Question

我在Class中有一个静态对象，其中Class是一个writer，然后是该Class的实例，需要引用其中一个静态对象。为了避免代码重复（我必须多次编写相同的代码，每个代码的唯一区别在于使用了哪些静态对象）。我的解决方案是有一个名为writer1的静态编写器和一个静态编写器调用writer7，然后有一个名为otherWriter的非静态编写器，它在writer1或writer7的构造函数中指向另一个编写器。
但是，当我访问otherWriter时，我不断收到NullPointer异常。错误和代码如下 - 任何想法？对不起，代码不是很整洁 - 这只是在这个阶段只是瞎傻的hacky。感谢

错误：

java.lang.NullPointerException
    at popl.PoplFormative.generalWrite(PoplFormative.java:108)
    at popl.PoplFormative.run(PoplFormative.java:51)

package popl;

import java.util.Arrays;

public class PoplFormative extends Thread {

    int currentIndex = 0;
    int lastIndexWritten = -1;

    static int data[];
    int writerId;

    static PoplFormative writer1;
    static PoplFormative writer7;

    PoplFormative otherWriter;

    public static void main(String[] args) {
        data = new int[10];
        writer1 = new PoplFormative(1);
        writer7 = new PoplFormative(7);
        writer1.start();
        writer7.start();
        try {
            Thread.sleep(600);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println(Arrays.toString(data));
    }

    public PoplFormative(int writer) {
        this.writerId = writer;
        if (writerId == 1) {
            this.otherWriter = PoplFormative.writer7;
        }
        else if (writerId == 7) {
            this.otherWriter = PoplFormative.writer1;
        }
        else {
            System.out.println("Big nasty error occurred");
            while (true) {
            }
        }
    }

    public void run() {
        try {
            generalWrite();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            System.out.println("Exception Thrown");
            e.printStackTrace();
        }
    }

    public void generalWrite() throws InterruptedException {
        while (currentIndex < 5) {
            //System.out.println(writerId + " writer has currentIndex; " + currentIndex);
            if (data[Math.max(lastIndexWritten,0)] == writerId || (lastIndexWritten == -1 && writerId == 7)) {
                write();
                synchronized (this) {
                    notify();
                }
                synchronized (otherWriter) {
                    otherWriter.wait();
                }
            }
            else {
                synchronized (otherWriter) {
                    otherWriter.wait();
                    write();
                }
                synchronized (this) {
                    notify();
                }
            }
            lastIndexWritten += 1;
            currentIndex += 1;
        }
        System.out.println(writerId + " has completed");
    }

    public void write() {
        System.out.println("writer: " + writerId + " currentIndex: " + currentIndex + " lastIndex: " + lastIndexWritten);
        data[currentIndex] = writerId;
    }

}

Answer 1

在main，您有

writer1 = new PoplFormative(1);

调用构造函数

public PoplFormative(int writer) {
    this.writerId = writer;
    if (writerId == 1) {
        this.otherWriter = PoplFormative.writer7;
    }
    else if (writerId == 7) {
        this.otherWriter = PoplFormative.writer1;
    }
    else {
        System.out.println("Big nasty error occurred");
        while (true) {
        }
    }
}

在这种情况下，执行此代码

if (writerId == 1) {
    this.otherWriter = PoplFormative.writer7;
}

但PoplFormative.writer7为null，因为它尚未初始化，因此otherWriter也变为null。

如果您需要这些循环引用，请考虑使用setter。

使用静态对象的第二个非静态引用时出现NullPointer异常

1 个答案: