Difference between a string and an array of characters

Question

How are these declarations different from each other?

String s="MY PROFESSION";
char c[]="MY PROFESSION";

What about the memory allocation in each case?

Answer 1

To correct compilation error replace with one of the below char[] statement

String s = "MY PROFESSION";
char c1[] = "MY PROFESSION".toCharArray();
char c2[] = { 'M', 'Y', ' ', 'P', 'R', 'O', 'F', 'E', 'S', 'S', 'I', 'O', 'N' };
StringBuilder sb = new StringBuilder("MY PROFESSION");
StringBuffer sbu = new StringBuffer("MY PROFESSION");

Following section compares above statement with each other

String Constant

String s="MY PROFESSION";

• "MY PROFESSION" is a constant and stored in String pool see Where does Java's String constant pool live, the heap or the stack?
• s is immutable i.e content of String is intact can not be modified.
• Size/Length of string is fixed (not possible to append)

Character Array

 char c1[]="MY PROFESSION".toCharArray();
 char c2[]={'M', 'Y', ' ', 'P', 'R', 'O', 'F', 'E', 'S', 'S', 'I', 'O', 'N'};

• c1 holds copy of String's underlying array (via System.arraycopy) and stored in heap space
• c2 is built on the fly in the stack frame by loading individual character constants
• c1 & c2 are mutable i.e content of Array can be modified. c2[0]='B'
• Size/Length of Array is fixed (not possible to append)

StringBuilder / StringBuffer

StringBuilder sb = new StringBuilder("MY PROFESSION");
StringBuffer sbu = new StringBuffer("MY PROFESSION");

• Both sb and sbu are mutable. sb.replace(0, 1, 'B');
• Both sb and sbu are stored in heap
• Size/Length can be modified. sb.append( '!');
• StringBuffer's methods are synchronised while StringBuilder's methods are not

Answer 2

The first one compiles. The second one doesn't.

A char[] is just that: an array of primitive numbers of type char. All it provides is a length attribute, and a way to get and set a char at a given index.

A String is an object, of type java.lang.String, which has a whole lot of useful methods for manipulating Strings. Internally, it uses a char array.

Another important feature of String is that it's immutable. You can pass a String to any method and be sure that this method won't change the contents of the String. That is not the case with a char array.

Regarding memory, a String consumes some more bytes, but that's generally not what should guide your design decisions: generally, using a char array is not what you should do.