Grouping and adding up BigDecimals using Java Stream API

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• How to create the table without @Id field in spring using jpa?

  I am using spring boot and jpa to create the table but I need to create table without @Id primary key column.It is not giving me to make the table without this @Id field. How to get this using spring data jpa?

  LetterDoc.java

  package com.ashwin.springsecurityangular.model;
  
  import java.io.Serializable;
  
  import javax.persistence.CascadeType;
  import javax.persistence.Entity;
  import javax.persistence.FetchType;
  import javax.persistence.GeneratedValue;
  import javax.persistence.GenerationType;
  import javax.persistence.Id;
  import javax.persistence.JoinColumn;
  import javax.persistence.ManyToOne;
  import javax.persistence.Table;
  
  import com.fasterxml.jackson.annotation.JsonIgnore;
  
  @Entity
  @Table(name = "letter_doc")
  public class LetterDoc implements Serializable {
  
      private static final long serialVersionUID = 1L;
  
      @ManyToOne(fetch = FetchType.EAGER, optional = false,cascade=CascadeType.PERSIST)
      @JoinColumn(name = "letterNo")
      @JsonIgnore
      private Letter letter;
  
      @ManyToOne(fetch = FetchType.EAGER, optional = false,cascade=CascadeType.PERSIST)
      @JoinColumn(name = "documentId")
      @JsonIgnore
      private Document document;
  
      private String docFile;
  
  //i omitted getters and setters and both constructor
  }
  

  It ask me the @Id field need.So there is error like:

  org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaConfiguration.class]: Invocation of init method failed; nested exception is org.hibernate.AnnotationException: No identifier specified for entity: com.ashwin.springsecurityangular.model.LetterDoc
  

  In this table i am avoided to make @Id field but it is not giving me.How to achieve this?
• Docker Compose + Spring Boot + Postgres connection, resulting in error

  I have a Java Spring Boot app which works with a Postgres database. I want to use Docker for both of them. Initially, I created a docker-compose.yml file as given below:

  version: '3'
  services:
    db:
      container_name: sample_db
      image: postgres
      volumes:
        - sample_db:/var/lib/postgresql/data
      ports:
        - "5432:5432"
      environment:
        - POSTGRES_DB="employee"
        - POSTGRES_USER="user"
        - POSTGRES_PASSWORD="password"
        - PGDATA=/var/lib/postgresql/data/pgdata
  
    web:
      build: .
      expose:
        - "8080"
      depends_on:
        - db
  volumes:
    sample_db: {}
  

  Then, in my Spring Boot app, inside the application.properties file I defined the following properties.

  spring.datasource.url=jdbc:postgresql://db:5432/employee
  spring.datasource.username=user
  spring.datasource.password=password
  

  along with this,I created a Dockerfile in my project directory, which looks like this:

  FROM openjdk:8-jdk-alpine
  EXPOSE 8080    
  ARG JAR_FILE=target/employee-0.0.1-SNAPSHOT.jar    
  COPY ${JAR_FILE} employee-demo.jar    
  ENTRYPOINT ["java","-jar","employee-demo.jar"]
  

  I issued these commands and ended up in the error as given below

  mvn clean package

  docker-compose up

  The problem is the connection string. I believe I have to change it to something else, but I don't know what it should be. I get the following error messages:

   Caused by: org.postgresql.util.PSQLException: The connection attempt failed.
  web_1  |    at org.postgresql.core.v3.ConnectionFactoryImpl.openConnectionImpl(ConnectionFactoryImpl.java:292) ~[postgresql-42.2.5.jar!/:42.2.5]
  web_1  |    at org.postgresql.core.ConnectionFactory.openConnection(ConnectionFactory.java:49) ~[postgresql-42.2.5.jar!/:42.2.5]
  web_1  |    at org.postgresql.jdbc.PgConnection.<init>(PgConnection.java:195) ~[postgresql-42.2.5.jar!/:42.2.5]
  web_1  |    at org.postgresql.Driver.makeConnection(Driver.java:454) ~[postgresql-42.2.5.jar!/:42.2.5]
  web_1  |    at org.postgresql.Driver.connect(Driver.java:256) ~[postgresql-42.2.5.jar!/:42.2.5]
  web_1  |    at com.zaxxer.hikari.util.DriverDataSource.getConnection(DriverDataSource.java:136) ~[HikariCP-3.2.0.jar!/:na]
  web_1  |    at com.zaxxer.hikari.pool.PoolBase.newConnection(PoolBase.java:369) ~[HikariCP-3.2.0.jar!/:na]
  web_1  |    at com.zaxxer.hikari.pool.PoolBase.newPoolEntry(PoolBase.java:198) ~[HikariCP-3.2.0.jar!/:na]
  web_1  |    at com.zaxxer.hikari.pool.HikariPool.createPoolEntry(HikariPool.java:467) ~[HikariCP-3.2.0.jar!/:na]
  web_1  |    at com.zaxxer.hikari.pool.HikariPool.checkFailFast(HikariPool.java:541) ~[HikariCP-3.2.0.jar!/:na]
  web_1  |    at com.zaxxer.hikari.pool.HikariPool.<init>(HikariPool.java:115) ~[HikariCP-3.2.0.jar!/:na]
  web_1  |    at com.zaxxer.hikari.HikariDataSource.getConnection(HikariDataSource.java:112) ~[HikariCP-3.2.0.jar!/:na]
  web_1  |    at org.hibernate.engine.jdbc.connections.internal.DatasourceConnectionProviderImpl.getConnection(DatasourceConnectionProviderImpl.java:122) ~[hibernate-core-5.3.7.Final.jar!/:5.3.7.Final]
  web_1  |    at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess.obtainConnection(JdbcEnvironmentInitiator.java:180) ~[hibernate-core-5.3.7.Final.jar!/:5.3.7.Final]
  web_1  |    at org.hibernate.resource.transaction.backend.jdbc.internal.DdlTransactionIsolatorNonJtaImpl.getIsolatedConnection(DdlTransactionIsolatorNonJtaImpl.java:43) ~[hibernate-core-5.3.7.Final.jar!/:5.3.7.Final]
  web_1  |    ... 44 common frames omitted
  web_1  | Caused by: java.net.UnknownHostException: sample_db
  web_1  |    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:184) ~[na:1.8.0_181]
  web_1  |    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392) ~[na:1.8.0_181]
  web_1  |    at java.net.Socket.connect(Socket.java:589) ~[na:1.8.0_181]
  web_1  |    at org.postgresql.core.PGStream.<init>(PGStream.java:70) ~[postgresql-42.2.5.jar!/:42.2.5]
  web_1  |    at org.postgresql.core.v3.ConnectionFactoryImpl.tryConnect(ConnectionFactoryImpl.java:91) ~[postgresql-42.2.5.jar!/:42.2.5]
  web_1  |    at org.postgresql.core.v3.ConnectionFactoryImpl.openConnectionImpl(ConnectionFactoryImpl.java:192) ~[postgresql-42.2.5.jar!/:42.2.5]
  web_1  |    ... 58 common frames omitted
  

  Am new to docker, If any issues please inform
• I'm having with a String based Binary tree and it's insertion

  The Binary tree does not use comparison rather, the user inputs the String name of the node they want to add the left or right child too, if the node already has a child for which ever of the two and it will say so and keep child.

  I'm having some difficulties, it doesn't stop it from overwriting a pre existing Node and it does not always remembers it own child.

  Please tell me I missed something simple or if I need to redo everything and if so, how should I do it this time.

  Thanks for the help in advanced.

      public class Node {
  
  private String name;
  private Node leftChild;
  private Node rightChild;
  private Node parent;
  
  public String getName() {
      return name;
  }
  
  public void setName(String name) {
      this.name = name;
  }
  
  public Node getLeftChild() {
      return leftChild;
  }
  
  public void setLeftChild(Node leftChild) {
      this.leftChild = leftChild;
  }
  
  public Node getRightChild() {
      return rightChild;
  }
  
  public void setRightChild(Node rightChild) {
      this.rightChild = rightChild;
  }
  
  public Node getParent() {
      return parent;
  }
  
  public void setParent(Node parent) {
      this.parent = parent;
  }
  
  public void displayNode() // display ourselves
  {
      System.out.println(name);
  }
  

  }

      public class Tree {
  
  private Node root;
  
  public Tree() {
      root = null;
  }
  
  public void insertRoot(String rootName) {
      if (root == null) {
          Node newNode = new Node();
          newNode.setName(rootName);
          root = newNode;
      } else {
          System.out.println("There is already a root");
      }
  }
  
  public void insertLeftChild(String parentName, String childName) {
      Node temp = new Node();
  
      Node current = parent(parentName, root, temp);
  
      if (current.getName() == null) {
          System.out.println("No such parent exists");
      } else if (current.getLeftChild() == null) {
  
          Node newNode = new Node();
          newNode.setName(childName);
  
          current.displayNode();
  
          newNode.setParent(current);
  
          current.setLeftChild(newNode);
          System.out.println("It worked");
      } else if (current.getLeftChild() != null) {
          System.out.println("They already have a left child");
      }
  }
  
  public void insertRightChild(String parentName, String childName) {
      Node temp = new Node();
  
      Node current = parent(parentName, root, temp);
  
      if (current.getName() == null) {
          System.out.println("No such parent exists");
      } else if (current.getRightChild() == null) {
          Node newNode = new Node();
          newNode.setName(childName);
  
          newNode.setParent(current);
          current.setRightChild(newNode);
      } else if (current.getRightChild() != null) {
          System.out.println("They already have a right child");
      }
  
  }
  
  public Node parent(String parentName, Node current, Node found) {
  
      if (current != null) {
          if (current.getName().equals(parentName)) {
              found.setName(parentName);
              found.setParent(current.getParent());
              found.setLeftChild(current.getLeftChild());
              found.setRightChild(current.getRightChild());
  
              return found;
          }
  
  
          parent(parentName, current.getLeftChild(), found);
  
          parent(parentName, current.getRightChild(), found); // right child
  
      }
  
      return found;
  }
  

  }
• Java 8 How to kick off a thread just before returning of a method

  I have a service written in Java Spring. This service is to return a computational result to the user. However, I also want to do some additional task (a side task) with that result and it should not affect the performance. So my question is how can I kick off another Thread to do the additional task without preventing returning the result back to the user.
• Inserting into database a datetime with the current UTC time

  I'm attempting to insert a datetime into a mysql database for the current UTC time via the GETUTCDATE() function in sql. It's failing with "FUNCTION GETUTCDATE DOES NOT EXIST".

  Is a way for me to get the current UTC time in sql datetime format from Java, and simply insert it as a string?

  Another big issue I'm having is I need to convert the above utc datetime object into local time zones and I don't really know how to do that through standard java api's.
• Android Studio Can't Change Language Level

  I'm working on a project in android studio, using libgdx, and I'm getting this error: However, when I compile it, it works just fine. I have changed all the gradle files I can think of, and they all are using java 8. When I press alt-enter, it promts to change language level to 8, but this does nothing when I select it. I have been searching around the internet for days, and would really like the android studio debugger to actually show the errors that occur at compile time, as this is not one of them.

  Thanks, Neil
• Java Stream Grouping multiple times and merging the key collisions

  So I'm struggling a bit with this lambda. I'm making a generic implementation so I need to split properly my code. I cannot use external hashmap to fill with the groups.

  The principle is I need to send notifications to the managers of each group of people while keeping tab of the list of people concerned by the notification

  Person has a Group which has multiple Managers. Each Group has multiple Person. Notifications can have different level thus different email template will be sent. My simplified query result would be :

  //Reminder 1
  Person1 | Group1 | Manager1 | Warning
  Person1 | Group1 | Manager2 | Warning
  Person1 | Group1 | Manager3 | Warning
  
  //Reminder 2
  Person2 | Group1 | Manager1 | Info
  Person2 | Group1 | Manager2 | Info
  Person2 | Group1 | Manager3 | Info
  
  //Reminder 3
  Person3 | Group2 | Manager4 | Info
  Person3 | Group2 | Manager5 | Info
  

  So how do I start from List to get a clean Stream<Reminder>

  class Reminder{
    Level level;
    Group group;
    Set<Person> notifiedPersons;
  }
  
  class Group{
    long id;
    Set<Manager> managers;
  }
  
  class Person{
    long id;
    String name;
  }
  

  I've tried to use groupingBy but the code get overly complicated. My final grouping key would be the Group ID + level of reminder.

  A YAML view of the final result would be:

  reminders:
    - group:
        id: 1
        managers:
          - Manager1
          - Manager2
          - Manager3
      notifiedPersons:
        - id: 1
          name: Person1
      level: Warning
    - group:
        id: 1
        managers:
          - Manager1
          - Manager2
          - Manager3
      notifiedPersons:
        - id: 2
          name: Person2
      level: Info
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        id: 2
        managers:
          - Manager4
          - Manager5
      notifiedPersons:
        - id: 3
          name: Person3
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• how to filter elements that throw exception during stream map

  I was following the article https://www.oreilly.com/ideas/handling-checked-exceptions-in-java-streams for extracting a method into a method, in order to handle exceptions, and noticed the examples given only look clean since they don't actually compile because they miss a return statement.
  Essentially I'm looking to have a parse method similar to this "divide" in the article example 3, but mine parses the list and in some scenarios it will throw errors log them and continue, like this:

  public List<String> validator(List<String> values) {
      return values.stream()
          .map(this::parse)
          .filter(Objects::nonNull)
          .collect(Collectors.toList());
  }
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      try{
           // returns something or throws exceptions
      } catch (Exception e ){
         log(e)
      }
      return null;
  }
  

  As you can see from this code I cheated the compilation error by returning a null at the end of the parse method that I then filter nulls on my validator before collecting, and it looks appalling. Is there a better way to skip the values in a clean easy way?
• Comparator.nullsLast does not avoid NullPointerException

  I want to sort a list of objects by one of nullable fields.

  In order to avoid NullPointerexception I use Comparator.nullsLast. But the exception still occurs:

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      public static void main(String[] args) {
          List<Bean> l = new ArrayList<>();
          for (int i=0;i<5;i++) {
              Bean b = new Bean("name_"+i,i);
              l.add(b);
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          l.get(2).setVal(null);
          System.out.println(l);
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          System.out.println(l);
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      static class Bean{
          String name;
          Integer val;
          // omit getters & setters & constructor
      }
  
  }
  

  How can I sort this kind of list?