Some useful stream operations

Separate Odd and Even numbers (List<Integer> to Map<String, List<Integer>>)

Map<String, List<Integer>> oddEvenMap = numbers.stream()  
        .collect(Collectors.groupingBy(i -> (i % 2 != 0 ? "Odd" : "Even")));

Find distinct values from Map<String, List<Integer>

Set<Integer> set = map.values().stream()
					.flatMap(Collection::stream)
					.collect(Collectors.toSet());

Biggest Odd and Even from List (List<Integer> to Map<String, Integer>)

Map<String, Integer> bigOddEven = numbers.stream().collect(Collectors.toMap(i -> (i % 2 == 0 ? "Even" : "Odd"),  Function.identity(), (i1 , i2) -> i1 > i2 ? i1 : i2));

Use value as key and create new map from existing

Map<String, Integer> map = new HashMap<>();  
map.put("a", 1);  
map.put("b", 1);  
map.put("c", 2);  
  
Map<Integer, List<String>> ans = map.keySet().stream().collect(Collectors.groupingBy(map::get));  
System.out.println(ans);

Output

{1=[a, b], 2=[c]}

Find frequency of each number in list

List<Integer> numbers = new ArrayList<>();  
numbers.add(1);  
numbers.add(2);  
numbers.add(3);  
numbers.add(4);  
numbers.add(4);  
  
Map<Integer, Long> ans = numbers.stream()  
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));  
System.out.println(ans);

Output

{1=1, 2=1, 3=1, 4=2}

Remove from map based on condition on values

Map<String, Integer> map = new HashMap<>();  
map.put("a", 1);  
map.put("b", 1);  
map.put("c", 2);  
  
map.values().removeIf(i -> i == 1);  
System.out.println(map);

Output

{c=2}

Distinct and Sum of numbers

Stream<Integer> distinct = numbers.stream().distinct();
int s = numbers.stream().mapToInt(Integer::intValue).sum();

IntSummaryStatistics for Average, Sum, Min/Max

IntSummaryStatistics intSummaryStatistics = numbers.stream().mapToInt(Integer::intValue).summaryStatistics();
double average = intSummaryStatistics.getAverage();
long sum = intSummaryStatistics.getSum();
int max = intSummaryStatistics.getMax();
int min = intSummaryStatistics.getMin();
long count = intSummaryStatistics.getCount();

Java 9 to 19

Java 8 is still extensively used in the industry and many applications will gradually shift to newer Java version, especially the LTS versions.

In this post we will take a look at the evolution happened in Java language from Java 9 to Java 19. Note that each version comes with many improvements, bug fixes and variety of features, we will cover the ones which are majorly used and can impact our day to day developement.

Java 9

Factory methods for collection

    List immutableL = List.of(1, 2, 3);
    Map immutableM = Map.of(1, "ONE", 2, "TWO", 3, "THREE")

JShell: Java Shell, or REPL (Read Evaluate Print Loop) to execute java constructs directly in command line.

Private methods in interface.

This will avoid code duplication and better separation of concern when it comes to implementing default and static methods in interface.

interface Student {
    private String joinNames(String firstName, String lastName) {
        return String.join(firstName, " ",lastName);
    }
    private static String schoolName() {
        return "Some School";
    }

    default String id(String firstName, String lastName) {
        String fullName = joinNames(firstName, lastName);
        return schoolName() + "\n" + fullName;
    }
}

Step in direction to optimize String concatenation.

For the given class,

public class Test {
    public static void main(String[] args) {
        String str = args[0] + " and " + args[1];
    }
}

If we compile and check the bytecode, we can notice significant different in the way concatenation is handled.

In Java 8,

➜  java git:(main) ✗ java -version 
openjdk version "1.8.0_362"
OpenJDK Runtime Environment (build 1.8.0_362-bre_2023_01_22_03_30-b00)
OpenJDK 64-Bit Server VM (build 25.362-b00, mixed mode)
➜  java git:(main) ✗ clear           
➜  java git:(main) ✗ java -version
openjdk version "1.8.0_362"
OpenJDK Runtime Environment (build 1.8.0_362-bre_2023_01_22_03_30-b00)
OpenJDK 64-Bit Server VM (build 25.362-b00, mixed mode)
➜  java git:(main) ✗ javac Test.java
➜  java git:(main) ✗ javap -c  Test 
Compiled from "Test.java"
public class Test {
  public Test();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class java/lang/StringBuilder
       3: dup
       4: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
       7: aload_0
       8: iconst_0
       9: aaload
      10: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      13: ldc           #5                  // String  and
      15: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      18: aload_0
      19: iconst_1
      20: aaload
      21: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: invokevirtual #6                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      27: astore_1
      28: return
}

In Java 9,

➜  java git:(main) ✗ java -version 
openjdk version "9"
OpenJDK Runtime Environment (build 9+181)
OpenJDK 64-Bit Server VM (build 9+181, mixed mode)
➜  java git:(main) ✗ javac Test.java
➜  java git:(main) ✗ javap -c  Test 
Compiled from "Test.java"
public class Test {
  public Test();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: aload_0
       1: iconst_0
       2: aaload
       3: aload_0
       4: iconst_1
       5: aaload
       6: invokedynamic #2,  0              // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
      11: astore_1
      12: return
}

Notice the multiple StringBuilder invocations in case of Java 8, which is replaced with makeConcatWithConstants in Java 9.

Java 10

Local variable type interface, use var to declare.

    var i = 1;
    var str = "Hello";
    var student = getStudent();

Static factory methods to create immutable copy of Collection,

    List<String> immutable = List.copyOf(otherList);

orElseThrow() in Optional

    Optional<Object> optional = Optional.ofNullable(null);
    optional.orElseThrow(() -> new RuntimeException("Something went wrong!"));

Java 11 (LTS)

Execute Java file directly without compiling with javac.

java command internally takes care of the compilation.

Some helper methods for String

String str = "Hello";
boolean isBlank = str.isBlank();
str.lines().forEach(System.out::println);
str = str.strip();
// str.stripLeading();
//  str.stripTrailing();

Removed deprecated packages,

java.xml.ws
java.xml.bind
java.activation
java.xml.ws.annotation
java.corba
java.transaction
java.se.ee
jdk.xml.ws
jdk.xml.bind

Make file read and write convenient,

Path path = Files.writeString(Files.createTempFile("temporary", ".txt"), "Something to write!");

String fileContent = Files.readString(path);
System.out.println(fileContent);

Java 12

Switch can be an expression, changes are in preview.

Before Java 12

Animals animal = Animals.COW;

switch (animal) {
    case COW:
    case GOAT:
        System.out.println("Herbivore");
        break;
    case TIGER:
    case LION:
        System.out.println("Carnivore");
        break;
}

Can now be reduced to,

String animalType = switch (animal) {
    case COW, GOAT -> "Herbivore";
    case TIGER,  LION -> "Carnivore";
};

No need to typecast for instanceof,

if(object instanceof String) {
    System.out.println(((String)object).toUpperCase());
}

but now you can do,

if(object instanceof String str) {
    System.out.println(str.toUpperCase());
}

Compare files,

try {
    Path filePath1 = Files.createTempFile("abc1", ".txt");
    Path filePath2 = Files.createTempFile("abc2", ".txt");

    Files.writeString(filePath1, "Hello!");
    Files.writeString(filePath2, "Hello! (Diff)");

    long mismatchIndex = Files.mismatch(filePath1, filePath2);
    if(mismatchIndex == -1) {
        System.out.println("Both files are same!");
    } else {
        System.out.println("Mismatch found at " + mismatchIndex);
    }
} catch (IOException e) {
    throw new RuntimeException(e);
}

Output

Mismatch found at 6

String Identation,

String str = "Hello";
for(int i = 0 ; i < 5; i++) {
    System.out.print(str.indent(i));
}

Output

Hello
 Hello
  Hello
   Hello
    Hello

Convenient method to transform String,

String numbers = "1:ONE,2:TWO,3:THREE";
Map<Integer, String> map = numbers.transform(input -> {
    Map<Integer, String> output =
        Arrays.stream(input.split(","))
            .collect(Collectors.toMap(i -> Integer.parseInt(i.split(":")[0]),
                i -> i.split(":")[1]));
    return output;
});
System.out.println(map);

Java 13

Text block support in String,

String textBlock = """
    I can write anything,
    without adding \\n in the String.
    """;
System.out.println(textBlock);

New Methods in String for format,

String anything = "Hello %d and %s".formatted(1, "ONE");

Intoduced yield in switch case, this will replace the break for cases where we want to return the number. Difference between yeild and return is that yeild will return the value to switch invocation while return will return the value to the caller of the method.

int answer = switch (number) {
    case 1:
    case 3:
    case 5:
    case 7:
        yield number;
    default:
        yield -1;
};

Java 14

Preview of records, a data class.

record Person(String name, int age){}

Can be used,

Person person = new Person("Human", 999);
System.out.printf("Person %s, age %d\n", person.name(), person.age());

Things to note about record

• Can not extend, can not be extended by class
• Can not be abstract
• Allows static fields and methods
• Instance fields can be declared during initialization.
• Declared fields are private and final

record Person(String name, int age){
    // int anything = 0; // Not allowed
    static int anything = 0;

    public String personDetails() {
        return String.format("Person %s, age %d\n", name(), age());
    }
    Person {
        if(name == "Human") {
            throw new RuntimeException("Invalid name");
        }
    }
}

Records can implement interfaces,

interface Human {
    public String personDetails();
}

record Person(String name, int age) implements Human{
    public String personDetails() {
        return String.format("Person %s, age %d\n", name(), age());
    }
}

It can support multiple constructors as well,

record Person(String name, int age){
    public Person() {
        this("Human", 9999);
    }
    public Person (int age) {
        this("Human", age);
    }
}

Allow trailing space in text block,

String textBlock = """
    I can write anything,
    without adding \\n in the String.\s\s\s
    """;
System.out.println(textBlock);

Java 15

Preview of sealed classes or interfacse,

to allow only specific types which can extend or implement respectively.

public abstract sealed class Animal permits Herbivore, Carnivore {
}

final class Herbivore extends Animal{}
sealed class Carnivore extends Animal{}

//class Unknown extends Animal{} // Not allowed to extend

Subclass of a sealed class must have either of the following modifiers,

• sealed : Will allow to be extended further by permitted classes.
• non-sealed : Will allow to be extended further by any classes.
• final : Will not allow to be extended further.

public abstract sealed class Animal permits Herbivore, Carnivore, Omnivore {
}

final class Herbivore extends Animal {}
sealed class Carnivore extends Animal permits  Tiger{}
non-sealed class Omnivore extends Animal {}

final class Tiger extends Carnivore{}

Records can implement the sealed interfaces,

sealed interface Food permits Creature {
    void doSomething();
} 
record Creature(String name) implements Food {
    @Override
    public void doSomething() {
        System.out.println("Anything");
    }
}

Java 16

Pattern matching in instanceof no longer makes variable implicitly final

if(object instanceof String) {
    object = String.format("Result %s", object); // Would give compile time error prior to Java 16.
    System.out.println(object.toUpperCase());
}

New Vector API, incubator.

int[] odd = {1, 3, 5, 7};
int[] even = {2, 4, 6, 8};
var vector1 = IntVector.fromArray(IntVector.SPECIES_128, odd, 0);
var vector2 = IntVector.fromArray(IntVector.SPECIES_128, even, 0);
var vector3 = vector1.add(vector2);
System.out.println(vector3);

Output

[3, 7, 11, 15]

Note that, to run the program you will need to add the module otherwise it won’t be visible.

 java --add-modules jdk.incubator.vector JavaMainClass

Java 17 (LTS)

null in switch,

switch (number) {
    case 1, 2, 3 -> System.out.println("Valid");
    case null -> System.out.println("Not available");
    default -> System.out.println("Invalid");
}

Pattern matching in switch,

String value = switch (obj) {
    case Integer i -> "Integer";
    case Long l    -> "Long";
    case Double d  -> "Double";
    case String s  -> "String";
    case null -> "NULL";
    default -> obj.toString();
};

Java 18

Introduce @snippet in JavaDoc to write code in comments,

/**
* {@snippet:
*  int a = 10;
* }
*/
public void testMethod() {

}

Finalization is deprecated.

The use of finalize() method is discouraged and the support will be removed in future.

Java 19

Preview Virtual Threads

lightweight threads which effectively shares the platform thread for optimal hardware utilisation.

ExecutorService executorService = Executors.newVirtualThreadPerTaskExecutor();

Structured Concurrency (Incubator)

Sounds interesting, it enables to consider multiple threads as a unit. This will going to give better control over multithreaded programs.

Basics of Spring Transactional

Why Spring Transaction?

• Provides consistent model across JTA (Java Transaction API), JPA (Java Persistence API), JDBC (Java Database Connectivity) and JDO (Java Data Objects)
• Declarative Transaction Management
• Simple API for programmatic transaction management - overcomes complexity of JTA.
• Makes the implementation easy to stub and test.

Transaction with JDBC

import java.sql.Connection;

//...
public void deleteSomething() {
    Connection connection = DriverManager.getConnection(host, user, password); 

    try (connection) {
        connection.setAutoCommit(false);
        Statement deleteStatement = connection.createStatement(); 
        String deleteQuery = "DELETE FROM SOMETHING WHERE SOMETHING_ID = 1";
        deleteStatement.executeQuery(deleteQuery); 
        connection.commit();
    } catch (SQLException e) {
        connection.rollback();
    }
}

//...

With raw JDBC we need to handle commit, rollback, savepoint all on our own. Spring takes away majority of boilerplate code so that we can focus on core business functionalities. 

Transaction with Spring

Above function can be implemented in following way with Spring (assuming the transaction management configuration is in place),

@Transactional
public void deleteSomething() {
    somethingRepository.deleteBySomethingId(1);
}

Spring Transaction Manager

Spring transaction manager mainly rely on definition and status. In definition the nature and behaviour of transaction is defined while the status helps to keep track of transaction journey statuses.

Different Supported transaction managers includes,

•  DataSourceTransactionManager
•  JtaTransactionManager
•  HibernateTransactionManager
•  JdbcTransactionManager
•  etc.

This Transactional annotation handles everything behind the scene for use. This annotation has two important configurations - Isolation and Propagation, which we will understand in detail.

Propagation

Transcation propagation defines the nature of transaction handling when there are multiple transactional annottated methods are invoked within the same spring transcation context. 

You can suppy the propagation in Transactional annotation,

@Transactional(propagation = Propagation.REQUIRES_NEW)

Following are supported propagation ways,

• REQUIRED  → Create new transcation or use existing.
• SUPPORTS  → Work with or without transaction.
• MANDATORY  → Do not create transaction but fail if transaction do not exist.
• REQUIRES_NEW  → Create a new transaction.
• NOT_SUPPORTED  → Execute without transaction, suspend if transaction exists.
• NEVER  → Execute without transaction, fail if transaction exists.
• NESTED  → One physical transaction with multiple savepoints to manage subtransactions.

Isolation

Isolation level configuration determines the visibility of data between transactions in case of concurrent operations on same database resource. It helps to ensure consistency and integrity of the data. Transaction isolation level is subject to underlying database you are using. We will refer to them in generic sense to get glimps of it.

You can suppy the isolation in Transactional annotation,

@Transactional(isolation = Isolation.DEFAULT)

The isolation levels tackles different concurrent transcation phenomena listed below,

• Dirty Read : Transaction read uncommitted data of other transaction.
• Non-repeatable read : Re-read the same data which is now modified by other transaction.
• Phantom Read : Re-execute the query which returns result set which changed by other transcation.
• Serialization Anomaly : Inconsistent state of data - group of transactions are committed sequentially with all possible ordering.

Following are supported isolations levels in Spring,

• DEFAULT → User default isolation level of underlying database.
• READ_UNCOMMITTED → Allows to read uncommitted data of other transaction. It can not prevent problems may arrise due to concurrency.
• READ_COMMITTED →  Allows to read committed data of other transaction. Prevents dirty read.
• REPEATABLE_READ → Prevents dirty read and non-repeatable read (and even phantom read) by returning unchanged data in repeatable reads within same transaction.
• SERIALIZABLE → Executes transactions sequentially. Solves all consurrency problems but performance suffers.

This can be a starting point for spring transaction learnings, if you have understood the propagation and isolation level well, it becomes easier to configure and debug accordingly. 

Virtual Threads : Java 21

Java 21 came with an exciting feature of Virtual Thread - Lightweight Threads. The idea is to achieve optimal hardware utilisation, which has been a bottleneck for the conventional java.lang.Thread - platform threads.

Platform threads are 1:1 mapped with OS Thread, which makes them powerful to accomplish all possible tasks. java.lang.Thread is a thin layer on actual OS threads, which are quite heavy. The number of thread application can support depends on the hardware capacity of the system.

Let’s say thread memory consumption is 1 MB, to support 1 million threads in concurrent application at the time of heavy load, you will need 1 TB of memory. One thread per request style of implementation also suffer due to this limitation - asynchronous programming can solve this upto some extent but it has its own drawbacks.

Virtual threads effectively shares the platform thread. Rather than holding the platform thread for entire lifetime, it runs short lived tasks and for the needed executions - not while it waits for I/O. This allows insane number of concurrent operations without need of additional threads or hardware capacity. This brings up the new way to deal with concurrency in Java applications.

Virtual threads can be created with java.lang.Thread builder approach,

Runnable function = () -> System.out.println("Something to execute in Virtual Thread!");
Thread virtual = Thread.startVirtualThread(function);

In addition to that, java.util.concurrent.ExecutorService also has factory method for Virtual threads,

ExecutorService virtualExecutor = Executors.newVirtualThreadPerTaskExecutor();

Virtual threads are daemon threads, join() to wait on the main thread. Thread local variables of virtual thread are not accessible to carrier thread or vice versa.

Deep vs Shallow Copy

In programming language like Java, everything revolves around objects. Lifecycle of object from creation to garbage collection is something that keeps happening continuously in any realtime application.

There are different ways you can use to create and Object of a Class. However, the object creation with new keyword is the straightforward one.

Car mercedes = new Mercedes();

While we keep creating objects on-demand, in some realtime scenarios we may need to create new object which is a copy of existing object. In that case new Object should hold same state as current Object is holding.

You may ask, what kind of scenarios require copying existing object?

Well, it completely depends on the software you are developing but in any application where you see copy option, whether it can be copying table row, copying form etc. such cases are good candidates to use object copying mechanism.

There are two approaches you can use to copy object,

• Shallow copy

• Deep copy

There are different methods to implement copying approach,

• Using clone method

• Copy constructor

• Static factory method

There are certain third party libraries available which provides methods to copy the objects.

For example, BeanUtils#copyProperties(Object destination, Object source)

Which option to choose majorly depends on your requirement.

Shallow copy

All the fields of existing object is copied to new object.

Consider following diagram, while copying Car object, company object reference is reused in the copy object, which simply means shallow copy only copies values (variable and object references).

Basically, it doesn’t create copy of objects referenced inside the object we want to copy, thus it is called shallow copy.

Following is a shallow copy example using copy constructor method,

class Car {

    private String model;

    private Company company;

    public Car(String model, Company company) {
        this.model = model;
        this.company = company;
    }

    public Car(Car carToCopyFrom) {
        this(carToCopyFrom.model, carToCopyFrom.company);
    }

}

Deep copy

All the fields along with the objects referenced by existing object are copied to new object.

Consider following diagram, company object is also copied and reference to this new object is used in car object. Note that all the referenced objects at any level (direct or indirect) are copied and referred in copy object.

Following is a deep copy example using copy constructor method,

class Car {

    private String model;

    private Company company;

    public Car(String model, Company company) {
        this.model = model;
        this.company = company;
    }

    public Car(Car carToCopyFrom) {
        this(carToCopyFrom.model, new Company(carToCopyFrom.company.getName()));
    }
}