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How to create immutable class in java

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instanceof java

Posts: 576
Nickname: instanceof
Registered: Jan, 2015

instanceof java is a java related one.
How to create immutable class in java Posted: Dec 24, 2015 11:20 AM
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Original Post: How to create immutable class in java
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  • In Java String and all wrapper classes are immutable classes.
  • So how to create custom immutable class in java?
  • Lets see how to make a class object immutable.

  Immutable class:

  • Make class final so that it should not be inherited.
  • All the variables should be private so should not be accessible outside of class. 
  • Make all variables final so that value can not be changed.
  • A constructor to assign values to variables in class.
  • Do not add any setter methods.

1. Java Program to create custom immutable class object in java.

  1. package com.instaceofjava;

  3.  
  4. public class ImmutableClass{
  5.   
  6. private final int a;
  7. private final int b;
  8.  
  9. ImmutableClass( int x, int y){
  10.  
  11.  a=x;
  12.  b=y; 

  14. } 
  15.  
  16. public getA(){
  17.  
  18.  return a;

  20. }
  21.  
  22. public getB(){
  23.  
  24.  return b;

  26. }
  27.  
  28. public static void main(String[] args) {
  29.  
  30. ImmutableClass obj= new ImmutableClass(10,20);
  31.  
  32. System.out.println("a="+obj.getA());
  33.  
  34. System.out.println("b="+obj.getB());
  35.  
  36. }
  37. }
Output:

  1. a=10
  2. b=20


String class in java: Immutable


  1. public final class String
  2.         implements java.io.Serializable, Comparable<String>, CharSequence
  3. {  
  4.   
  5. //String class variables

  7.   private final char value[];
  8.   private final int offset;
  9.   private final int count;
  10.   private int hash; // Default to 0
  11.   private static final ObjectStreamField[] serialPersistentFields =
  12.       new ObjectStreamField[0];
  13.   
  14. //String class constructor
  15. public String(String original) {
  16.  
  17.             int size = original.count;
  18.              char[] originalValue = original.value;
  19.              char[] v;
  20.              if (originalValue.length > size) {
  21.                 // The array representing the String is bigger than the new
  22.               // String itself.  Perhaps this constructor is being called
  23.                 // in order to trim the baggage, so make a copy of the array.
  24.                  int off = original.offset;
  25.                 v = Arrays.copyOfRange(originalValue, off, off+size);
  26.             } else {
  27.                  // The array representing the String is the same
  28.                  // size as the String, so no point in making a copy.
  29.                  v = originalValue;
  30.            }
  31.             this.offset = 0;
  32.              this.count = size;
  33.             this.value = v;
  34.         } 

  36. }



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