当同一 class 中的另一个属性的值更新时，如何动态更新 java 属性

Question

我有一个名为 Project 的 Java class。 它具有三个属性（除其他外）： goal 、 amountRaised和percentageRaised 。

goal是项目旨在筹集的资金数额； amountRaised顾名思义是到目前为止筹集的金额； percentageRaised是筹集的资金相对于目标的百分比。

我希望percentageRaised在goal或amountRaised更改时随时更新。

我的问题是，如果我为percentageRaised声明一个 setter 方法（私有），它会自动计算还是需要在某处调用它？

或者，像这样在初始化时添加逻辑是否是一种不好的做法：

public class Project {

    @JsonProperty
    private String id;

    @NotBlank(message = "Please specify the amount of money you intend to raise")
    @JsonProperty
    private double goal;

    @JsonProperty
    private double amountRaised = 0;

    @JsonProperty
    private int percentageRaised = (int) ((amountRaised/goal) * 100);
}

Answer 1

最简单的方法是在两个属性的设置器中执行一个计算方法。

public class Project {

    @JsonProperty
    private String id;

    @NotBlank(message = "Please specify the amount of money you intend to raise")
    @JsonProperty
    private double goal;

    @JsonProperty
    private double amountRaised = 0;

    @JsonProperty
    private int percentageRaised = (int) ((amountRaised/goal) * 100);

    public void setGoal(double goal) {
        this.goal = goal;
        this.calculatePercentageRaised();
    }

    public void setAmountRaised(double amountRaised) {
        this.amountRaised = amountRaised;
        this.calculatePercentageRaised();
    }
    
    private void calculatePercentageRaised() {
      //check not null
      if (amountRaised!=null && goal!=null && goal != 0) {
        this.percentageRaised = (int) ((amountRaised/goal) * 100);
      }
    }
}

Answer 2

这可以在基本的 java beans PropertyChangeListener的帮助下实现

package p1;

import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;
import java.io.Serializable;

public class Project implements PropertyChangeListener, Serializable {

    private final PropertyChangeSupport pcs = new PropertyChangeSupport(this);
    private String id;
    private double goal = 1;
    private double amountRaised = 0;
    private double percentageRaised = 0;

    public Project() {
        pcs.addPropertyChangeListener(this);
    }

    public static void main(String[] args) {
        Project project = new Project();
        project.setAmountRaised(20);
        System.out.printf("\n1: now, amountraised= is %f, goal is %f, percent is %f",
            project.amountRaised, project.goal, project.percentageRaised);
        project.setGoal(50);

        System.out.printf("\n2: now, amountraised= is %f, goal is %f, percent is %f",
            project.amountRaised, project.goal, project.percentageRaised);
        project.setGoal(100);
        System.out.printf("\n3: now, amountraised= is %f, goal is %f, percent is %f",
            project.amountRaised, project.goal, project.percentageRaised);
    }

    public void setGoal(double goal) {
        var old = this.goal;
        this.goal = goal;
        pcs.firePropertyChange("goal", old, goal);
    }

    public void setAmountRaised(double amountRaised) {
        var old = this.amountRaised;
        this.amountRaised = amountRaised;
        pcs.firePropertyChange("amountRaised", old, amountRaised);
    }

    public void updatePercentageRaised() {
        this.percentageRaised = ((amountRaised / goal) * 100);
        ;
    }

    @Override
    public void propertyChange(PropertyChangeEvent evt) {
        updatePercentageRaised();
    }
}

Output：

1: now, amountraised= is 20.000000, goal is 1.000000, percent is 2000.000000
2: now, amountraised= is 20.000000, goal is 50.000000, percent is 40.000000
3: now, amountraised= is 20.000000, goal is 100.000000, percent is 20.000000

参考： https://docs.oracle.com/javase/tutorial/javabeans/writing/properties.html 。

https://kodejava.org/how-do-i-listen-for-beans-property-change-event/