Lecture 9: String

In the section you will learn how to

How to look for a class(Use java docuement)

Use java doucment : Java 1.5 API Doc

Example:

You want to use an object that represents a point. You guess that there is a java class related to point. You look for the word Point in the classes list and bingo! You find it here  .

How to use the class?

Example:

You find out all the contructors for the class Point, and how to get the (x,y)-coordinate of a point.
You must read the text book p134-146 or   Characters and Strings(and its links) (only read the part about Strig)

Sample program for String class

// this program reverse the string 
public class StringsDemo {
    public static void main(String[] args) {
        String string = "I am a string";
        int len = string.length();
        String dest = "";

        for (int i = (len - 1); i >= 0; i--) {
            dest += string.charAt(i);
        }
        System.out.println(dest);
    }
}

How to create a String

A string is often created from a string literal--a series of characters enclosed in double quotes. e.g. "Something in the quotes".
when it encounters the  string literal, the Java platform creates a String object with value "Something in the quotes".
A common way to create a string is
String string = "I am a string";
Here's an example of creating a string from a character array: 
char[] helloArray = { 'h', 'e', 'l', 'l', 'o' };
helloString = new String(helloArray);
System.out.println(helloString);
The last line of this code snippet displays: hello .

You can find other ways of constructing String in the Java document.

Add Strings

You can add string together like
String helloworld = "Hello" + "World";
You can also add String and primitive type together e.g
 public class Info {
    public static void main(String[] args) {
    int myAge = 18;
    String myName = "Charles";
    String myInfo = myName + " is " + myAge + " years old.";
    System.out.println(myInfo);
 }
}
The output of the program is 
Charles is 18 years old.
Or as in the StringDemo program 
dest += string.charAt(i);

Other useful methods

Look through the methods in Java documents we can find some useful methods.
Here are some examples:

Methods for changing strings

 
String concat(String str)
String replace(char oldChar, char newChar)
String toLowerCase()
 
String toUpperCase()
String trim()

Methods for searching

 
int indexOf(int ch)
int indexOf(int ch, int fromIndex)
int lastIndexOf(int ch)
int lastIndexOf(int ch, int fromIndex)
 
int indexOf(String str)
int indexOf(String str, int fromIndex)
int lastIndexOf(String str)
int lastIndexOf(String str,int fromIndex)




Methods for testing

 
boolean startsWith(String prefix, int toffset)
boolean startsWith(String prefix)
 
boolean endsWith(String suffix)
boolean equalsIgnoreCase(String anotherString)

Methods for accessing

 
String substring(int beginIndex)
String substring(int beginIndex, int endIndex)
 
char charAt(int index)
public char[] toCharArray()

Test for equality

The correct way to test for equality is to use the equals (String string) methods.
e.g.
public class EqualTest {
    public static void main(String[] args) {
        String string1 = "string1";
        String string2 = "string2";
        if(string1.equals(string2)) {
            System.out.println("string1 is equal to string2");
        } else {
            System.out.println("string1 is not equal to string2"); 
        }
    }
}
The usual test expression if(string1 == string2) has a different meaning. It tests if they refer to the same object.
.

Questions

public class Question {
 /** returns the number of occurences of char ch in the string */
 public static int numOfChar(String string, char ch) {
 // your codes here
 }

 /** returns a new string resulting from removing all occurences of char ch */
 public static String remove(String string, char ch) {
 // your codes here
 }

 /** returns a new string resulting from moving the first n characters to the end */
 public static String move(String string, int n) {
 // your codes here
 }

 /** returns the number represented by the string */
  public static int stringToNumber(String string) {
         // we assume the string consists of only digits
         // your codes here
 } 
}