How to convert a String to CharSequence?

288
votes

How to convert String to CharSequence in Java?

6
javastringcharsequence
The question is kind of non sensical. Converting a String to a CharSequence is like converting a flute to a musical instrument. A String already is a CharSequence. The String class implements the CharSequence interface. – Jeff Scott Brown
The OP's confusion might stem from not knowing what it means to program to an interface – Raedwald
@JeffScottBrown the question actually makes sense, it's a legitimate wonder to anyone reading through the Android or Java doc and missing the detail that CharSequence is not a class. Your comment helped me, but if the question had not been worded the way it is, i wouldn't have found it and have spent more time looking for an answer than it's really worth ;) – Kheldar
I came to this question in the context of a groovy Problem: Exception groovy.lang.MissingMethodException: No signature of method: static java.util.regex.Pattern.matcher() is applicable for argument types: (java.lang.String) – BlackEye
Here's how I'd advice @JeffScottBrown to reword his comment- "The String class implements (IS-A) the CharSequence interface so you can pass in a String in place of a CharSequence. It's like how you could refer to a flute as a musical instrument because a flute IS-A musical instrument" – Vighnesh

6 Answers

280
votes

Since String IS-A CharSequence, you can pass a String wherever you need a CharSequence, or assign a String to a CharSequence:

CharSequence cs = "string";
String s = cs.toString();
foo(s); // prints "string"

public void foo(CharSequence cs) { 
  System.out.println(cs);
}

If you want to convert a CharSequence to a String, just use the toString method that must be implemented by every concrete implementation of CharSequence.

68
votes

Straight answer:

String s = "Hello World!";

// String => CharSequence conversion:

CharSequence cs = s;  // String is already a CharSequence

CharSequence is an interface, and the String class implements CharSequence.

13
votes

CharSequence is an interface and String is its one of the implementations other than StringBuilder, StringBuffer and many other.

So, just as you use InterfaceName i = new ItsImplementation(), you can use CharSequence cs = new String("string") or simply CharSequence cs = "string";

4
votes

You can use 

CharSequence[] cs = String[] {"String to CharSequence"};
1
votes

Attempting to provide some (possible) context for OP's question by posting my own trouble. I'm working in Scala, but the error messages I'm getting all reference Java types, and the error message reads a lot like the compiler complaining that CharSequence is not a String. I confirmed in the source code that String implements the CharSequence interface, but the error message draws attention to the difference between String and CharSequence while hiding the real source of the trouble:

scala> cols
res8: Iterable[String] = List(Item, a, b)

scala> val header = String.join(",", cols)
<console>:13: error: overloaded method value join with alternatives:
  (x$1: CharSequence,x$2: java.lang.Iterable[_ <: CharSequence])String <and>
  (x$1: CharSequence,x$2: CharSequence*)String
 cannot be applied to (String, Iterable[String])
       val header = String.join(",", cols)

I was able to fix this problem with the realization that the problem wasn't String / CharSequence, but rather a mismatch between java.lang.Iterable and Scala's built-in Iterable.

scala> val header = String.join(",", coll: _*)
header: String = Item,a,b

My particular problem can also be solved via the answers at Scala: join an iterable of strings

In summary, OP and others who come across similar problems should parse the error messages very closely and see what other type conversions might be involved.

0
votes

That's a good question! You may get into troubles if you invoke API that uses generics and want to assign or return that result with a different subtype of the generic type. Java 8 helps to transform:

    List<String> input = new LinkedList<>(Arrays.asList("a", "b", "c"));
    List<CharSequence> result;
    
//    result = input; // <-- Type mismatch: cannot convert from List<String> to List<CharSequence>
    result = input.stream().collect(Collectors.toList());
    
    System.out.println(result);