C ++和Java中的字符串之间的区别

Question

In C++ I can do something like this...

String s = "abc";
char c = s[i]; // works fine...

But in Java , if I try doing the same, it throws an error. Why?.

In java , to achieve the above, I have to do :

s.toCharArray(); 

How is the implementation of Strings in C++ different from that in Java?

Answer 1

In java, to achieve the above, I have to do : s.toCharArray();

Not really. You can use charAt instead:

char c = s.charAt(i);

Basically, C++ allows user-defined operators - Java doesn't. So the String class doesn't expose any sort of "indexing" operator; that only exists for arrays, and a String isn't an array. (It's usually implemented using an array, but that's a different matter.)

EDIT: As noted in comments, the + operator is special-cased for strings - right in the language specification. The same could have been done for [] , but it isn't - and as it's not in the language specification, and Java doesn't support overloaded operators, it can't be performed in library code. (For example, you can't give custom behaviour to + for any other class.)

Answer 2

The difference is that C++ has operator overloading, and uses it to access the string contents.

They both store the string characters in such a way as you cannot change them.

Answer 3

The reason that it is possible to write

string s = "abc";
char c = s[i]; 

in C++ is that the string class has overloaded the indexing operator (say [] operator) which allows programmers to access characters of a string object the same way that they access an element of an array, despite the fact that a string object is not an array.

Java, on the other hand, does not allow operator overloading of any kind (the only exception is the + operator that is overloaded for strings) and hence, the indexing operator is not and can not be overloaded for String objects. In Java, to access a character of a string, you need to use accessor member methods, such as charAt . You can also invoke the toCharArray method of the String class, which returns to you an array of the characters of the string object and you can use the indexing operator with this returned value:

char c = s.toCharArray()[i];

Answer 4

See the method String#charAt

Returns the char value at the specified index. An index ranges from 0 to length() - 1. The first char value of the sequence is at index 0, the next at index 1, and so on, as for array indexing.

If the char value specified by the index is a surrogate, the surrogate value is returned.

public char charAt(int index)

Answer 5

in c++ strings are already treated as array of characters, but in java String is a built in class. it is different from array of characters.

Answer 6

In C++, a string is typically just an array of (or a pointer to) chars, terminated with a NULL (\\0) character. You can process a string by indexing also as you would process any array.
But in Java , a strings are not arrays. Java strings are objects of type java.lang.String so You cannot process them by indexing .