1. Strings
A string is a sequence of characters that is treated as a single value. Strings are objects.
We have been using strings all along. Every time you use a “” string, a string object is created automatically. For example, to display text
Blotgrunge ... out to reality ... HelloWorld.java
String is a class in the java.lang package.
System.out.println("Hello, how are you?");
We have also seen the string concatenation operation. For example, the following code reads in a person’s name and greets the user:
//assume inputBox and messageBox are declared
//and initialized correctly
String name;
name = inputBox.getString("What is your name?");
messageBox.show("Hello, " + name + ". Nice to meet you.");

2. Explicit String Objects
A declaration and object creation are needed for instances of the String class. For example,
String name1,name2,name3;
name1 = "Frooot";
name2 = new String(name1);
name3 = new String({’F’,’r’,’o’,’o’,’t’});
Blitititi … out to reality … StringVar.java
Blitititi … out to reality … StringVar2.java

3. Concatenating Strings
The + operator concatenates strings.
A new string object is created - the operands are not affected. Old strings are garbage collected.
Gadzook … out to reality … StringCat.java
Note that System.out.print really does this, not “and also print”.
Gadzook … back to reality … StringVar.java
Gadzook … back to reality … StringVar2.java

4. String variables are References - 1
Code
String word1, word2;
word1 = "Java”;
word2 = word1; A
Both word1 and word2 are allocated memory (to store references), but the objects themselves are not yet created, so they both contain null.
word1 L
word2
State of
Memory L
After is executed A

5. String variables are References - 2
Code
String word1, word2;
word1 = "Java”;
word2 = word1; B
One String object is created and assigned to word1, so word1 contains the address of this object.
word1
word2
word1
String
Java
word2 L
State of
Memory
After is executed B

6. String variables are References - 3
Content of word1, which is an address, is assigned to word2, making word2 refer to the same object.
Code
String word1, word2;
word1 = "Java”;
word2 = word1; C
word1
word2
String
Java
After is executed C
State of
Memory
Gadzook … out to reality … StringAlias.java
Gadzook … out to reality … StringAlias2.java

7. Equality (==) vs. equals – Case 1
String
Java
word1
String
Java
word2
word1 and word2 point to different objects having the same string.
false
word1 == word2
word1.equals( word2 )
true

8. Equality (==) vs. equals – Case 2
String
Java
word1
String
Bali
word2
word1 and word2 point to different objects with different strings.
false
word1 == word2
word1.equals( word2 )
false

9. Equality (==) vs. equals - Case 3
word1
word2
String
Java
word1 and word2 point to the same object.
true
word1 == word2
word1.equals( word2 )
true
Quixote … out to reality … StringJavas.java

10. Determining the Size 7 Error!
We determine the number of characters in a String with the length method.
String name = "Sumatra";
String str2 = "";
String str3;
name.length(); 7
str2.length();
Error because no object is created for str3, so it is a null.
Remember that a String is an object. If we simply declare, for example,
String str3;
and forget to create a String object, then str3 is a null object. You will get a NullPointerException if you try to a method of non-existent object, such as str3 in this example.
str3.length();
Error!

11. Accessing Individual Elements
Individual characters in a String accessed with the charAt method.
String name = “Sumatra”; 1 2 3 4 5 6 S u m a t r
name
name.charAt( 3 )
Olipidoo … out to reality … StringVowels.java
Olipidoo … out to reality … StringWords.java

12. Other Useful String Operators
Method
Meaning
compareTo
Compares the two strings. str1.compareTo( str2 )
substring
Extracts the a substring from a string. str1.substring( 1, 4 )
trim
Removes the leading and trailing spaces. str1.trim( )
valueOf
Converts a given primitive data value to a string. String.valueOf( )
startsWith
Returns true if a string starts with a specified prefix string. str1.startsWith( str2 )
endsWith
Returns true if a string ends with a specified suffix string. str1.endsWith( str2 )
Here are some examples:
String str1 = “Java”, str2 = “ Wow “;
str1.compareTo( “Hello” ); //returns positive integer
//because str1 >= “Hello”
str1.substring( 1, 4 ); //returns “ava”
str2.trim( ) //returns “Wow”, str2 stays same
str1.startsWith( “Ja” ); //returns true
str1.endsWith( “avi” ); //returns false

13. Command line Strings
The formal arguments to the main method receive strings from the command line arguments.
When running a program, supply command line arguments after the program name, e.g.,
java MyJavaProgram cat 27 'Java is great!'
has three command line arguments.
Gadzook … out to reality … MyJavaProgram.java
When we write
String name;
we must say “name is a variable of type String whose value is a reference to an instance of String” to be precise. However, when the value of a variable X is a reference to an instance of class Y, we usually say “X is an instance of Y” or “X is a Y object.” For example, we say canvas is a DrawingBoard object

14. Strings and Methods
Strings are reference variables, so (like arrays) formal parameters point to the actual parameter data
Strings returned from methods point to the data created in the method
Likidylik … out to reality … StringMethod.java

15. Strings are Immutable
A String object is immutable, which means that once a String object is created, is cannot be changed.
It is not possible to add, delete, or modify characters of a String object.
The methods of the String class, such as toUpperCase and substring, do not modify the original string; they return a new string.
Java adopts this immutability restriction to implement an efficient memory allocation scheme for managing String objects.
Oieeeeiooo … out to reality … StringImmut.java

16. StringBuffer
Creating a new string from the old one will work for most cases, but sometimes manipulating the content of a string directly is more convenient.
Manipulation means operations such as replacing a character, appending a string with another string, deleting a portion of a string, and so forth.
Java has a StringBuffer class for this.
StringBuffers are created from strings (no shorthand)
Ay … out to reality … StringBufferX.java
Ay … out to reality … StringBufferMake.java