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Trail: Learning the Java Language
Lesson: Object and Data Basics

## Java Strings Are First-Class Objects

> By convention, C and C++ strings are null-terminated array of characters; there is no real entity in C and C++ that is a string. Java strings are first-class objects.

> Strings as objects provides several advantages to the programmer:

> - The manner in which you obtain strings and elements of strings is consistent across all strings and all systems.
> - Since the programming interface for the `String` and `StringBuffer` classes is well-defined, Java strings function predictably every time.
> - The `String` and `StringBuffer` classes do extensive runtime checking for boundary conditions. They catch errors for you.
> To illustrate why this is an important feature of Java, let's look at a small example. This C function copies the contents of `src` into `dest`.

> >     int myStrCopy(char *dest, char *src)
> >     {
> >         for ( ; *src != '\0'; src++, dest++)
> >             *dest = *src;
> >     }

### C `String`s Behave Unpredictably

> In the example shown above, the developer uses pointer arithmetic to step through both strings copying one into the other. While allowing programmers to inspect arbitrary memory locations through pointers is a powerful tool, this power can be the source of many errors. One fruitful source of errors is pointers that stray off the end of an array. The `myStrCopy` function above has such an error: the `for` loop in the function does not check the length of `dest`, and if `src` is longer than `dest` the string copy writes right over the end of `dest`. Here's a program that tickles the bug.

> >     main() {
> >         char *s = "HotJava is Cool!";
> >         char t[] = "Java is Cool!";

> >         printf("%s, %s\n", s, t);
> >         myStrCopy(t, s);
> >         printf("%s, %s\n", s, t);
> >     }

> On my machine, the program prints:

> >     HotJava is Cool!, HotJava is Cool!%s, %s

> `myStrCopy` writes over the end of `dest` thereby corrupting whatever was stored in the memory after it. Note: `%s, %s` are the characters that happened to be stored in the memory location after `dest` and will probably be different when you run the program on your machine.

> Sure, the error in `myStrCopy` can be fixed easily. But errors like this are often difficult to find.

### Java Strings Are Predictable

> Java strings are first-class objects deriving either from the `String` class or the `StringBuffer` class. This makes finding and fixing an entire class of common and frustrating programming errors such as the one illustrated above trivial.

> Here's the program above (including the error) rewritten in the Java language:

> >     class strcpy {
> >         public static void main(String[] args) {
> >             String s = "HotJava is Cool!";
> >             StringBuffer t = new StringBuffer("Java is Cool!");

> >             System.out.println(s + ", " + t);
> >             myStrCopy(t, s);
> >             System.out.println(s + ", " + t);
> >         }

> >         static void myStrCopy(StringBuffer dest, String src) {
> >             int i, len = src.length();

> >             for (i = 0; i < len; i++)
> >                 dest.setCharAt(i, src.charAt(i));
> >         }
> >     }

> Notice that this translation uses the `String` class, the `StringBuffer` class, and the methods appropriate for obtaining specific characters instead of character arrays and pointers.

> Like the C version, the Java language version of the `myStrCopy` method loops over the length of `src` and never checks the length of `dest`. Thus, when `src` is longer than `dest`, the method tries to obtain characters beyond the end of `dest`. However, when you run the Java language version, you'll see the following runtime error message:

> >     Exception in thread "main" java.lang.StringIndexOutOfRangeException String index out of range: 13
> >         at java.lang.Exception.< init >(Exception.java)
> >         at java.lang.StringIndexOutOfRangeException.< init >(StringIndexOutOfRangeException.java)
> >         at java.lang.StringBuffer.setCharAt(StringBuffer.java)
> >         at strcpy.myStrCopy(strcpy.java:23)
> >         at strcpy.main(strcpy.java:15)

> The primary difference between the Java language version of this program and the C version is that the Java program will reliably and obviously crash, whereas the C program will do something obscure.

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