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The value of s1 after the concat method is executed remains an unchanged "abc". s2 is
equal to "abcdef" since concat returns a new object with the concatenated result, which
is assigned to s2. The main advantage of String immutability is that the Java compiler
can save space by sharing Strings. If a program repeatedly performs String
concatenation, when processing a file, for example, then the repeated creation of new
objects becomes inefficient. To cater for this, Java provides a mutable StringBuffer
class, which we discuss in Chapter 4.
We can convert all primitive data types to a String using the
java.lang.String.valueOf method. For example, the following converts an int to a
String:
int count = 123;
String countString = String.valueOf(count);
2.5.6 Arrays
An array contains a collection of elements, all of which have the same type. Arrays can be
of any type. An array declaration is of the form
datatype variable_name [] ;
or
datatype [] variable_name;
We have already seen an array, args, of type String in the Circle program. If we
wish to declare an array, intArray, say, of type int, enter either
int [] intArray;
or
int intArray[];
This statement only declares the variable intArray. To create or define an array, that is,
reserve storage in memory to hold the array, we need to use the new keyword. For example,
intArray = new int [2];
will create the array intArray with two elements and initialize it with zero values, the
default value for numbers. The reason for the new keyword is that an array is actually an
object in Java. Objects in Java are created using the new keyword, as we shall see in
Chapter 4.
Array elements are counted from zero, so