问题
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?
回答1:
From here: http://docs.oracle.com/javase/tutorial/information/glossary.html
field
- A data member of a class. Unless specified otherwise, a field is not static.
property
- Characteristics of an object that users can set, such as the color of a window.
attribute
- Not listed in the above glossary
variable
- An item of data named by an identifier. Each variable has a type, such as int or Object, and a scope. See also class variable, instance variable, local variable.
回答2:
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 :)
回答3:
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.
回答4:
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)
回答5:
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.
回答6:
Variables are comprised of fields and non-fields.
Fields can be either:
- Static fields or
- non-static fields also called instantiations e.g. x = F()
Non-fields can be either:
- local variables, the analog of fields but inside a methods rather than outside all of them, or
- 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
}
}
回答7:
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
回答8:
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.
回答9:
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.
来源：https://stackoverflow.com/questions/10115588/what-is-the-difference-between-field-variable-attribute-and-property-in-java