问题

我已经编写了下面的程序来打印奇数和偶数：

public class PrintEvenOdd {

    public static void main(String[] args) {
        CurrentValue currentValue = new CurrentValue();
        Thread oddThread = new Thread(new PrintOdd(10, currentValue));
        Thread evenThread = new Thread(new PrintEven(10, currentValue));
        oddThread.start();
        evenThread.start();
    }
}

class CurrentValue {

    private int current = 0;

    public int getCurrent() {
        return current;
    }

    public void setCurrent(Integer current) {
        this.current = current;
    }
}

class PrintOdd implements Runnable {

    private int noOfValuesToPrint;
    private CurrentValue currentValue;

    public PrintOdd(int noOfValuesToPrint, CurrentValue currentValue) {
        this.noOfValuesToPrint = noOfValuesToPrint;
        this.currentValue = currentValue;
    }

    public void run() {
        while (true) {
            synchronized (currentValue) {
                System.out.println("Inside Print odd");
                int current = currentValue.getCurrent();
                System.out.println("Value of current in odd is " + current);
                while (current % 2 != 0) {
                    try {
                        System.out.println("Value of current in odd is " + current + "and value of current % 2  is "
                                + current % 2);
                        System.out.println("odd waiting");
                        currentValue.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("Odd no. is " + ++current);
                currentValue.setCurrent(current);
                currentValue.notify();
                System.out.println("Notify executed from odd");
            }
        }
    }
}

class PrintEven implements Runnable {

    private int noOfValuesToPrint;
    private CurrentValue currentValue;

    public PrintEven(int noOfValuesToPrint, CurrentValue currentValue) {
        this.noOfValuesToPrint = noOfValuesToPrint;
        this.currentValue = currentValue;
    }

    public void run() {
        while (true) {
            synchronized (currentValue) {
                System.out.println("Inside Print even");
                int current = currentValue.getCurrent();
                System.out.println("Value of current in even is " + current);
                while (current % 2 == 0) {
                    try {
                        System.out.println("even waiting");
                        currentValue.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("Even no. is " + ++current);
                currentValue.setCurrent(current);
                currentValue.notify();
                System.out.println("Notify executed from even");
            }
        }
    }
}

它给我输出的结果是：

**Inside Print odd**

**Value of current in odd is 0**

**Odd no. is 1**

**Notify executed from odd**

**Inside Print odd**

**Value of current in odd is 1**

**Value of current in odd is 1and value of current % 2  is 1**

**odd waiting**

**Inside Print even**

**Value of current in even is 1**

**Even no. is 2**

**Notify executed from even**

**Inside Print even**

**Value of current in even is 2**

**even waiting**

**Value of current in odd is 1and value of current % 2  is 1**

**odd waiting**

我期望两个线程交替打印奇数和偶数，使用`wait`和`notify`机制。我做错了什么？我还尝试过将`current`变量设为`volatile`，但输出结果相同。

public class PrintEvenOdd {
public static void main(String[] args) {
CurrentValue currentValue = new CurrentValue();
Thread oddThread = new Thread(new PrintOdd(10, currentValue));
Thread evenThread = new Thread(new PrintEven(10, currentValue));
oddThread.start();
evenThread.start();
}
}
class CurrentValue {
private int current = 0;
public int getCurrent() {
return current;
}
public void setCurrent(Integer current) {
this.current = current;
}
}
class PrintOdd implements Runnable {
private int noOfValuesToPrint;
private CurrentValue currentValue;
public PrintOdd(int noOfValuesToPrint, CurrentValue currentValue) {
this.noOfValuesToPrint = noOfValuesToPrint;
this.currentValue = currentValue;
}
public void run() {
while (true) {
synchronized (currentValue) {
System.out.println("Inside Print odd");
int current = currentValue.getCurrent();
System.out.println("Value of current in odd is " + current);
while (current % 2 != 0) {
try {
System.out.println("Value of current in odd is " + current + "and value of current % 2  is "
+ current % 2);
System.out.println("odd waiting");
currentValue.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("Odd no. is " + ++current);
currentValue.setCurrent(current);
currentValue.notify();
System.out.println("Notify executed from odd");
}
}
}
}
class PrintEven implements Runnable {
private int noOfValuesToPrint;
private CurrentValue currentValue;
public PrintEven(int noOfValuesToPrint, CurrentValue currentValue) {
this.noOfValuesToPrint = noOfValuesToPrint;
this.currentValue = currentValue;
}
public void run() {
while (true) {
synchronized (currentValue) {
System.out.println("Inside Print even");
int current = currentValue.getCurrent();
System.out.println("Value of current in even is " + current);
while (current % 2 == 0) {
try {
System.out.println("even waiting");
currentValue.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("Even no. is " + ++current);
currentValue.setCurrent(current);
currentValue.notify();
System.out.println("Notify executed from even");
}
}
}
}

答案1

得分: 1

在这个条件中 while (current % 2 != 0)（以及在 PrintEven 中相反的条件），current 的值没有被更新。请改为使用 while (currentValue.getCurrent() % 2 != 0)；要么移除 current 变量，要么在循环中更新它。