- We have studied String before
-
See: click here
But the presentation was very vague (not transparent) because we have not learned about instance methods
- Now that we have studied user-defined classes and instance methods, we can now fully understand String objects
- The String class
- String is a
user-defined class
that is created by the
programmers who
implemented the
Java library
- Here is the
beginning of
the description of the
String class
in the Java deocumentation:
- String is a
user-defined class
that is created by the
programmers who
implemented the
Java library
- Defining
String variables:
- Using the object definition syntax
(See:
click here)
String s; // s is an identifier s = new String( "Hello World " ); // creates a String object and // calls a constructor method
- The designer of the
Java language wanted to
treat Strings special and
they made the
Java compiler recognize
a short hand notation for
String creation:
String s; // s is an identifier s = "Hello World "; // Short hand for: new String("Hello World");
- Using the object definition syntax
(See:
click here)
- How a String object
is stored:
- How objects are
stored in general:
- A String is
an object and
it is stored in a similar way:
A String typed variable contains an address (= reference) to the String object
The String object stores the characters in the string.
- How objects are
stored in general:
- Some instance methods on
String typed variables:
- length():
returns the length of the string
Examples:
String s = "Abc", t = "Hello"; s.length() returns 3 t.length() returns 5
- charAt(n):
returns the charcater
at position n in the string
Examples:
012 01234 String s = "Abc", t = "Hello"; s.charAt(1) returns 'b' t.charAt(1) returns 'e'
- substring(i,j):
returns the substring
from position i
upto (not including)
position j of the string
Examples:
012 01234 String s = "Abc", t = "Hello"; s.substring(0,2) returns "Ab" t.substring(1,4) returns "ell"
- Concatenating strings:
- Using an instance method
(defined inside the
String class):
String s = "Abc", t = "Hello"; s.concat("xyz") returns "Abcxyz" t.concat(s) returns "HelloAbc"
Note: We would normally store the return value in a String variable String r; r = t.concat(s); // r = "HelloAbc" - The designers
of the Java language wanted to
treat Strings special
and they made the Java compiler recognize a
short hand notation
for String concatenation:
String s = "Abc", t = "Hello"; s + "xyz" // Synonym for: s.concat("xyz") t + s // Synonym for: t.concat("s)
Note: We would normally store the return value in a String variable String r; r = t + s // Same as: r = t.concat(s);
- Using an instance method
(defined inside the
String class):
- length():
returns the length of the string
- Fact:
- String is a
class in Java
In other words, someone wrote:
public class String { instance variables... instance methods such as: publich char charAt() { ... } public int length() { ... } public String substring(int a, int b) { ... } }
- All operations on
Strings are implemented as
instance methods
- String is a
class in Java
- The general format to
use an operation on a String object:
String s1; s1.StringOperationMethod( parameters )Meaning of the method call:
- The name of the operation is
StringOperationMethod
- The operation is to be
applied to
the implicit parameter
s1
- The operation StringOperationMethod may require additional parameters to perform the task
- The name of the operation is
StringOperationMethod
- We will next study a small number of useful String operations