Website Notes: Ch.4 - The Java Char and String Classes.

1. Character class static methods.

Character Class Methods
Character class is used mostly for static methods to test char values.
b =	Character.isDigit(c)	true if c is digit character.
b =	Character.isLetter(c)	true if c is letter character.
b =	Character.isLetterOrDigit(c)	true if c is letter or digit.
b =	Character.isLowerCase(c)	true if c is lowercase char.
b =	Character.isUpperCase(c)	true if c is uppercase char.
b =	Character.isWhitespace(c)	true if c is space, tab, ....
c =	Character.toLowerCase(c)	Lowercase version of c.
c =	Character.toUpperCase(c)	Uppercase version of c.

// This program reads a single character from the keyboard and displays it
public class DemonstrateRead1
{
public static void main(String[] args)
{
char entry;

// Prompt for a keyboard input and read it
System.out.print("Enter a character: ");
entry = Keyboard.readChar();

// Display the keyboard character typed
System.out.println();
System.out.println("You typed " + entry);
}
}

2. Strings and Concatenation.

The + operator joins two strings together. If either operand is String, the other is converted to String and concatenated with it. This is the most common way to convert numbers to Strings. If a non-String object is part of the concatenation, its string representation is produced by calling its toString() method. When you write a class that might have a sensible representation as a string, it's often useful for debugging purposes to define a toString() method. If no such method is defined, the default version from Object is used.

"abc" + "def" "abcdef" "abc" + 4 "abc4" "1" + 2 "12" "xyz" + (2+2 == 4) "xyztrue" 1 + "2.5" "12.5"

//Here is the standard way of getting text.

public class Echo3 {
public static void main (String[] args)
{

String message; // Creates a varible called message for input

System.out.print ("Enter the message : ");
message = Keyboard.readString();

System.out.print("You ");
System.out.println("entered : " + message);

} // method main
} // end of public class Echo

 

3. String methods.

Below are some of the most common String methods that we will be using.

In all of these prototypes, i and j are int indexes into a string, s and t are

Strings, and c is a char.

Length
i =	s.length()	length of the string s.
Comparison (note: use these instead of == and !=)
i =	s.compareTo(t)	compares to s. returns <0 if s<t, 0 if ==, >0 if s>t
i =	s.compareToIgnoreCase(t)	same as above, but upper and lower case are same
b =	s.equals(t)	true if the two strings have equal values
b =	s.equalsIgnoreCase(t)	same as above ignoring case
b =	s.startsWith(t)	true if s starts with t
b =	s.startsWith(t, i)	true if t occurs starting at index i
b =	s.endsWith(t)	true if s ends with t
Searching -- Note: All "indexOf" methods return -1 if the string/char is not found
i =	s.indexOf(t)	index of the first occurrence of String t in s.
i =	s.indexOf(t, i)	index of String t at or after position i in s.
i =	s.indexOf(c)	index of the first occurrence of char c in s.
i =	s.indexOf(c, i)	index of char c at or after position i in s.
i =	s.lastIndexOf(c)	index of last occurrence of c in s.
i =	s.lastIndexOf(c, i)	index of last occurrence of c on or before i in s.
i =	s.lastIndexOf(t)	index of last occurrence of t in s.
i =	s.lastIndexOf(t, i)	index of last occurrence of t on or before i in s.
Getting parts
c =	s.charAt(i)	char at position i in s.
s1 =	s.substring(i)	substring from index i to the end of s.
s1 =	s.substring(i, j)	substring from index i to BEFORE index j of s.
Creating a new string from the original
s1 =	s.toLowerCase()	new String with all chars lowercase
s1 =	s.toUpperCase()	new String with all chars uppercase
s1 =	s.trim()	new String with whitespace deleted from front and back
s1 =	s.replace(c1, c2)	new String with all c2s replaced by c1s.
Static Methods for Converting to String
s =	String.valueOf(x)	Converts x to String, where x is any type value (primitive or object).

4. Convert from type X to type Y

　             integer to String :

int i = 42;

String str = Integer.toString(i);

or

String str = "" + i

double to String :

String str = Double.toString(i);

long to String :

String str = Long.toString(l);

float to String :

String str = Float.toString(f);

String to integer :

str = "25";

int i = Integer.valueOf(str).intValue();

or

int i = Integer.parseInt(str);

String to double :

double d = Double.valueOf(str).doubleValue();

String to long :

long l = Long.valueOf(str).longValue();

or

long l = Long.parseLong(str);

String to float :

float f = Float.valueOf(str).floatValue();

decimal to binary :

int i = 42;

String binstr = Integer.toBinaryString(i);

decimal to hexadecimal :

int i = 42;

String hexstr = Integer.toString(i, 16);

or

String hexstr = Integer.toHexString(i);

hexadecimal (String) to integer :

int i = Integer.valueOf("B8DA3", 16).intValue();

or

int i = Integer.parseInt("B8DA3", 16);

ASCII code to String :

int i = 64;

String aChar = new Character((char)i).toString();

integer to ASCII code (byte) :

char c = 'A';

int i = (int) c; // i will have the value 65 decimal

To extract Ascii codes from a String :

String test = "ABCD";

for ( int i = 0; i < test.length(); ++i ) {

char c = test.charAt( i );

int i = (int) c;

System.out.println(i);

}　

integer to boolean :

b = (i != 0);

boolean to integer :

i = (b)?1:0;

 

5. Here is an idea for a String Program:

public class How4

{public static void main(String[] args){

int x=0;
String greeting="Hello, how are you feeling today?";
String reply="I am feeling ";
String tofind="how are you";
String lookfor;
boolean found=false;

x=(int)(3.0*Math.random());

System.out.println(greeting);
System.out.println();
System.out.print(reply);

for(int i=0;i<greeting.length()-11;i++)
{ lookfor=greeting.substring(i,i+11);

if(lookfor.equals(tofind))
{found=true; break;}}

if((x==0)&&found)
{System.out.println("fine.");}

if((x==1)&&found)
{System.out.println("happy.");}

if((x==2)&&found)
{System.out.println("sad.");}

System.out.println();
} }

Anti-Phishing Game

ASCII Table

ARCHIVES:   CH.1   CH.2  CH.3

0. MIT Artificial Intelligence

1. Sun Microsystems Java

2. Elements of Java

3. University of Virginia Oracle

4. Review of Java

5. Honda ASIMO