Do we need to explicitly call save() method in Hibernate or JPA to save entities?

By Wayan in Java Persistence API 0 Comment

No, you typically don’t need to explicitly call the save() method in Hibernate or JPA to save entities. In JPA, when you modify a managed entity (an entity that was retrieved or persisted by the entity manager), the changes are automatically synchronized with the database when the transaction commits. Hibernate, being an implementation of JPA, follows this behavior.

Here’s how it generally works:

  1. Persisting new entities: When you create a new entity object and persist it using EntityManager.persist() (or Session.save() in Hibernate), the entity becomes managed by the persistence context. Any changes made to this entity within the scope of the transaction will be automatically synchronized with the database upon transaction commit. 
    entityManager.persist(entity);
  2. Updating existing entities: When you retrieve an entity from the database (either by EntityManager.find() or through a query), any changes made to this managed entity will also be synchronized with the database upon transaction commit. You don’t need to call any explicit save method. 
    Entity entity = entityManager.find(Entity.class, id);
entity.setSomeProperty(newValue);
// Changes to 'entity' are automatically synchronized with the database upon transaction commit
  3. Automatic dirty checking: Hibernate/JPA employs the concept of dirty checking. It tracks the changes made to managed entities within a transaction. When the transaction commits, it automatically detects the changes and synchronizes them with the database. 
    // Entity retrieved and modified within a transaction
Entity entity = entityManager.find(Entity.class, id);
entity.setSomeProperty(newValue);
// Changes to 'entity' are automatically tracked and synchronized with the database upon transaction commit

Explicitly calling save() might be necessary in specific cases where you’re dealing with detached entities (entities that are not managed by the persistence context) or if you’re operating outside a transaction boundary, but in general usage within a transaction, it’s not required.

How do I check if a character is a whitespace in Java?

By Wayan in Core API, Lang Package 0 Comment

Whitespace characters in Java (or programming in general) aren’t just the space ' ' character. It also includes other characters that create some form of space or break in the text. The most common ones include:

  • space ' '
  • tab '\t'
  • newline '\n'
  • carriage return '\r'
  • form feed '\f'.

All these characters fall into the category of whitespace characters.

Now, if we want to check if a character in Java is one of these whitespace characters, we can make use of the built-in method Character.isWhitespace(char ch). Character is a class in Java that provides a number of useful class (i.e., static) methods for working with characters. And the isWhitespace() method is one of them which checks if the provided character is a whitespace character.

Here is a simple code snippet:

package org.kodejava.lang;

public class CharacterIsWhitespace {
    public static void main(String[] args) {
        char ch = ' ';

        if (Character.isWhitespace(ch)) {
            System.out.println(ch + " is a whitespace character.");
        } else {
            System.out.println(ch + " is not a whitespace character.");
        }
    }
}

This code first defines a character ch and then uses Character.isWhitespace(ch) to check if it is a whitespace character. The isWhitespace() method returns true if the given character is a space, new line, tab, or other whitespace characters, false otherwise.

Here’s a little more expansive example:

package org.kodejava.lang;

import java.util.Arrays;
import java.util.List;

public class CharacterIsWhitespaceDemo {
    public static void main(String[] args) {
        List<Character> characters = Arrays.asList(' ', '\t', '\n', '\r', '\f', 'a', '1');
        for (char ch : characters) {
            if (Character.isWhitespace(ch)) {
                System.out.println("'" + ch + "' is a whitespace character.");
            } else {
                System.out.println("'" + ch + "' is not a whitespace character.");
            }
        }
    }
}

Output:

' ' is a whitespace character.
'   ' is a whitespace character.
'
' is a whitespace character.
' is a whitespace character.
'' is a whitespace character.
'a' is not a whitespace character.
'1' is not a whitespace character.

In this code snippet, we are checking and outputting whether each character in a list of characters is a whitespace character or not. The list includes a space, a tab, newline, carriage return, form feed, an alphabetic character, and a digit. The isWhitespace() method identifies correctly which ones are the whitespace characters.

The Character.isWhitespace(char ch) method in Java also considers Unicode whitespace. It checks for whitespace according to the Unicode standard. The method considers a character as a whitespace if and only if it is a Unicode space separator (category “Zs”), or if it is one of the following explicit characters:

  • U+0009, HORIZONTAL TABULATION (‘\t’)
  • U+000A, LINE FEED (‘\n’)
  • U+000B, VERTICAL TABULATION
  • U+000C, FORM FEED (‘\f’)
  • U+000D, CARRIAGE RETURN (‘\r’)

Here is an example of checking Unicode whitespace:

package org.kodejava.lang;

public class CharacterIsWhitespaceUnicode {
    public static void main(String[] args) {
        char ch = '\u2003';  // EM SPACE

        if (Character.isWhitespace(ch)) {
            System.out.println("Character '" + ch + "' (\\u2003) is a whitespace character.");
        } else {
            System.out.println("Character '" + ch + "' (\\u2003) is not a whitespace character.");
        }
    }
}

Output:

Character ' ' (\u2003) is a whitespace character.

In this example, \u2003 is a Unicode representation of the “EM SPACE” character, which is a type of space character in the Unicode standard. The isWhitespace() method correctly identifies it as a whitespace character.

How to remove map’s entry set elements in certain condition?

By Wayan in Util Package 0 Comment

In Java, you can use the removeIf() method to remove elements from a Set-based in a certain condition. Here’s how you can do it:

  • First, get the entry set from the map. The entry set is a Set<Map.Entry<K,V>>.
  • Then, call removeIf() on this set.
  • The removeIf() method takes a predicate, which is a condition that is checked against every element in the set.
  • If the predicate is true for a given element, that element is removed.

Here is the Java code:

package org.kodejava.util;

import java.util.HashMap;
import java.util.Map;

public class MapEntrySetRemoveIf {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("One", 1);
        map.put("Two", 2);
        map.put("Three", 3);
        map.put("Four", 4);

        // Remove entry with key "Two"
        map.entrySet().removeIf(entry -> entry.getKey().equals("Two"));

        map.entrySet().forEach(System.out::println);
    }
}

Output:

One=1
Four=4
Three=3

This will remove the map entry with “Two” as its key. You can replace entry.getKey().equals("Two") with any condition you desire.

Please note that this operation may throw ConcurrentModificationException if the map is structurally modified at any time after the iterator is created. Make sure you’re aware of concurrent modifications when using this method.

How do I merge the entries of two separate map objects?

By Wayan in Util Package 0 Comment

You can use putAll() method provided by the Map interface to merge entries of two separate Map objects. The putAll() method copies all the mappings from the specified map to the current map. Pre-existing mappings in the current map are replaced by the mappings from the specified map.

Here is a Java code example:

package org.kodejava.util;

import java.util.HashMap;
import java.util.Map;

public class MapPutAllExample {
    public static void main(String[] args) {
        Map<String, String> map1 = new HashMap<>();
        Map<String, String> map2 = new HashMap<>();

        map1.put("key1", "value1");
        map1.put("key2", "value2");
        map2.put("key3", "value3");

        System.out.println("Map1: " + map1);
        System.out.println("Map2: " + map2);

        map1.putAll(map2);

        System.out.println("Merged Map: " + map1);
    }
}

Output:

Map1: {key1=value1, key2=value2}
Map2: {key3=value3}
Merged Map: {key1=value1, key2=value2, key3=value3}

If you want to merge two maps but want to provide a specific behavior in case where a key is present in both maps, you might use Map.merge() available since Java 8.

Let’s assume that you want to concatenate the string values of the map where map keys are the same:

package org.kodejava.util;

import java.util.HashMap;
import java.util.Map;

public class MapMergeExample {
    public static void main(String[] args) {
        Map<String, String> map1 = new HashMap<>();
        Map<String, String> map2 = new HashMap<>();

        map1.put("key1", "value1");
        map1.put("key2", "value2");
        map2.put("key1", "value3");
        map2.put("key3", "value4");

        map2.forEach(
                (key, value) -> map1.merge(key, value, (v1, v2) -> v1.concat(",").concat(v2))
        );

        // Output: {key1=value1,value3, key2=value2, key3=value4}
        System.out.println(map1);
    }
}

Output:

{key1=value1,value3, key2=value2, key3=value4}

In this example, the Map.merge() method is called for each key-value pair in map2. If map1 already contains a value for the key, it will replace the value with the result of the lambda expression (v1, v2) -> v1.concat(",").concat(v2). This lambda expression tells Java to concatenate the existing and new values with a comma in between. If map1 doesn’t contain the key, it will simply put the key-value pair from map2 into map1.

So, in conclusion, putAll() is straightforward and simply puts all entries from one map to the other, possibly overwriting existing entries. merge(), On the other hand, allows specifying a behaviour for combining values of duplicate keys, providing more control and flexibility when merging maps.

How do I use replace() and replaceAll() methods of Map?

By Wayan in Util Package 0 Comment

In Java, the Map interface provides the methods replace() and replaceAll(), which are used to replace existing entries in the map.

Replace:

replace(K key, V value) is a method that replaces the entry for the specified key only if it is currently mapped to some value. It returns the old value associated with the specified key or null if the key is not in the map.

Here is a simple usage of the replace() method:

package org.kodejava.util;

import java.util.HashMap;
import java.util.Map;

public class MapReplaceExample {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("key1", "v1");
        map.replace("key1", "value1");

        // Output: New value of key1: value1
        System.out.println("New value of key1: " + map.get("key1"));
    }
}

Output:

New value of key1: value1

replace(K key, V oldValue, V newValue) replaces the entry for the specified key only if currently mapped to the specified value. This variant of replace() method provides additional check for existing value, which can prevent data corruption in concurrent environment without additional synchronization.

ReplaceAll:

replaceAll(BiFunction<? super K,? super V,? extends V> function) is a method that replaces each entry’s value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.

Here is a simple usage of the replaceAll() method:

package org.kodejava.util;

import java.util.HashMap;
import java.util.Map;

public class MapReplaceAllExample {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("key1", "v1");
        map.put("key2", "v2");

        map.replaceAll((k, v) -> v.toUpperCase());

        // Output: {key1=V1, key2=V2}
        System.out.println(map);
    }
}

Output:

{key1=V1, key2=V2}

In this example, the replaceAll() method is used to replace every value in the map with its uppercase version. The provided function should be non-interfering and stateless.

The benefits of using replace() and replaceAll() methods are:

  • They are more concise and expressive.
  • They can improve code readability and maintainability.
  • They are beneficial while working in a multi-thread environment because they provide additional safety without additional synchronization.