Strings

Character: A class that can hold a single character
String: A class that can hold multiple characters. String objects CANNOT be changed.
StringBuffer: A class that can hold multiple characters. StringBuffer objects CAN be changed.

Characters

Characters are specified using the single quote mark ' - usually located blow the @ symbol on your keyboard. I.e.

'a'

This specifies a single 'a' character.

This following is invalid because it does not specify a single keyboard character:-

'ab'

Here are some more valid characters:-

'1' 'w' '%' '\n'

the last character '\n' denotes a newline character.

You may be used to creating the primitive char data types, i.e.

char ch = 'a';

but you can use the Character class instead if you want a Character object:-

Characterch = new Character ('a');

String Class

Strings are sequences of Unicode characters. In Java we use the String class and it's methods for working with String objects. The String class is used for creating strings whose value will NOT change. If you want to be able to modify a String after you have created it then you must use the StringBuffer class instead.

Creating Strings

Strings are specified by placing characters between double quotes.

Here are some valid Strings:-

"hello" "1234" "I am 3"

Often Strings are created from String literals:-

System.out.println("Hello World!");

In this case the String "Hello World" is created for you and then destroyed after the line is executed. You do not have a permanent reference to the String "Hello World". You can create a string with a variable referencing it in two ways.

Normally we create objects in Java using the new keyword. We can create String objects in the same way-

String str = new String ("Hello World!");

But there is a shortcut way of creating String objects without using the new keyword:-

String str = "Hello World!";

In both examples given I have a variable str which references a String object with the value "Hello World!"

String Methods Overview

Here are some useful methods you can use with Strings.

~~String Length~~

length()

find the number of characters in the String

~~Comparing Strings~~

`boolean equals(Object obj)`	returns true if one String has the same value as another String (I.e. have the same set of characters)
`boolean equalsIgnoreCase(str)`	returns true if one String has the same value as another String str ignoring case considerations (I.e. have the same set of characters)

~~Searching Strings~~

`int indexOf(char or String);`	returns the index of the first occurrence of char ch or String str within a String.
`int indexOf(char or string, int index);`	returns the index of the first occurrence of char ch or String str within a String, starting the search at the specified index.
`int lastIndexOf(char or String);`	returns the index of the last occurrence of char ch ore String str within a String.
`int lastIndexOf(char or String, int index);`	returns the index of the last occurrence of of char ch or String str within a String, starting the search backwards from the specified index.

~~Extracting characters and substrings from Strings~~

`String subString(int beginIndex)`	returns a new String which is a substring of the original String starting at the specified beginIndex.
`String subString(int beginIndex, int endIndex)`	returns a new String which is a substring of the original String starting at the specified beginIndex and ending at the specified endIndex.
`char charAt(int index)`	returns the character at the specified index.

~~Converting other data types to Strings~~

static String valueOf(some datatype)

returns the String representation of the argument. the argument may be a boolean, character, double, float, int, or long.

Now let's have a look at some of these methods in more detail.

String Concatenation

You can create new String by joining (concatenating) other Strings together using the + operator. Here is an example:-

String str1 = "Hello";

// note the space before the W in World

Stringstr2 = " World!";

System.out.println(str1 + str2);

This would display "Hello World!".

Or we could have used a third String variable:-

Stringstr3 = str1 + str2;

System.out.println(str3);

When you use the + concatenation operator, if one of the operands is a String but the other operand is a number then Java will attempt to convert the number to a String and then join the two, i.e.

Expression Resultant Value

"1" + 2 "12"

"1" + 2 + 3 "123"

1 + 2 3 //this is not a String

"I am " + 3 "I am 3"

String Length

You can find out the number of characters in a String using the length() method:-

Stringstr1 = "I am a string";

System.out.println(str1.length());//prints 13

Try copying, compiling and running the following code:-

import java.awt.*; import javax.swing.*;

public class StringLength extends JApplet { public void init() { // Create two string objects

String str1 = new String ("I am a string");

String str2 = new String ("I am a string");

JOptionPane.showMessageOption(this,

"The length of str1 is " + str1.length());

}

String Comparison

Let's have another look at some methods used for comparing Strings.

~~Comparing Strings~~

`boolean equals(Object obj)`	returns true if one String has the same value as another String (I.e. have the same set of characters)
`boolean equalsIgnoreCase(str)`	returns true if one String has the same value as another String str ignoring case considerations (I.e. have the same set of characters)

So, you can compare Strings to see if one Strings value matches another using the equals() method:-

Stringstr1 = new String ("I am a string");

Stringstr2 = new String ("I am a string");

if ( str1.equals(str2) ) //would return true

You may come across expressions such as:-

if ( str1 == str2 )

but you may not get the result you are after.

To compare Strings for equality, DON'T use ==. The ==operator checks to see if two objects are exactly the same object, whereas the equals() methods checks to see if two String objects contain the same value (have exactly the same characters in them).

Here is an example:-

//returns false because str1 is not the same object as str2

if ( str1 == str2 )

Now try copying, compiling and running the following code:-

import java.awt.*; import javax.swing.*;

public class StringCompare extends JApplet { public void init() { // Create two string objects

String str1 = new String ("I am a string");

String str2 = new String ("I am a string");

if ( str1.equals(str2) )

JOptionPane.showMessageOption(this,"str1 equals str2");

else

JOptionPane.showMessageOption(this,"str1 does not equal str2");

}

Searching Strings

Let's have another look at some methods used for searching Strings.

~~Searching Strings~~

`int indexOf(char or String);`	returns the index of the first occurrence of char ch or String str within a String.
`int indexOf(char or string, int index);`	returns the index of the first occurrence of char ch or String str within a String, starting the search at the specified index.
`int lastIndexOf(char or String);`	returns the index of the last occurrence of char ch ore String str within a String.
`int lastIndexOf(char or String, int index);`	returns the index of the last occurrence of of char ch or String str within a String, starting the search backwards from the specified index.

You can search a String for a particular character or set of characters using...

indexOf: returns the index of the first occurrence of a character or String within a String.
lastIndexOf: returns the index of the last occurrence of a character or String within a String.

For example:-

"I am a string".indexOf ('a');

returns the number 2 since this is the location of the first occurrence of 'a' in "I am a string". You should note that index numbers start at 0 and not 1.

You can also use indexOf to locate the first occurrence of a set of characters (a String) within another String:-

"I am a string".indexOf ("I am");

returns the number 0 since this is the location of the first occurrence of "I am" within "I am a string".

Now:-

"I am a string".lastIndexOf ('a');

returns the number 6 since this is the location of the last occurrence of 'a' in "I am a string".

if the specified character or set of characters cannot be found...

then both indexOf and lastIndexOf return the value -1.

There is an alternative way of using the indexOf method. You can specify where in the String the method should start searching for the character:-

"I am a string".indexOf ('a'); // returns 2

but

"I am a string".indexOf ('a',3); // returns 6

since I specified that the search for the character 'a' should start at index 3 within the string "I am a string".

You can also search for a set of one or more characters (a String) within another String, specifying where to start the search from:-

"I am a string".indexOf ("a",3); // returns 6

String Extraction

Let's have another look at some methods used for extracting characters and substrings from Strings.

~~Extracting characters and substrings from Strings~~

`String subString(int beginIndex)`	returns a new String which is a substring of the original String starting at the specified beginIndex.
`String subString(int beginIndex, int endIndex)`	returns a new String which is a substring of the original String starting at the specified beginIndex and ending at the specified endIndex.
`char charAt(int index)`	returns the character at the specified index.

You can extract a character from a String using the charAt method:-

For example:-

// ch is assigned the character 'a'

char ch = "I am a string".charAt(5);

Here is another example:-

String str = "I am a String";

// ch is assigned the character 'a'

char ch = str.charAt(5);

You can extract a substring from a String using the subString method:-

For example:-

// newStr is assigned the String "a string"

String newStr = "I am a string".subString(5);

Here subString(5) specifies a substring of "I am a string" starting at index 5, giving a new String "a string". The variable newStr is assigned to reference this substring.

Here is another example:-

String str = "I am a String";

// newStr is assigned the String "a string"

String newStr = str.subString(5);

As we have seen, when using the subString method we must specify a starting index from which to extract a new string. We can also specify an end index:-

// newStr is assigned the String "am"

String newStr = "I am a string".subString(2,4);

Converting To a String

Let's have another look at methods used for converting other data types to Strings

~~Converting other data types to Strings~~

static String valueOf(some datatype)

returns the String representation of the argument. the argument may be a boolean, character, double, float, int, or long.

The valueOf method is overloaded which means it can be used to convert various types to a String. Let's look at a few examples:-

String s;

s = String.valueOf(5); // integer conversion. s = "5"

s = String.valueOf(3.141); // double conversion. s = "3.141"

s = String.valueOf('z'); // character conversion. s = "z"

s = String.valueOf(true); // boolean conversion. s = "true"

valueOf is a class method

Notice the declaration:- static String valueOf(some datatype)

The static keyword means the method is a class method, in other words it is used by the String class and not an instance of a String class, i.e.

String.valueOf(5); //notice the class String uses the valueOf method

Strings Coding Example

StringBuffer Class

A StringBuffer is like a String, but unlike a String it CAN be modified. If you want to be able to modify a String after you have created it then you must use the StringBuffer class instead.

At any point in time a StringBuffer contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls.

Creating StringBuffers

StringBuffers are created by using the new keyword:-

StringBuffer sb = new StringBuffer();

This creates a StringBuffer which is initially empty but can hold up to 14 characters.

Another way of creating a StringBuffer with a capacity for a stated number of characters is:-

StringBuffer sb = new StringBuffer(30);

Yet another way of creating a StringBuffer is to specify the set of characters you want it to store:-

StringBuffer sb = new StringBuffer("hello World!");

StringBuffer Capacity

StringBuffer objects are created with space for 16 extra characters over and above those stored with it at creation time. If necessary, this space is increased automatically.

StringBuffer Methods Overview

Here are some useful methods you can use with StringBuffers.

~~StringBuffer Length~~

length()

find the number of characters in the StringBuffer

~~Comparing StringBuffers~~

As you will see, you have to convert a StringBuffer to a String to use comparison methods

~~Searching StringBuffers~~

As you will see, you have to convert a StringBuffer to a String to use searching methods

~~Extracting characters and substrings from StringBuffers~~

`String subString(int beginIndex)`	returns a new String which is a substring of the StringBuffer starting at the specified beginIndex.
`String subString(int beginIndex, int endIndex)`	returns a new String which is a substring of the StringBuffer starting at the specified beginIndex and ending at the specified endIndex.
`char charAt(int index)`	returns the character at the specified index.

~~Modifying StringBuffers~~

`StringBuffer append(some type)`	adds the specified characters to the end of the StringBuffer. This method is overloaded to accept various types - char[]. boolean, int, long. double, float, object, String.
`StringBuffer insert(int index, some type)`	inserts the specified characters into the StringBuffer at the element specified by index. This method is overloaded so the second parameter can be a char[]. boolean, int, long. double, float, object or String.
`StringBuffer delete(int startIndex,int endIndex)`	deletes the characters from elements startIndex to endIndex
`StringBuffer deleteCharAt(int index)`	deletes the character at index
`StringBuffer replace(int startIndex,int endIndex, String s)`	replace the characters between startIndex and endIndex with the specified String s.
`String toString()`	returns a String equivalent of a StringBuffer

Now let's have a look at some of these methods in more detail.

StringBuffer Length

You can find out the number of characters in a StringBuffer using the length() method:-

StringBuffer sb1 = new StringBuffer("cheese");

System.out.println(sb1.length());//prints 6

StringBuffer Extraction

Let's have another look at some methods used for extracting characters and substrings from StringBuffers.

~~Extracting characters and substrings from StringBuffers~~

`String subString(int beginIndex)`	returns a new String which is a substring of the StringBuffer starting at the specified beginIndex.
`String subString(int beginIndex, int endIndex)`	returns a new String which is a substring of the StringBuffer starting at the specified beginIndex and ending at the specified endIndex.
`char charAt(int index)`	returns the character at the specified index.

You can extract a character from a StringBuffer using the charAt method:-

For example:-

StringBuffer sb1 = new StringBuffer( "green cheese");

// ch is assigned the character 'r'

char ch = sb1.charAt(1);

You can extract a substring from a StringBuffer using the subString method:-

For example:-

StringBuffer sb1 = new StringBuffer( "green cheese");

// newStr is assigned the String "cheese"

String newStr = sb1.subString(6);

Here subString(6) specifies a substring of "green cheese" starting at index 6, giving a new String "cheese". The variable newStr is assigned to reference this new String.

As we have seen, when using the subString method we must specify a starting index from which to extract a new string. We can also specify an end index:-

StringBuffer sb1 = new StringBuffer( "green cheese");

// newStr is assigned the String "he"

String newStr = sb1.subString(7,9);

Modifying StringBuffers

Let's have another look at some methods used for modifiying StringBuffers.

~~Modifying StringBuffers~~

`StringBuffer append(some type)`	adds the specified characters to the end of the StringBuffer. This method is overloaded to accept various types - char[]. boolean, int, long. double, float, object, String.
`StringBuffer insert(int index, some type)`	inserts the specified characters into the StringBuffer at the element specified by index. This method is overloaded so the second parameter can be a char[]. boolean, int, long. double, float, object or String.
`StringBuffer delete(int startIndex,int endIndex)`	deletes the characters from elements startIndex to endIndex
`StringBuffer deleteCharAt(int index)`	deletes the character at index
`StringBuffer replace(int startIndex,int endIndex, String s)`	replace the characters between startIndex and endIndex with the specified String s.
`StringBuffer setCharAt(int index,int char ch)`	replaces the character at index with the specified character ch.
`String toString()`	returns a String equivalent of a StringBuffer

You can add one or more characters to the end of a StringBuffer using the append method:-

For example:-

StringBuffer sb1 = new StringBuffer( "the cow");

sb1.append(" jumped"); //changes sb1 to "the cow jumped"

You can insert one or more characters into a StringBuffer using the insert method:-

For example:-

StringBuffer sb1 = new StringBuffer( "the cow");

sb1.insert(3, " fat"); //changes sb1 to "the fat cow"

You can delete one or more characters from a StringBuffer using the delete method:-

For example:-

StringBuffer sb1 = new StringBuffer( "the cow");

//delete the characters from index 0 to index 3

sb1.delete(0,3); //changes sb1 to "cow"

You can delete a single character from a StringBuffer using the deleteCharAt method:-

For example:-

StringBuffer sb1 = new StringBuffer( "??");

//delete the characters from index 0 to index 3

sb1.deleteCharAt(?); //changes sb1 to "cow"

You can replace a set of characters within a StringBuffer using the replace method. The section within StringBuffer specified by startIndex to endIndex is deleted then the new string is inserted at startIndex.

For example:-

StringBuffer sb1 = new StringBuffer( "ABCD");

//deletes the characters from index 1 to 3 then inserts "bc"

sb1.replace(1,3,"bc"); //changes sb1 to "AbcD"

If the deleted section is smaller than the replacement string then the Str is expanded in size to cope with this.

For example:-

StringBuffer sb1 = new StringBuffer( "ABCD");

//deletes the characters from index 1 to 3 then inserts "bcd"

sb1.replace(1,3,"bcd"); //changes sb1 to "AbcdD"

You can replace a single character within a StringBuffer using the setCharAt method.

For example:-

StringBuffer sb1 = new StringBuffer( "ABCD");

sb1.replace(2,'X'); //changes sb1 to "ABXD"

You can return a String equivalent of a StringBuffer using the toString method.

For example:-

StringBuffer sb1 = new StringBuffer( "ABCD");

//set newStr to reference a String with a value of "ABCD"

String newStr = sb1.toString();

Searching StringBuffers

There are no methods for searching within the StringBuffer class. You have to convert a StringBuffer to a String and use the searching methods of the String class.

For example:-

StringBuffer sb1 = new StringBuffer( "ABCD");

sb1.toString().indexOf("B"); //returns 1

StringBuffers Comparison

There are no methods for comparison within the StringBuffer class. You have to convert a StringBuffer to a String and use the comparison methods of the String class.

For example:-

StringBuffer sb1 = new StringBuffer( "ABCD");

sb1.toString().equals("ABCD"); //returns true

StringBuffer Coding Example

Try copying, compiling and running the following code:-

import java.awt.*; import javax.swing.*; import java.awt.event.*; public class Keyboard extends JApplet implements ActionListener { StringBuffer txtInput;//declare a StringBuffer JTextField txtDisplay;//declare a text field //create an array of 90 buttons JButton[] keyArray = new JButton[90]; public void init(){ char AsciiChar; //can hold characters of ASCII code //set layout of applet content pane getContentPane().setLayout(new FlowLayout());

//create an instance of a StringBuffer txtInput = new StringBuffer(""); //create an instance of a text field txtDisplay = new JTextField(30); //add the text field to the applet content pane getContentPane().add (txtDisplay); // create grid layout panel with 4 rows without

//specifying the number of columns JPanel akeyboard = new JPanel(new GridLayout(6,0)); // loop for creating 90 buttons for(AsciiChar = 33; AsciiChar <= 122; AsciiChar++) { // create a button JButton aButton = new JButton ( "" + (AsciiChar)); // add the button to the button array keyArray[AsciiChar-33] = aButton; // add each button to the panel akeyboard.add(keyArray[AsciiChar-33]); // add an action listener to each button keyArray[AsciiChar-33].addActionListener(this); // set the action command for each button keyArray[AsciiChar-33].setActionCommand("" + (AsciiChar)); } //add the panel to the applet content pane getContentPane().add (akeyboard); } public void actionPerformed(ActionEvent e){

//append the user input to the StringBuffer txtInput.append(e.getActionCommand()); //display the StringBuffer contents in the text field txtDisplay.setText(txtInput.toString()); } }