Does String's toString() method have any practical purpose?

Question

I was reading the docs and noticed it. Never imaginated it. The description:

This object (which is already a string!) is itself returned.

Besides filling conventions or using computing resources, what does a .toString() in Java do on a String that using the String itself wouldn't? Why doesn't it simply inherit the .toString() from class java.lang.Object?

EDIT:

I understand that in situations of polymorphism an own toString() method has to exist since it overrode its parent's toString(). What I want to know in the first question is if there is any situation where you'll get something different between using stringVariable/"String value" and using stringVariable.toString()/"String value".toString().

Ex. gr.: An output operation like System.out.println(stringVariable.toString()); or a value assignment like stringVariable = "String value".toString();.

Solution 1

Besides filling conventions or using computing resources, what does a .toString() in Java do on a String that using the String itself wouldn't?

It means it gives an appropriate result when called polymorphically.

Why doesn't it simply inherit the .toString() from class java.lang.Object?

Because that wouldn't give the same result, and almost certainly not the desired result.

Object.toString() is meant to give a reasonably useful string representation of the object. The Object implementation gives information about the type and a value which can be used for crude identity hints (diagnostically useful, but that's all). That's clearly not the most useful string representation for a string - the string itself is.

While I would say that it's a pity that toString is defined in quite a woolly way (it's not clear whether the result is meant for machine, developer or user consumption), it feels obvious to me that a String would return itself in the implementation.

Solution 2

Practical example:

public abstract class MetadataProcessor {

protected void processMetadata() {    
   Map<String, Object> metadata = getMetadata();
   for(String key : metadata.keySet()) {
      if(metadata.get(key) instanceof Date) {
      processDate(metadata.get(key));
   } else { //String or any other object type
      processString(metadata.get(key).toString());
   }
}

private void processDate(Date date) {
   (...)
}

private void processString(String string) {
   (...)
}

/**
* contains document's metadata, values must be String or Date
**/
protected abstract Map<String, Object> getMetadata();
}

If String's .toString() wouldn't return the String itself, an additional if(foo instanceof String) would be needed in above code.

Solution 3

Obviously old, but another answer that avoids the need of (foo instanceof String) is when dealing with generics.

It'd be annoying to get a string representation of every object, but then string is the odd man out requiring extra code to get the same result.

Solution 4

public final class String
extends Object
implements Serializable, Comparable<String>, CharSequence

see: https://docs.oracle.com/javase/10/docs/api/java/lang/String.html

All object has toString() method (see https://docs.oracle.com/javase/10/docs/api/java/lang/Object.html#toString() ). String is a type of object, therefore, it also need a method toString() for implementing method from its base class.

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Comments

[Louis Wasserman]

Effective Java item 10 discusses some reasons to avoid fully specifying toString(): because someone will inevitably decide it's a good idea to parse it.

[Jon Skeet]

@TrevorTrovalds: Doesn't the documentation already answer that? It returns "this object" - so no, there's no situation where using foo and foo.toString() will give a different result where foo is a non-null reference to a String.

[Jon Skeet]

@TrevorTrovalds: Um, ish - although I wouldn't use it for that purpose.

[ryvantage]

@Trevor... I'm scratching my head trying to envision a scenario where throwing a NPE with str.toString() would be better than a if str == null check...

[JohnDoDo]

@user985358: Trevor, you recently unaccepted this answer as well as others. Is it something wrong with this answer or the others or had you other reasons to unnaccept so many answers?

[halfer]

@JohnDoDo: the OP may not see your comment there, since they have no comments in this thread (they may have deleted them since you posted). I'll put a note on the question.