31: String class

Learning Objectives
After completing this session, you will be able to:
‰  Write Java programs using String class
The String Class: Example
1 class StringDemo {
2 public static void main(String args[]) {
3 String name = "Jonathan";
4 System.out.println("name: " + name);
5 System.out.println("3rd character of name: " +
6 name.charAt(2));
7 /* character that first appears alphabetically
8 has lower unicode value */
9 System.out.println("Jonathan compared to Solomon: "
10 + name.compareTo("Solomon"));
11 System.out.println("Solomon compared to Jonathan: "
12 + "Solomon".compareTo("Jonathan"));
13 /* 'J' has lower unicode value compared to 'j' */
14 System.out.println("Jonathan compared to jonathan: " +
15 name.compareTo("jonathan"));
16 System.out.println("Jonathan compared to jonathan
17 (ignore case): " + name.compareToIgnoreCase("jonathan"));
18 System.out.println("Is Jonathan equal to Jonathan? " +
name.equals("Jonathan"));
20 //continued...
21 System.out.println("Is Jonathan equal to jonathan? " +
22 name.equals("jonathan"));
23 System.out.println("Is Jonathan equal to jonathan
(ignore case)? " + name.equalsIgnoreCase("jonathan"));
25 char charArr[] = "Hi XX".toCharArray();
26 /* Need to add 1 to the endSrc index of getChars */
27 "Jonathan".getChars(0, 2, charArr, 3);
28 System.out.print("getChars method: ");
29 System.out.println(charArr);
30 System.out.println("Length of name: " +
31 name.length());
32 System.out.println("Replace a's with e's in name: " +
33 name.replace('a', 'e'));
34 /* Need to add 1 to the endIndex parameter of
35 substring*/
36 System.out.println("A substring of name: " +
37 name.substring(0, 2));
38 //continued...
39 System.out.println("Trim \" a b c d e f \": \"" +
40 " a b c d e f ".trim() + "\"");
41 System.out.println("String representation of boolean
42 expression 10>10: " + String.valueOf(10>10));
43 /* toString method is implicitly called in the println
44 method*/
45 System.out.println("String representation of boolean
46 expression 10<10: " + (10<10));
47 /* Note there's no change in the String object name
48 even after applying all these methods. */
49 System.out.println("name: " + name);
50 }
51 }
Try It Out
Problem Statement:
Write a program that illustrates the usage of various methods available in the String class.
Code:
public class StringApp {
public static void main(String args[]) {
String s = " Java 2 Certification ";
System.out.println(s);
System.out.println(s.toUpperCase());
System.out.println(s.toLowerCase());
System.out.println("[" + s + "]");
s = s.trim();
System.out.println("[" + s + "]");
s = s.replace('J', 'X');
s = s.replace('C', 'Y');
s = s.replace('2', 'Z');
System.out.println(s);
How It Works:
‰  This program performs several manipulations of a string s, which is initially set to "
Java 2 Certification ".
‰  It prints the original string and then printsuppercase and lowercase versions of it,
illustrating the use of the toUpperCase() and toLowerCase() methods.
‰  It prints the string enclosed between two braces to show that it contains leading and
trailing spaces.
‰  It then trims away these spaces using the trim() method and reprints the string to show
that these spaces were removed.
‰  The program uses the replace() method to replace 'J‘ , 'C‘ , and '2' with 'X', 'Y', and 'Z',
and prints out the string to show the changes.
‰  The replace() method is case sensitive.
‰  It uses the indexOf() method to get the indices of 'X', 'Y', and 'Z' within s.
‰  It uses the toCharArray() to convert the string to a char array.
‰  It then uses the indices to put 'J', 'C', and '2' back in their proper locations within the
character array.
‰  The String() constructor is used to construct a new string from the character array.
‰  The new string is assigned to s and is printed.
‰  The output of the program is as follows:
Java 2 Certification
JAVA 2 CERTIFICATION
java 2 certification
[ Java 2 Certification ]
[Java 2 Certification]
Xava Z Yertification
Java 2 Certification
Tips and Tricks:
What will happen if you concatenate null with a string?
Solution:
‰  String concatenation process will add a string with the value of “null”, if an object
reference is null and that object is appearing in a concatenation expression by itself.
But if you try to access its members or methods, then a NullPointerException is
thrown.
‰  The same is true for arrays. Array name is replaced with null, but trying to index it
when the null of it throws a NullPointerException.
Summary
‰  String methods to remember are charAt(), concat(), equalsIgnoreCase(), length(),
replace(), substring(), toLowerCase(),toString(), toUpperCase(), and trim().
‰  Strings have a method: length(); arrays have an attribute named length.
‰  When the JVM finds a String literal, it is added to the String literal pool.
‰  Remember that chained methods are evaluated from left to right.
Test Your Understanding
Differentiate the methods compareTo(), equals(), and equalsIgnoreCase().