How to create Immutable class?
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There
are many immutable classes like String, Boolean, Byte, Short, Integer, Long, Float,
Double etc. In short, all the wrapper classes and String class is immutable.
We can also create immutable class by creating final class that have final
data members as the example given below:
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Example to
create Immutable class
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In
this example, we have created a final class named Employee. It have one final
datamember, a parameterized constructor and getter method.
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1. public final class Employee{
2. final String pancardNumber;
3.
4. public Employee(String pancardNumber){
5. this.pancardNumber=pancardNumber;
6. }
7.
8. public String getPancardNumber(){
9. return pancardNumber;
10. }
11.
12. }
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The
above class is immutable because:
·
The instance variable of the class is
final i.e. we cannot change the value of it after creating an object.
·
The class is final so we cannot create
the subclass.
·
There is no setter methods i.e. we have
no option to change the value of the instance variable.
These
points makes this class as immutable.
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Java toString() method
If you want to represent any object
as a string, toString() method comes into existence.
The toString() method returns
the string representation of the object.
If you print any object, java
compiler internally invokes the toString() method on the object. So overriding
the toString() method, returns the desired output, it can be the state of an
object etc. depends on your implementation.
Advantage
of Java toString() method
By overriding the toString()
method of the Object class, we can return values of the object, so we don't
need to write much code.
Understanding
problem without toString() method
Let's see the simple code that
prints reference.
1. class Student{
2. int rollno;
3. String name;
4. String city;
5.
6. Student(int rollno, String name, String city){
7. this.rollno=rollno;
8. this.name=name;
9. this.city=city;
10. }
11.
12. public static void main(String args[]){
13. Student s1=new Student(101,"Raj","lucknow");
14. Student s2=new Student(102,"Vijay","ghaziabad");
15.
16. System.out.println(s1);//compiler writes here s1.toString()
17. System.out.println(s2);//compiler writes here s2.toString()
18. }
19. }
Output:Student@1fee6fc
Student@1eed786
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As
you can see in the above example, printing s1 and s2 prints the hashcode
values of the objects but I want to print the values of these objects. Since
java compiler internally calls toString() method, overriding this method will
return the specified values. Let's understand it with the example given
below:
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Example of Java toString() method
Now let's see the real example
of toString() method.
1. class Student{
2. int rollno;
3. String name;
4. String city;
5.
6. Student(int rollno, String name, String city){
7. this.rollno=rollno;
8. this.name=name;
9. this.city=city;
10. }
11.
12. public String toString(){//overriding the toString() method
13. return rollno+" "+name+" "+city;
14. }
15. public static void main(String args[]){
16. Student s1=new Student(101,"Raj","lucknow");
17. Student s2=new Student(102,"Vijay","ghaziabad");
18.
19. System.out.println(s1);//compiler writes here s1.toString()
20. System.out.println(s2);//compiler writes here s2.toString()
21. }
22. }
Output:101 Raj lucknow
102 Vijay ghaziabad
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