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:

Table of Contents

Example to create Immutable class

In this example, we have created a final class named Employee. It have one final datamember, a parameterized constructor and getter method.

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. }

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

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
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:

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

StringTokenizer in Java

The java.util.StringTokenizer class allows you to break a string into tokens. It is simple way to break string.
It doesn’t provide the facility to differentiate numbers, quoted strings, identifiers etc. like StreamTokenizer class. We will discuss about the StreamTokenizer class in I/O chapter.
Constructors of StringTokenizer class
There are 3 constructors defined in the StringTokenizer class.

Constructor	Description
StringTokenizer(String str)	creates StringTokenizer with specified string.
StringTokenizer(String str, String delim)	creates StringTokenizer with specified string and delimeter.
StringTokenizer(String str, String delim, booleanreturnValue)	creates StringTokenizer with specified string, delimeter and returnValue. If return value is true, delimiter characters are considered to be tokens. If it is false, delimiter characters serve to separate tokens.

The 6 useful methods of StringTokenizer class are as follows:

Public method	Description
booleanhasMoreTokens()	checks if there is more tokens available.
String nextToken()	returns the next token from the StringTokenizer object.
String nextToken(String delim)	returns the next token based on the delimeter.
booleanhasMoreElements()	same as hasMoreTokens() method.
Object nextElement()	same as nextToken() but its return type is Object.
int countTokens()	returns the total number of tokens.

Simple example of StringTokenizer class

Let’s see the simple example of StringTokenizer class that tokenizes a string “my name is khan” on the basis of whitespace.

1. import java.util.StringTokenizer; 
2. public class Simple{ 
3. public static void main(String args[]){ 
4. StringTokenizer st = new StringTokenizer("my name is khan"," "); 
5. while (st.hasMoreTokens()) { 
6. System.out.println(st.nextToken()); 
7. } 
8. } 
9. }

Output:my
name
is
khan

Example of nextToken(String delim) method of StringTokenizer class

1. import java.util.*; 
2. 
3. public class Test { 
4. public static void main(String[] args) { 
5. StringTokenizer st = new StringTokenizer("my,name,is,khan"); 
6. 
7. // printing next token 
8. System.out.println("Next token is : " + st.nextToken(",")); 
9. } 
10. }

Output:Next token is : my
StringTokenizer class is deprecated now. It is recommended to use split() method of String class or regex (Regular Expression).

How to reverse String in Java

There are many ways to reverse String in Java. We can reverse String using StringBuffer, StringBuilder, iteration etc. Let’s see the ways to reverse String in Java.
1) By StringBuilder / StringBuffer
File: StringFormatter.java

1. public class StringFormatter { 
2. public static String reverseString(String str){ 
3. StringBuilder sb=new StringBuilder(str); 
4. sb.reverse(); 
5. return sb.toString(); 
6. } 
7. }

File: TestStringFormatter.java

1. public class TestStringFormatter { 
2. public static void main(String[] args) { 
3. System.out.println(StringFormatter.reverseString("my name is khan")); 
4. System.out.println(StringFormatter.reverseString("I am sonoo jaiswal")); 
5. } 
6. }

Output:
nahksiemanym
lawsiajoonos ma I

2) By Reverse Iteration

File: StringFormatter.java

1. public class StringFormatter { 
2. public static String reverseString(String str){ 
3. char ch[]=str.toCharArray(); 
4. String rev=""; 
5. for(int i=ch.length-1;i>=0;i--){ 
6. rev+=ch[i]; 
7. } 
8. return rev; 
9. } 
10. }

File: TestStringFormatter.java

1. public class TestStringFormatter { 
2. public static void main(String[] args) { 
3. System.out.println(StringFormatter.reverseString("my name is khan")); 
4. System.out.println(StringFormatter.reverseString("I am sonoo jaiswal")); 
5. } 
6. }

Output:
nahksiemanym
lawsiajoonos ma I

So, this brings us to the end of blog. This Tecklearn ‘How to create Immutable class in Java’ blog helps you with commonly asked questions if you are looking out for a job in Java Programming. If you wish to learn Java and build a career Java Programming domain, then check out our interactive, Java and JEE Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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