为什么我的Java方法changeVolume（）会返回相同的结果，尽管值发生了变化？

Question

运行代码时，我的音量变量没有更改。 我尝试更改代码和新变量名称的顺序，但输出未更改。 changeVolume（）函数没有执行它应该做的事情。

请帮助我弄清楚为什么它不起作用。贝娄是我的代码和运行它的屏幕截图：

import java.awt.Color;
public class HeadPhone{
//Three constants
public static final int LOW = 1;
public static final int MEDIUM = 2;
public static final int HIGH = 3;

//a private int data filed for volume    
private int volume;
//and a pluggedIn value
private boolean pluggedIn;
//A private String data field for manufacturer
private String manufacturer;
//A private Color data field
private Color headPhoneColor;
//A private String field 
private String headPhoneModel;

//setters and getters for the declared values
public void setVolume(int volume){
    this.volume = volume;
}
public int getVolume(){
    return volume;
}
public void setPluggedIn(boolean pluggedIn){
    this.pluggedIn = pluggedIn;
}
public boolean getPluggedIn(){
    return pluggedIn;
}
public void setManufacturer(String manufacturer){
    this.manufacturer = manufacturer;
}
public void setHeadPhoneColor(Color headPhoneColor){
    this.headPhoneColor = headPhoneColor;
}
public Color getHeadPhoneColor(){
    return headPhoneColor;
}
public void setHeadPhoneModel(String headPhoneModel){
    this.headPhoneModel = headPhoneModel;
}
public String getHeadPhoneModel(){
    return headPhoneModel;
}

//A no argument default constructor
public HeadPhone(){
    volume = MEDIUM;
    pluggedIn = false;
    manufacturer = "Apple";
    headPhoneColor = Color.WHITE;
    headPhoneModel = "Mic-Enabled";        
}

//toString method that returns 
//the current set of headphones    
public String toString() {
    String headPhoneColorString = ""; 
    if (headPhoneColor == Color.RED){
        headPhoneColorString = "Red";
    }
    if (headPhoneColor == Color.BLUE){
        headPhoneColorString = "Blue";
    }
    if (headPhoneColor == Color.BLACK){
        headPhoneColorString = "Black";
    }
    if (headPhoneColor == Color.CYAN){
        headPhoneColorString = "Cyan";
    }   
    if (headPhoneColor == Color.YELLOW){
        headPhoneColorString = "Yellow";
    }
    if (headPhoneColor == Color.WHITE){
        headPhoneColorString = "White";
    }

    String pluggedUnplugged = (pluggedIn == true) ? "plugged in" : "unplugged";

    return "\n\n Volume Set To: " + volume + 
           "\n Headphone set is: " + pluggedUnplugged +
           "\n Manufacturer: " + manufacturer +
           "\n Headphone Color: " + headPhoneColorString +
           "\n Headphone Model: " + headPhoneModel;               
}
//a changeVolume method to adjust headphones volume
public void changeVolume(int volume) {
    if (volume <= LOW){
        volume = LOW;
    }else if (volume >= HIGH){
        volume = HIGH;
    }else{
        volume = MEDIUM;
    }            
    }       

    //main class
    public static void main(String[] args) {
    HeadPhone defaultHeadphoneSet= new HeadPhone();        
    System.out.println(defaultHeadphoneSet);

    HeadPhone set1 = new HeadPhone();
    set1.changeVolume(HIGH);
    set1.setPluggedIn(true);
    set1.setManufacturer("Beats");
    set1.setHeadPhoneColor(Color.RED);
    set1.setHeadPhoneModel("Wireless");
    System.out.println(set1);

    HeadPhone set2 = new HeadPhone();
    set2.changeVolume(HIGH);
    set2.setPluggedIn(true);
    set2.setManufacturer("BOSS");
    set2.setHeadPhoneColor(Color.BLACK);
    set2.setHeadPhoneModel("Noisecancelling");
    System.out.println(set2);

    HeadPhone set3 = new HeadPhone();
    set3.changeVolume(1);
    set3.setPluggedIn(false);
    set3.setManufacturer("SkullCandy");
    set3.setHeadPhoneColor(Color.CYAN);
    set3.setHeadPhoneModel("Taptic");
    System.out.println(set3);    
    }
    }

Screenshot of me compiling and running the code