This might sound stupid to you,
but why do I need to define an empty constructor in my #Entitys?
Every tutorial I saw said : every entity needs an empty constructor.
But Java always give you a default invisible empty constructor (if you don't redefine one).
Let me clarify..
What I understood by "need" was write.
Meaning: always write an empty constructor in your entity.
example:
#Entity
public class MyEntity implements Serializable {
#Id
private String str;
public MyEntity(){}
//here getter and setter
}
But Java always gives you this empty constructor when you don't redefine it (write an other one with parameters).
In this case writing this empty constructor seems useless.
An empty constructor is needed to create a new instance via reflection by your persistence framework. If you don't provide any additional constructors with arguments for the class, you don't need to provide an empty constructor because you get one per default.
You can also use the #PersistenceConstructor annotation which looks like following
#PersistenceConstructor
public Movie(Long id) {
this.id = id;
}
to initialise your entity if Spring Data is present in your project. Thus you can avoid the empty constructor as well.
But java always give you a default invisible empty constructor (if you
don't redefine one).
This statement is true only when you don't provide any constructor in your class. If an argument constructor is provided in your class, then JVM will not add the no-argument constructor.
Explicitly defining a default constructor is not necessary unless you provide another constructor for the entity. If you provide another constructor, aside from one with the default constructor's signature, the default constructor will not be created.
Since JPA implementations rely upon the existence of a default constructor it is then necessary to include the default constructor that will be omitted.
As you specified the "JPA" tag, I assume your question applies to JPA only and not empty constructors in general.
Persitence frameworks often use reflection and more specifically Class<T>.newInstance() to instantiate your objects, then call getters/setters by introspection to set the fields.
That is why you need an empty constructor and getters/setters.
See this StackOverflow question about empty constructors in Hibernate.
Actually you don't need to write it. You have it by default. Sometimes you can create private constructor to prevent users to use default
public class MyClass{
private MyClass(){}
}
For singelton patterns, for example you can block using default constructor.
Sometimes, when you use Gson plugin to convert String Json data to Object, it demands to write default constructor, otherwise it doesn't work
All the answers are fine.
But let's talk with code. Following snippets of code will give you more clarity.
PersonWithImplicitConstructor.java
public class PersonWithImplicitConstructor {
private int id;
private String name;
}
First we have to compile the .java file
javac PersonWithImplicitConstructor.java
Then class file will be generated.
Running the javap on top this class file will give you the following information.
javap PersonWithImplicitConstructor.class
Compiled from "PersonWithImplicitConstructor.java"
public class PersonWithImplicitConstructor {
public PersonWithImplicitConstructor();
}
NOTE: If you want more information, you can use -p flag on javap.
The next java file will have parameterised constructor only.
PersonWithExplicitConstructor.java
public class PersonWithExplicitConstructor {
private int id;
private String name;
public PersonWithExplicitConstructor(int id, String name) {
this.id = id;
this.name = name;
}
}
javac PersonWithExplicitConstructor.java
javap PersonWithExplicitConstructor.class
Compiled from "PersonWithExplicitConstructor.java"
public class PersonWithExplicitConstructor {
public PersonWithExplicitConstructor(int, java.lang.String);
}
PersonWithBothConstructors.java
public class PersonWithBothConstructors {
private int id;
private String name;
public PersonWithBothConstructors() {
}
public PersonWithBothConstructors(int id, String name) {
this.id = id;
this.name = name;
}
}
javac PersonWithBothConstructors.java
javap PersonWithBothConstructors.class
Compiled from "PersonWithBothConstructors.java"
public class PersonWithBothConstructors {
public PersonWithBothConstructors();
public PersonWithBothConstructors(int, java.lang.String);
}
Java not always give you a default invisible empty constructor if your class got argument constructor, you have to define the empty constructor by your own.
From the JPA tag, I suppose that you are working with Java beans. Every bean needs to have the following properties:
Getters and setters for all its main instance variables.
An empty constructor.
All its instance variables should preferably be private.
Thus the statement : "every entity needs an empty constructor".
Related
I was following the udemy android app development course, In the course, we were writing code to Create A listview and get some data when the user clicks on the list, to do that the teacher uses a thing called Constructor I know how constructors work theoretically but can't understand the way it works in code. It will be great if someone can explain what these lines of codes do.
edit: Full Code is here https://github.com/atilsamancioglu/A14-LandmarkBook
import android.graphics.Bitmap;
public class Globals {
private static Globals instance;
private Bitmap chosenImage;
private Globals(){
}
public void setData(Bitmap chosenImage){
this.chosenImage=chosenImage;
}
public Bitmap getData(){
return this.chosenImage;
}
public static Globals getInstance() {
if(instance==null){
instance = new Globals();
}
return instance;
}
}
Constructors are special methods invoked when an object is created and are used to initialize them.
A constructor can be used to provide initial values for object attributes.
You can think of constructors as methods that will set up your class by default, so you don’t need to repeat the same code every time.
In your codes, you can define the constructor as below(it may be unrelated, it's just an example):
private Globals(int id){
return chosenImage.setId(id);
}
The constructor is called when you create an object using the new keyword:
Globals objectGlobe = new Globals(000008);
Also a single class can have multiple constructors with different numbers of parameters.
The setter methods inside the constructors can be used to set the attribute values.
It's not bad to be mentioned that; Java automatically provides a default constructor, so all classes have a constructor, whether one is specifically defined or not.
I recently came across a part of code which has annotation #PortableProperty used for variables declared in a DTO. Can anyone please explain what it does and the need for using it?
Portable marks a class as being eligible for use by a PofAnnotationSerializer. This annotation is only permitted at the class level and is a marker annotation with no members. The following class illustrates how to use Portable and PortableProperty annotations.
A PortableProperty marks a member variable or method accessor as a POF serialized attribute. Whilst the value() and codec() can be explicitly specified they can be determined by classes that use this annotation. Hence these attributes serve as hints to the underlying parser.
Quoted this from doc found here and here. Check this for more details.
Example:
public class Book
extends ExternalIdEntity
implements Serializable, PortableObject {
#PortableProperty(1)
private String id;
#PortableProperty(3)
private String parentId;
#PortableProperty(4)
private String name;
#PortableProperty(5)
private String legalEntityId;
...
}
I'm using IReport (JasperStudio plugin for Eclipse) and I'm trying to create a report with a JavaBean as source.
Suppose I have these two classes:
public class MyClass {
private String myClassAttribute;
// getter and setter for myClassAttribute
}
public class AnotherMyClass {
private String anotherMyClassAttribute;
private MyClass myClass;
// getter and setter for anotherMyClassAttribute
// getter and setter for myClass
}
If I choose AnotherMyClass as JavaBeanSource I can set only fields from that class (anotherMyClassAttribute), I didn't find a way to set a text to getMyClass().getmyClassAttribute().
Do JavaBeans stop at level one or is there a way to use attribute from other classes between references?
Thanks.
In report define field $F{myClass} with type MyClass
In text field use expression $F{myClass}.getMyClassAttribute()
No, it doesn't stop at level one, you may go as deep as you want. You may use the attribute like myClass.myClassAttribute. And for setting a value to it, myClass.myClassAttribute = "some value"
Is there some way of using magic methods in Java like there is in PHP with __call?
For instance:
class foo {
#Setter #Getter
int id;
#Getter
Map <String, ClassInFoo> myMap;
protected class ClassInFoo {
#Setter #Getter
String name;
}
#Setter
String defaultKey;
}
I'm using Project Lombok annotations for getter and setter methods to simplify the code.
Let's consider that that my map contains several items mapped by String and the defaultKey defines the default one.
What I would like is to be able to call foo.getName() which would return the default name as foo.myMap.get(defaultKey).getName().
The reason I can't just write all the getters manually is that the Foo class is in fact inherited with generics and the the inner class might be different.
I sort of need something like:
function Object __call(method) {
if (exist_method(this.method)
return this.method();
else
return this.myMap.get(defaultKey).method();
}
Is this somehow possible in Java?
EDIT:
I made a more precise example of what I am trying to achieve here: https://gist.github.com/1864457
The only reason of doing this is to "shorthand" the methods in the inner class.
You absolutely can through reflection by using its features like
public Method getMethod(String name, Class<?>... parameterTypes)
that can be used to see if a class has some methods defined but I don't see how your problem couldn't be solved with a proper use of interfaces, inheritance and overriding of methods
Features like reflection are provided to manage certain, otherwise unsolvable, issues but Java is not PHP so you should try to avoid using it when possible, since it's not in the philosophy of the language.
Isn't it the whole point of inheritance and overriding?
Base class:
public Object foo() {
return this.myMap.get(defaultKey).method();
}
Subclass:
#Overrides
public Object foo() {
return whateverIWant;
}
My question in short: how do I detect if a java annotation is present (and in the right place) for a given user class/object.
Details of the "problem"
Lets say I have two java classes:
public class Line {
private List<Cell> cells;
public Line(Object... annotatedObjects) {
// check if annotations #Line and #Cell are present in annotatedObjects.
}
// getter/setter for cells.
}
public class Cell {
// some members
// some methods
}
A Line object holds Cells.
I also have two annotations, like:
public #interface Line {
// some stuff here
}
public #interface Cell {
// some stuff here
}
I also have a bunch of user classes (two will do for this example) that contain the #Line and #Cell annotations I specified, like:
#Line(name="pqr", schema="three")
public class AUserClass {
#Cell
private String aString;
}
#Line(name="xyz", schema="four")
public class AnotherUserClass {
#Cell(name="birthday")
private Date aDate;
}
The problem: When I instantiate a new Line object, I want to be able to pass the user classes/objects into the Line constructor. The Line constructor then finds out if the passed user classes/objects are valid classes that can be processed. Only user classes that have a #Line annotation for the class, and at least one #Cell annotation for its members are valid objects that can be passed into the constructor of the Line object.
All other passed objects are invalid. The moment a valid user object is passed, all the available members that are tagged as #Cell in that object are transformed to Cell objects and added to the cells list.
My questions:
is this possible to detect the annotations in this object/class at runtime, and only for THIS passed object (I don't want to scan for annotations on the classpath!)?
is it possible to detect the datatype of the #Cell tagged members? This is needed because the Cell class doesn't accept all datatypes.
is it possible to retrieve the actual member name (specified in the java file) so that the user doesn't have to specify the members Cell name. I want the user to be able to write #Cell (without a name) and #Cell(name="aName"), and when only #Cell is specified, the name of the member is used instead. I have no idea if this information is still available at runtime using reflection.
How to detect if the annotations are in the right place?If code is tagged like this, then the object should be ignored (or maybe an exception is thrown)?
#Cell // oh oh, that's no good :(
public class WrongClass {
// some members
}
Could you provide some startup code, so I know a little to get going with this problem. I am really new to annotations and reflection. BTW: I am using the latest jvm 1.6+
Thank you for your kind help!
First you need to have retention policy on your annotations so you can read them with reflection
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.TYPE)
public static #interface Line {
}
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.FIELD)
public static #interface Cell {
}
Second you need to test if the class has the Line annotation with isAnnotationPresent(annotationClass). This method is accessible from java.lang.Class and a java.lang.reflect.Field.
NOTE: that you need to retrieve the fields that are private with class.getDeclaredField(fieldName).
3. I don't think you can make an annotation have a default value based on a propertyName but you can make name optional by providing a default String name() default DEFAULT and check for that value when iterating through the fields and either use the value stored in name() or the propertyName
Q.1 :is this possible to detect the annotations in this object/class at runtime, and only for THIS passed object (I don't want to scan for annotations on the classpath!)?
Yes it is very well possible using isAnnotationPresent
#Deprecated
public class AnnotationDetection {
public static void main(String[] args) {
AnnotationDetection annotationDetection = new AnnotationDetection();
System.out.println(annotationDetection.getClass().isAnnotationPresent(Deprecated.class));
}
}
Note that annotation which are scoped to retain at Runtime will be available only,