Immutability in Java

Hello, Habr. In anticipation of the imminent start of the Oracle Java Programmer Certification Preparation (OCAJP) course, we have prepared a traditional translation of the material for you.



We also invite everyone to participate in the open demo lesson "Constructors and Initialization Blocks". On this free webinar, we:

- We will disassemble the constructor for parts

- Define the finalists (final variables)

- Put things in order (initializations)

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An immutable class is a class that, after initialization, cannot change its state. That is, if the code contains a reference to an instance of an immutable class, then any changes in it lead to the creation of a new instance.

For a class to be immutable, it must meet the following requirements:

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import java.util.Map;
public final class MutableClass {
  private String field;
  private Map<String, String> fieldMap;
public MutableClass(String field, Map<String, String> fieldMap) {
  this.field = field;
  this.fieldMap = fieldMap;
}
public String getField() {
  return field;
}
public Map<String, String> getFieldMap() {
  return fieldMap;
}
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

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import java.util.HashMap;
import java.util.Map;
public class App {
public static void main(String[] args) {
  Map<String, String> map = new HashMap<>();
  map.put("key", "value");
    
  //   "" 
  MutableClass mutable = new MutableClass("this is not immutable", map);
  //      map ==  
  mutable.getFieldMap().put("unwanted key", "another value");
  mutable.getFieldMap().keySet().forEach(e ->  System.out.println(e));
}
}
//   
unwanted key
key
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

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import java.util.HashMap;
import java.util.Map;
public class AlmostMutableClass {
  private String field;
  private Map<String, String> fieldMap;
public AlmostMutableClass(String field, Map<String, String> fieldMap) {
  this.field = field;
  this.fieldMap = fieldMap;
}
public String getField() {
  return field;
}
public Map<String, String> getFieldMap() {
  Map<String, String> deepCopy = new HashMap<String, String>();
  for(String key : fieldMap.keySet()) {
    deepCopy.put(key, fieldMap.get(key));
  }
  return deepCopy;
}
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

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import java.util.HashMap;
import java.util.Map;
public class App {
public static void main(String[] args) {
  Map<String, String> map = new HashMap<>();
  map.put("good key", "value");
    
  //   "" 
  AlmostMutableClass almostMutable = new AlmostMutableClass("this is not immutable", map);
  
  //       
  //      map
  System.out.println("Result after modifying the map after we get it from the object");
  almostMutable.getFieldMap().put("bad key", "another value");
  almostMutable.getFieldMap().keySet().forEach(e -> System.out.println(e));
  
  System.out.println("Result of the object's map after modifying the initial map");
  map.put("bad key", "another value");
  almostMutable.getFieldMap().keySet().forEach(e -> System.out.println(e));
    
  }
}
//   
Result after modifying the map after we get it from the object
good key
Result of the object's map after modifying the initial map
good key
bad key
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

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public AlmostMutableClass(String field, Map<String, String> fieldMap) {
  this.field = field;      
  Map<String, String> deepCopy = new HashMap<String, String>();
  for(String key : fieldMap.keySet()) {
    deepCopy.put(key, fieldMap.get(key));
  }
  this.fieldMap = deepCopy;
}
//   
Result after modifying the map after we get it from the object
good key
Result of the object's map after modifying the initial map
good key
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

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