CharSequence VS String in Java?

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.

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.

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:

1. 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.

2. 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 ==

3. 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.

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();
}

CharSequence is a readable sequence of char values which implements String. it has 4 methods

1. charAt(int index)
2. length()
3. subSequence(int start, int end)
4. toString()

Please refer documentation CharSequence documentation