创建具有类字段或简单字段的pojo的良好实践是什么

Question

What is good practice to create pojo as having Class fields or simple fields. I am creating pojo like this.

public class StatusDTO {

private String id;
private int totalNodes;
private int totalServlets;
private boolean status;
public String getId() {
    return id;
}

public void setId(String id) {
    this.id = id;
}


public int getTotalNodes() {
    return totalNodes;
}

public void setTotalNodes(int totalNodes) {
    this.totalNodes = totalNodes;
}

public int getTotalServlets() {
    return totalServlets;
}

public void setTotalServlets(int totalServlets) {
    this.totalServlets = totalServlets;
}

public boolean isStatus() {
    return status;
}

public void setStatus(boolean status) {
    this.status = status;
}

} someone recommanded me to do like this as below

public class StatusDTO {

private String id;
private boolean status;
private Total total;

public Total getTotal() {
    return total;
}

public void setTotal(Total total) {
    this.total = total;
}
public String getId() {
    return id;
}

public void setId(String id) {
    this.id = id;
}

public boolean isStatus() {
    return status;
}

public void setStatus(boolean status) {
    this.status = status;
}
public static class Total {
    private int nodes;
    private int servlets;
    public int getNodes() {
        return nodes;
    }
    public void setNodes(int nodes) {
        this.nodes = nodes;
    }
    public int getServlets() {
        return servlets;
    }
    public void setServlets(int servlets) {
        this.servlets = servlets;
    }

}

}

what difference does it make? what is good practice among those two? I am using this class to set db info and send info to web socket client(stomp)

Answer 1

Well, one important thing in a good Java application is separation of concerns, for example in an airport application a service that give the last flight of a customer should not require as parameter an object with the first name, the last name, the social security number, the marital status, the gender or whatever other information about the customer that are completely useless (or should be) in retrieving the customer last flight, such that you need to have an object Customer (with all customer information) and another object CustomerId (with only the necessary bits to get the flights).

Another example is for a online shop application, a service that calculate the total price of the basket should not require all the information about all articles (photos, description, specifications, ...) in the basket but only the prices and the discounts which should be enclosed in another object.

Here you have to decide if the concerns of your Total (you need a better name) object could be taken separately of the concerns of your StatusDTO object, such that a method could require only the Total object without the associated StatusDTO object. If you can take them separately then you should have separate objects, if you can't then it's unnecessary.

Answer 2

The answer, as always in such questions, is: It depends.

Simple classes like the first one have the advantage that they are simpler and smaller. The advantage on the second attempt is that if your class, maybe now, maybe later, gets extended, it might be easier if you create a separate Total class.

Good Objectoriented Programming, and Java is strongly OO, almost always requires you to put everything into it's own class.

As a rule of thumb, I create a separate class if:

• there is some functionality you to your fields.
• you have more then two, mabye three fields related to each other (eg connectionHost, connectionPort)
• it's just a model class (eg Customer, Article)
• I can use the field in multiple other classes

Of course there are more but those are some of the most important ones (comment if you think there is another important one I forgot to mention).