Chapter 8 Strings and Text I/O

The String Class

· Constructing a String:

· String message = "Welcome to Java“;

· String message = new String("Welcome to Java“);

· String s = new String();

· Obtaining String length and Retrieving Individual Characters in a string

· String Concatenation (concat)

· Substrings (substring(index), substring(start, end))

· Comparisons (equals, compareTo)

· String Conversions

· Finding a Character or a Substring in a String

· Conversions between Strings and Arrays

· Converting Characters and Numeric Values to Strings

Constructing Strings

String newString = new String(stringLiteral); 

String message = new String("Welcome to Java");

Since strings are used frequently, Java provides a shorthand initializer for creating a string:

String message = "Welcome to Java";

Strings Are Immutable

A String object is immutable; its contents cannot be changed. Does the following code change the contents of the string?

String s = "Java";

s = "HTML";

Interned Strings

Since strings are immutable and are frequently used, to improve efficiency and save memory, the JVM uses a unique instance for string literals with the same character sequence. Such an instance is called interned. You can also use a String object’s intern method to return an interned string. For example, the following statements:

Finding String Length

Finding string length using the length() method:

message = "Welcome";

message.length() (returns 7)

Retrieving Individual Characters in a String

F Do not use message[0]

Index starts from 0

String Concatenation

String s3 = s1.concat(s2);

String s3 = s1 + s2;

s1 + s2 + s3 + s4 + s5 same as

(((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5);

Extracting Substrings

You can extract a single character from a string using the charAt method. You can also extract a substring from a string using the substring method in the String class.

String s1 = "Welcome to Java";

String s2 = s1.substring(0, 11) + "HTML";

String Comparisons

F equals

String s1 = new String("Welcome“);

String s2 = "welcome";

if (s1.equals(s2)){

// s1 and s2 have the same contents

}

if (s1 == s2) {

// s1 and s2 have the same reference

}

String Comparisons, cont.

F compareTo(Object object)

String s1 = new String("Welcome“);

String s2 = "welcome";

if (s1.compareTo(s2) > 0) {

// s1 is greater than s2

}

else if (s1.compareTo(s2) == 0) {

// s1 and s2 have the same contents

}

else

// s1 is less than s2

String Conversions

The contents of a string cannot be changed once the string is created. But you can convert a string to a new string using the following methods:

F toLowerCase

F toUpperCase

F trim

F replace(oldChar, newChar)

Finding a Character or a Substring in a String

"Welcome to Java".indexOf('W') returns 0.

"Welcome to Java".indexOf('x') returns -1.

"Welcome to Java".indexOf('o', 5) returns 9.

"Welcome to Java".indexOf("come") returns 3.

"Welcome to Java".indexOf("Java", 5) returns 11.

"Welcome to Java".indexOf("java", 5) returns -1.

"Welcome to Java".lastIndexOf('a') returns 14.

Convert Character and Numbers to Strings

The String class provides several static valueOf methods for converting a character, an array of characters, and numeric values to strings. These methods have the same name valueOf with different argument types char, char[], double, long, int, and float. For example, to convert a double value to a string, use String.valueOf(5.44). The return value is string consists of characters ‘5’, ‘.’, ‘4’, and ‘4’.

Example:
Finding Palindromes

FObjective: Checking whether a string is a palindrome: a string that reads the same forward and backward.

import javax.swing.JOptionPane;
 
public class CheckPalindrome {
 /** Main method */
  public static void main(String[] args) {
 // Prompt the user to enter a string
 String s = JOptionPane.showInputDialog("Enter a string:");
 
 // Declare and initialize output string
 String output = "";
 
 if (isPalindrome(s))
 output = s + " is a palindrome";
 else
 output = s + " is not a palindrome";
 
 // Display the result
 JOptionPane.showMessageDialog(null, output);
 }
 
 /** Check if a string is a palindrome */
 public static boolean isPalindrome(String s) {
 // The index of the first character in the string
 int low = 0;
 
 // The index of the last character in the string
 int high = s.length() - 1;
 
 while (low < high) {
 if (s.charAt(low) != s.charAt(high))
 return false; // Not a palindrome
 
 low++;
 high--;
 }
 
 return true; // The string is a palindrome
 }
}

The Character Class

Examples

Character charObject = new Character('b');

charObject.compareTo(new Character('a')) returns 1

charObject.compareTo(new Character('b')) returns 0

charObject.compareTo(new Character('c')) returns -1

charObject.compareTo(new Character('d') returns –2

charObject.equals(new Character('b')) returns true

charObject.equals(new Character('d')) returns false

Example: Counting Each Letter in a String

This example gives a program that counts the number of occurrence of each letter in a string. Assume the letters are not case-sensitive.

import javax.swing.JOptionPane;
 
public class CountEachLetter {
 /** Main method */
 public static void main(String[] args) {
 // Prompt the user to enter a string
 String s = JOptionPane.showInputDialog("Enter a string:");
 
 // Invoke the countLetters method to count each letter
 int[] counts = countLetters(s.toLowerCase());
 
 // Declare and initialize output string
 String output = "";
 
 // Display results
 for (int i = 0; i < counts.length; i++) {
 if (counts[i] != 0)
 output += (char)('a' + i) + " appears " +
 counts[i] + ((counts[i] == 1) ? " time\n" : " times\n");
 }
 
 // Display the result
 JOptionPane.showMessageDialog(null, output);
 }
 
 // Count each letter in the string
 public static int[] countLetters(String s) {
 int[] counts = new int[26];
 
 for (int i = 0; i < s.length(); i++) {
 if (Character.isLetter(s.charAt(i)))
 counts[s.charAt(i) - 'a']++;
 }
 
 return counts;
 }
}