Java How to Program Fourth Edition - DCC

540 Strings and Characters Chapter 10
Software Engineering Observation 10.1
In most cases, it is not necessary to make a copy of an existing String object. String objects
are immutable—their character contents cannot be changed after they are created. Also,
if there are one or more references to a String object (or any object for that matter),
the object cannot be reclaimed by the garbage collector. Thus, a String reference cannot
be used to modify a String object or to delete a String object from memory as in other
programming languages, such as C or C++. 10.1
Line 27 instantiates a new String object and assigns it to reference s3 using class
String’s constructor that takes a character array as an argument. The new String
object contains a copy of the characters in the array.
Line 28 instantiates a new String object and assigns it to reference s4 using class
String’s constructor that takes a char array and two integers as arguments. The second
argument specifies the starting position (the offset) from which characters in the array
are copied. The third argument specifies the number of characters (the count) to be copied
from the array. The new String object contains a copy of the specified characters in the
array. If the offset or the count specified as arguments result in accessing an element
outside the bounds of the character array, a StringIndexOutOfBoundsException
is thrown. We discuss exceptions in detail in Chapter 14.
Line 29 instantiates a new String object and assigns it to reference s5 using class
String’s constructor that receives a byte array and two integers as arguments. The
second and third arguments specify the offset and count, respectively. The new
String object contains a copy of the specified bytes in the array. If the offset or the
count specified as arguments result in accessing an element outside the bounds of the
character array, a StringIndexOutOfBoundsException is thrown.
Line 30 instantiates a new String object and assigns it to reference s6 using class
String’s constructor that takes a byte array as an argument. The new String object
contains a copy of the bytes in the array.
Line 31 instantiates a new String object and assigns it to reference s7 using class
String’s constructor that receives a StringBuffer as an argument. A String-
Buffer is a dynamically resizable and modifiable string. The new String object contains
a copy of the characters in the StringBuffer. Line 22 creates a new object of class
StringBuffer using the constructor that receives a String argument (in this case
"Welcome to Java Programming") and assign the new object to reference buffer.
We discuss StringBuffers in detail later in this chapter. The screen capture for the program
displays the contents of each String.
10.4 String Methods length, charAt and getChars
The application of Fig. 10.2 presents String methods length, charAt and get-
Chars, which determine the length of a String, get the character at a specific location
in a String and get the entire set of characters in a String, respectively.
Line 28 uses String method length to determine the number of characters in
String s1. Like arrays, Strings always know their own size. However, unlike arrays,
Strings do not have a length instance variable that specifies the number of elements
in a String.
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