The best way to map @ManyToMany

The best way to map @ManyToMany

   June 29, 2021       Ravi Yasas

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@ManyToMany

Many to many relationships can be easily mapped by creating another table using @JoinTable as follows.

Student entity

@Data

@Table(name = "Student")

@Entity

public class Student implements Serializable {

    @Id

    @GeneratedValue(strategy = GenerationType.AUTO)

    private Integer id;

    private String studentName;

    @ManyToMany

    @JoinTable(

        name = "student_task",

        joinColumns = @JoinColumn(name = "task_id"),

        inverseJoinColumns = @JoinColumn(name = "student_id")

    )

    private Set<Task> tasks = new HashSet<>();

}

Task entity

@Entity

@Data

@Table(name = "tasks")

public class Task implements Serializable {

    @Id

    @GeneratedValue(strategy = GenerationType.AUTO)

    private Integer id;

    private String taskName;

    @ManyToMany(mappedBy = "tasks")

    private Set<Student> students = new HashSet<>();

}

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How to avoid NullPointerException

How to avoid NullPointerException

   June 29, 2021       Ravi Yasas

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java.lang.NullPointerException

When can it be thrown?

According to the JavaDoc, the following scenarios can be found.

• Calling the instance method of a null object.
• Accessing or modifying the field of a null object.
• Taking the length of null as if it were an array.
• Accessing or modifying the slots of null as if it were an array.
• Throwing null as if it were a Throwable value.

How to avoid it?

• Use Optional in Java 8
• Call equals() and equalsIgnoreCase() methods on known objects.
• Use valueOf() over toString()

public class NullCheckDemo {

   public static void main(String args[]){

       Integer i = null;

       System.out.println(i.toString());      //This throws NullPointerException

       System.out.println(String.valueOf(i)); //This returns null

   }

}

• Use null-safe methods and libraries. Ex: Apache Commons StringUtils.
• Use @NotNull and @Nullable annotations
• Always check for nulls.

public class NullDemo {

    public static void main(String args[]){

        Integer i = null;

        if(i != null)

            System.out.println(i);

        else

            System.out.println("error occurred");

    }

}

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Throw vs throws in exception handling

Throw vs throws in exception handling

   June 26, 2021       Ravi Yasas

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Throw vs throws

Throw	Throws
Uses inside a method when it is required to throw an exception	This is used in the method signature in which methods that exceptions can occur.
Only one exception can be thrown	Throws can be used to declare multiple exceptions
Used to handle unchecked exceptions	Used to handle checked exceptions
Usually used to handle custom exceptions	Throws means, it says this method can be thrown these exceptions, when you use this method, you need to handle it.

 

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