[java] What is the difference between field, variable, attribute, and property in Java POJOs?

When referring to internal private variables of Java POJOs that have getters/setters, I've used the following terms:

• field
• variable
• attribute
• property

Is there any difference between the above? If so, what is the correct term to use? Is there a different term to use when this entity is persisted?

The question is old but another important difference between a variable and a field is that a field gets a default value when it's declared.A variable, on the other hand, must be initialized.

The difference between a variable, field, attribute, and property in Java:

A variable is the name given to a memory location. It is the basic unit of storage in a program.

A field is a data member of a class. Unless specified otherwise, a field can be public, static, not static and final.

An attribute is another term for a field. It’s typically a public field that can be accessed directly.

• In NetBeans or Eclipse, when we type object of a class and after that dot(.) they give some suggestion which are called Attributes.

A property is a term used for fields, but it typically has getter and setter combination.

My understanding is as below, and I am not saying that this is 100% correct, I might as well be mistaken..

A variable is something that you declare, which can by default change and have different values, but that can also be explicitly said to be final. In Java that would be:

public class Variables {

    List<Object> listVariable; // declared but not assigned
    final int aFinalVariableExample = 5; // declared, assigned and said to be final!

    Integer foo(List<Object> someOtherObjectListVariable) {
        // declare..
        Object iAmAlsoAVariable;

        // assign a value..
        iAmAlsoAVariable = 5;

        // change its value..
        iAmAlsoAVariable = 8;

        someOtherObjectListVariable.add(iAmAlsoAVariable);

        return new Integer();
    }
}

So basically, a variable is anything that is declared and can hold values. Method foo above returns a variable for example.. It returns a variable of type Integer which holds the memory address of the new Integer(); Everything else you see above are also variables, listVariable, aFinalVariableExample and explained here:

A field is a variable where scope is more clear (or concrete). The variable returning from method foo 's scope is not clear in the example above, so I would not call it a field. On the other hand, iAmAlsoVariable is a "local" field, limited by the scope of the method foo, and listVariable is an "instance" field where the scope of the field (variable) is limited by the objects scope.

A property is a field that can be accessed / mutated. Any field that exposes a getter / setter is a property.

I do not know about attribute and I would also like to repeat that this is my understanding of what variables, fields and properties are.

If your question was prompted by using JAXB and wanting to choose the correct XMLAccessType, I had the same question. The JAXB Javadoc says that a "field" is a non-static, non-transient instance variable. A "property" has a getter/setter pair (so it should be a private variable). A "public member" is public, and therefore is probably a constant. Also in JAXB, an "attribute" refers to part of an XML element, as in <myElement myAttribute="first">hello world</myElement>.

It seems that a Java "property," in general, can be defined as a field with at least a getter or some other public method that allows you to get its value. Some people also say that a property needs to have a setter. For definitions like this, context is everything.

• variable - named storage address. Every variable has a type which defines a memory size, attributes and behaviours. There are for types of Java variables: class variable, instance variable, local variable, method parameter

//pattern
<Java_type> <name> ;

//for example
int myInt;
String myString;
CustomClass myCustomClass;

• field - member variable or data member. It is a variable inside a class(class variable or instance variable)

• attribute - in some articles you can find that attribute it is an object representation of class variable. Object operates by attributes which define a set of characteristics.

CustomClass myCustomClass = new CustomClass();
myCustomClass.myAttribute = "poor fantasy"; //`myAttribute` is an attribute of `myCustomClass` object with a "poor fantasy" value

• property - field + bounded getter/setter. It has a field syntax but uses methods under the hood. Java does not support it in pure form. Take a look at Objective-C, Swift, Kotlin

For example Kotlin sample:

//field - Backing Field
class Person {
    var name: String = "default name"
        get() = field
        set(value) { field = value }
}

//using
val person = Person()
person.name = "Alex"    // setter is used
println(person.name)    // getter is used

[Swift variable, property...]

If you take clue from Hibernate:

Hibernate reads/writes Object's state with its field. Hibernate also maps the Java Bean style properties to DB Schema. Hibernate Access the fields for loading/saving the object. If the mapping is done by property, hibernate uses the getter and setter.

It is the Encapsulation that differentiates means where you have getter/setters for a field and it is called property, withthat and we hide the underlying data structure of that property within setMethod, we can prevent unwanted change inside setters. All what encapsulation stands for...

Fields must be declared and initialized before they are used. Mostly for class internal use.

Properties can be changed by setter and they are exposed by getters. Here field price has getter/setters so it is property.

class Car{
 private double price;
 public double getPrice() {…};
 private void setPrice(double newPrice) {…};
}

<class name="Car" …>
<property name="price" column="PRICE"/>
</class>

Similarly using fields, [In hibernate it is the recommended way to MAP using fields, where private int id; is annotated @Id, but with Property you have more control]

class Car{
  private double price;
}
<class name="Car">
<property name=" price" column="PRICE" access="field"/>
</class>

Java doc says: Field is a data member of a class. A field is non static, non-transient instance variable. Field is generally a private variable on an instance class.

Actually these two terms are often used to represent same thing, but there are some exceptional situations. A field can store the state of an object. Also all fields are variables. So it is clear that there can be variables which are not fields. So looking into the 4 type of variables (class variable, instance variable, local variable and parameter variable) we can see that class variables and instance variables can affect the state of an object. In other words if a class or instance variable changes,the state of object changes. So we can say that class variables and instance variables are fields while local variables and parameter variables are not.

If you want to understand more deeply, you can head over to the source below:-

http://sajupauledayan.com/java/fields-vs-variables-in-java

Dietel and Dietel have a nice way of explaining fields vs variables.

“Together a class’s static variables and instance variables are known as its fields.” (Section 6.3)

“Variables should be declared as fields only if they’re required for use in more than one method of the class or if the program should save their values between calls to the class’s methods.” (Section 6.4)

So a class's fields are its static or instance variables - i.e. declared with class scope.

Reference - Dietel P., Dietel, H. - Java™ How To Program (Early Objects), Tenth Edition (2014)

Variables are comprised of fields and non-fields.

Fields can be either:

1. Static fields or
2. non-static fields also called instantiations e.g. x = F()

Non-fields can be either:

1. local variables, the analog of fields but inside a methods rather than outside all of them, or
2. parameters e.g. y in x = f(y)

In conclusion, the key distinction between variables is whether they are fields or non-fields, meaning whether they are inside a methods or outside all methods.

Basic Example (excuse me for my syntax, I am just a beginner)

Class {    
    //fields    

    method1 {              
         //non-fields    

    }    
}

Yes, there is.

Variable can be local, field, or constant (although this is technically wrong). It's vague like attribute. Also, you should know that some people like to call final non-static (local or instance) variables

"Values". This probably comes from emerging JVM FP languages like Scala.

Field is generally a private variable on an instance class. It does not mean there is a getter and a setter.

Attribute is a vague term. It can easily be confused with XML or Java Naming API. Try to avoid using that term.

Property is the getter and setter combination.

Some examples below

public class Variables {

    //Constant
    public final static String MY_VARIABLE = "that was a lot for a constant";

    //Value
    final String dontChangeMeBro = "my god that is still long for a val";

    //Field
    protected String flipMe = "wee!!!";

    //Property
    private String ifYouThoughtTheConstantWasVerboseHaHa;

    //Still the property
    public String getIfYouThoughtTheConstantWasVerboseHaHa() {
        return ifYouThoughtTheConstantWasVerboseHaHa;
    }

    //And now the setter
    public void setIfYouThoughtTheConstantWasVerboseHaHa(String ifYouThoughtTheConstantWasVerboseHaHa) {
        this.ifYouThoughtTheConstantWasVerboseHaHa = ifYouThoughtTheConstantWasVerboseHaHa;
    }

}

There are many more combinations, but my fingers are getting tired :)