在这种情况下使用charAt函数有什么问题？

Question

I was trying to copy a word using this manner. I am not sure,I was following right manner handling String .

The code is:

 public static void main(String args[])
   {
      String str="Hello";
      int i=0;
      String copy = "";
      while (str.charAt(i) !='\0')
      {
          copy = copy + str.charAt(i);
          i++;
      }

      System.out.println(copy);
   }

Running this code results in Exception :

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 5
    at java.lang.String.charAt(Unknown Source)
    at ReverseWord.main(ReverseWord.java:15)

Am I using charAt() and checking null in right way? Or,I have wrong conception about String handling in Java ?

Answer 1

You are using String s in the wrong way (for Java !) lets clarify some basic points to use String's in Java:

• String are immutable . This means each time you modify it JVM will create a new object. That is a lot of resources, so, in order of better programming you shouldn't use concatenation in String s, to make concatenation use StringBuilder .
• Strings does not end with any special symbol , this could happen in some filetypes but not at Strings objects, so you have to get size with length() and use it to iterate if necessary.
• Always take a loop at the API of any Java object to know its functionallities:
  StringAPI7
StringAPI8
• To loop a String char by char you can do it with for , while and several more ways (splitting, converting...):

For loop example:

for (int i = 0; i < str.length(); i++)

While example:

while (i < str.length()) {

Said that... Take a look at this code working and using what is explained:

public static void main(String[] args) {
    String str = "Hello";
    int i = 0;
    // string builder is a mutable string! :)
    StringBuilder copy = new StringBuilder();
    // we iterate from i=0 to length of the string (in this case 4)
    while (i < str.length()) {
        // same than copy = copy + str.charAt(i)
        // but not creating each time a new String object
        copy.append(str.charAt(i));
        // goto next char
        i++;
    }

    // print result 
    System.out.println(copy);
}

UPDATE

thanks... but while I am trying this to find a reverse did not get result

If what you want is to reverse the String (your code didn't do that, you must write copy = str.charAt(i) + copy; ) is much easier with StringBuilder . Take a look at this example:

public static void main(String[] args) {
    String str = "Hello";
    StringBuilder copy = new StringBuilder(str);
    copy.reverse();
    System.out.println(copy);
}

Answer 2

FIrst way:

Use the below code:

for (int i = 0; i < str.length(); i++) {
    copy = copy+str.charAt(i);
}

Second Way:

Convert String into char[] . then convert it back to String .

char[] ch = str.toCharArray();
String copy = new String(ch);
System.out.println(copy);