String is one of the most used class in java programing language , let see what makes it very useful class in java as well as its properties.
So lets Start with
1) What is String ?
So String is a Class in Java , which is basically the array of characters , In Java String is Class
,not a primitive datatype as in C language.
Package Name -
java.lang.String
2) How can you say that String is immutable in java ?
So we should first understand what is immutable means , So immutable means when any object is not permit itself to be modified or changed. so how string is immutable in java , lets see by example
class StringManupulation
{
public static void main (String[] args) throws java.lang.Exception
{
// your code goes here
String str1= new String("Program");
str1.concat("In Java");
System.out.println(str1);
}
}
Output :- Program
Logic -
As String is immutable it is not allowing the object to be modified as the object initial refer to Program and after concat it with another string , It is not allowing itself to modified
class StringManupulation
{
public static void main (String[] args) throws java.lang.Exception
{
// your code goes here
String str1= new String("Program");
str1+=str1.concat("In Java");
System.out.println(str1);
}
}
Output : Program In Java
Logic:- As we are changing the reference of the Str1 to Str1+ "In java" Object so the String str1 is now pointing to different object hows valus is "Program In Java".
Difference btw string and string buffer ?
- String is immutable and StringBuffer is mutable
- immutable - means the string will not be modify after created
Why String is immutable?
String object are cached in
string pool and these strings are shared so changing values has risk.
security reasons-
- if string is mutable then user could have changed the path after logging.
- would have changed the db url
- Mutable string cause problem in reflection also.
Immutability with examples
Non Changing behaviour is called immutability
Example-
String s = new String("programinjava");
s.concat(".com");
System.out.println(s);
Second Example:-
String s1 = new String("programinjava");
String s2= new String ("programinjava");
System.out.println(s1==s2)
System.out.println(s.equals(s2))
Output:
In String class .equals Method meant for content check
Explanation:-
String s = new String("programinjava");
- 2 object created , One is in Heap area and other one is in String constant pool ,s is pointing to Heap Object
String s= "programinjava" ;
- 1 object created in string constant pool
- Garbage collector is not allowed to access String pool area.
Memory area in java
There are 5 memory area are there in java -
- Heap(garbage collected is applicable only for heap)
- Method Area (SCP is in Method area)
- Stack
- PC Register
- Native Method stacks
Whenever we use
new operator with string , it will compulsory create object in heap and put it in Method Area
Example-
How many object will create ?
String s1 = new String("programinjava"); // 1 h 1 scp
String s2 = new String("programinjava");// 1 h , pre scp
String s3="programinjava";// previous scp
String s4 ="programinjava";// previous scp
There are total 3 object;// 2 in heap and 1 in scp
Runtime opertion on string will create object in heap
Examples-
String s = new String("programinjava");
s =s.concat(".com");
Explanation:
Here ,
"programinjava" will be in
heap and
scp
s.concat will create one
string constant in scp "
.com"
and create object "
programinjava.com" in heap area
NOTE1-For every string constant , one object will be placed in scp area.
NOTE2 - At Runtime if an object is required to placed only in heap area but not in scp area.
Mutable with examples
StringBuffer sb = new StringBuffer("programinjava");
sb.append(".com");
System.out.println(sb);
Output:
programinjava.com
StringBuffer sb1 = new StringBuffer("durga");
StringBuffer sb2 = new StringBuffer("durga");
System.out.println(sb1==sb2); //false
System.out.println(sb1.equals(sb2) ); //false
In StringBuffer Class .equals method meant for reference check
If you have any problem in understanding this , Please let me know in Comment section.
Thanks For Reading
-Noeik
