSequence  the calling programmer may pass a String object  or may pass something else such as a StringBuffer or StringBuilder  Your method s code will simply use whatever is passed  calling any of the CharSequence methods    The closest you would get to converting is if your code receives a CharSequence and you know you need a String  Perhaps your are interfacing with old code written to String class rather than written to the CharSequence interface  Or perhaps your code will work intensively with the text  such as looping repeatedly or otherwise analyzing  In that case  you want to take any possible performance hit only once  so you call toString up front  Then proceed with your work using what you know to be a single piece of text entirely in memory   Twisted history  Note the comments made on the accepted Answer  The CharSequence interface was retrofitted onto existing class structures  so there are some important subtleties  equals    amp  hashCode     Notice the various versions of Java  1  2  4  amp  5  tagged on the classes interfaces   quite a bit of churn over the years  Ideally CharSequence would have been in place from the beginning  but such is life   My class diagram below may help you see the big picture of string types in Java 7 8  I m not sure if all of these are present in Android  but the overall context may still prove useful to you

User · Answer

CharSequence is a readable sequence of char values which implements String  it has 4 methods       charAt int index  length   subSequence int start  int end  toString     Please refer documentation CharSequence documentation

User · Answer

I believe it is best to use CharSequence  The reason is that String implements CharSequence  therefore you can pass a String into a CharSequence  HOWEVER you cannot pass a CharSequence into a String  as CharSequence doesn t not implement String  ALSO  in Android the EditText getText   method returns an Editable  which also implements CharSequence and can be passed easily into one  while not easily into a String  CharSequence handles all

User · Answer

CharSequence  A CharSequence is an interface  not an actual class  An interface is just a set of rules  methods  that a class must contain if it implements the interface  In Android a CharSequence is an umbrella for various types of text strings  Here are some of the common ones    String  immutable text with no styling spans  StringBuilder  mutable text with no styling spans  SpannableString  immutable text with styling spans  SpannableStringBuilder  mutable text with styling spans     You can read more about the differences between these here    If you have a CharSequence object  then it is actually an object of one of the classes that implement CharSequence  For example   CharSequence myString    hello   CharSequence mySpannableStringBuilder   new SpannableStringBuilder      The benefit of having a general umbrella type like CharSequence is that you can handle multiple types with a single method  For example  if I have a method that takes a CharSequence as a parameter  I could pass in a String or a SpannableStringBuilder and it would handle either one   public int getLength CharSequence text        return text length        String  You could say that a String is just one kind of CharSequence  However  unlike CharSequence  it is an actual class  so you can make objects from it  So you could do this   String myString   new String      but you can t do this   CharSequence myCharSequence   new CharSequence       error   CharSequence is abstract  cannot be instantiated   Since CharSequence is just a list of rules that String conforms to  you could do this   CharSequence myString   new String      That means that any time a method asks for a CharSequence  it is fine to give it a String    String myString    hello   getLength myString      OK          public int getLength CharSequence text        return text length        However  the opposite is not true  If the method takes a String parameter  you can t pass it something that is only generally known to be a CharSequence  because it might actually be a SpannableString or some other kind of CharSequence   CharSequence myString    hello   getLength myString      error          public int getLength String text        return text length

User · Answer

CharSequence is an interface and String implements it  You can instantiate a String but you could not do that for CharSequence since it is an interface  You can find other implementations in CharSequence in the official Java website

User · Answer

In general using an interface allows you to vary the implementation with minimal collateral damage  Although java lang String are super popular it may be possible that in certain contexts one may want to use another implementation  By building the API around CharSequences rather than Strings the code gives one the opportunity to do that

User · Answer

An issue that DO arise in practical Android code is that comparing them with CharSequence equals is valid but does not necessarily work as intended   EditText t    EditText  getView R id myEditText      Contains  OK  Boolean isFalse   t getText   equals  OK       will always return false    Comparison should be made by    OK   contentEquals t GetText

User · Answer

Strings are CharSequences  so you can just use Strings and not worry  Android is merely trying to be helpful by allowing you to also specify other CharSequence objects  like StringBuffers

[java] CharSequence VS String in Java?

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