java为什么inc/dec频道什么都不做？

Question

在下面的代码中，我的测试表明tvObject.incChannel(...)和tvObject.decChannel(...)方法不会改变代码中的任何内容。为什么?

package lab2;

public class TV {

    private int channelNumber;
    private int volumeLevel;
    private String status = "ON";

    public TV(int channel, int vol, String stat) {

        status = stat;
        setChannelNumber(channel);
        setVolumeLevel(vol);
    }
    public void setChannelNumber(int channel) {
        if (status.equalsIgnoreCase("ON")) {
            channelNumber = channel;
        } else if (status.equalsIgnoreCase("OFF")) {
            System.out.println("Error TV is off");
        }   
    }
    public int getChannelNumber() {
        return channelNumber;
    }
    public void setVolumeLevel(int vol) {
        if (status.equalsIgnoreCase("ON")) {
            volumeLevel = ((vol >= 0 && vol <= 100)?vol:0);
        } else if (status.equalsIgnoreCase("OFF")) {
            System.out.println("Error TV is off");
        }       
        volumeLevel = ((vol >= 0 && vol <= 100)?vol:0);
    }
    public int getVolumeLevel() {
            return volumeLevel; 
    }
    public void incVol(int vol) {
        if (status.equalsIgnoreCase("ON")) {
            volumeLevel = ((vol >= 0 && vol <= 98)?vol += 2:volumeLevel);
        } else if (status.equalsIgnoreCase("OFF")) {
            System.out.println("Error TV is off");
        }   
    }
    public void decVol(int vol) {
        if (status.equalsIgnoreCase("ON")) {
            volumeLevel = ((vol >= 2 && vol <= 100)?vol -= 2:volumeLevel);
        } else if (status.equalsIgnoreCase("OFF")) {
            System.out.println("Error TV is off");
        }           
    }

    // does not do anything
    public void incChannel(int channel) {
        if (status.equalsIgnoreCase("ON")) {
            channelNumber = channel++;
        } else if (status.equalsIgnoreCase("OFF")) {
            System.out.println("Error TV is off");
        }       
    }

    // does not do anything
    public void decChannel(int channel) {
        if (status.equalsIgnoreCase("ON")) {
            channelNumber = channel--;
        } else if (status.equalsIgnoreCase("OFF")) {
            System.out.println("Error TV is off");
        }       
    }
    public void Toggle() {
        //String status = "ON";  //default on

        if (status.equalsIgnoreCase("ON")) {
            status = "OFF";
        } else if (status.equalsIgnoreCase("OFF")) {
            status = "ON";
        }       
    }
}


// code to test

package lab2;

public class Test {

    public static void main(String[] args) {

            TV tvObject = new TV(53, 35, "ON");
            //TV tvObject2 = new TV(3, -10, "on");
            //TV tvObject3 = new TV(3, -10, "OFF");

            tvObject.Toggle();

            tvObject.setChannelNumber(53);
            tvObject.incChannel(53);

            tvObject.setVolumeLevel(35);
            tvObject.decVol(35);

            System.out.println(tvObject.getChannelNumber());
            System.out.println(tvObject.getVolumeLevel());
    }
}

Answer 1

It should be like this:

public void incChannel(int channel) {
        if (status.equalsIgnoreCase("ON")) {
            channelNumber = ++channel; // use either this
            // channelNumber = channel+1; // or this
        } else if (status.equalsIgnoreCase("OFF")) {
            System.out.println("Error TV is off");
        }       
    }

public void decChannel(int channel) {
        if (status.equalsIgnoreCase("ON")) {
            channelNumber =  channel; // use either this
            // channelNumber = channel-1; // or this
        } else if (status.equalsIgnoreCase("OFF")) {
            System.out.println("Error TV is off");
        }       
    }

channelNumber = channel++表示首先将通道分配给channelNumber，然后增加其值

channelNumber = ++channel表示首先增加通道值，然后将其分配给cannelNumber

Answer 2

后缀增量运算符（因此，变量名之后的“++”）首先计算表达式，然后才递增

因此，a channelNumber = channel++等价于以下内容

channelNumber = channel;
channel = channel + 1;

因为channel只是方法中的一个参数，所以它的增加不会改变任何东西（后面的方法中不会使用它），从而将语句简化为channelNumber = channel;表达式

最简单的代码修复程序如下：

// so it already works
public void incChannel(int channel) {
    if (status.equalsIgnoreCase("ON")) {
        channelNumber = ++channel;
    } else if (status.equalsIgnoreCase("OFF")) {
        System.out.println("Error TV is off");
    }       

。。。因为前缀增量运算符（“++”在变量名之前）首先增加，然后才计算。在这种情况下，有问题的语句的工作原理如下

channel = channel + 1;
channelNumber = channel;

这可能就是你想要的