我有一个关于下面发布的代码的问题。为什么我需要在 SumArray 对象的声明前面加上“static”这个词才能使这段代码正常运行？为什么不保证（由它自己）一次为一个线程调用同步方法？事实上，如果我删除“静态”一词，那么 SumArray 方法将被 2 个线程调用可互换。尽管如此，两个线程的总和将是正确的。
class SumArray {
private int sum;
synchronized int sumArray(int numbers[]) {
sum = 0;
for (int i = 0; i < numbers.length; i++) {
sum += numbers[i];
System.out.println("Current Sum for " + Thread.currentThread().getName() + " equals " + sum);
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
System.out.println("Main Thread has been stopped");
}
}
return sum;
}
}
class MyThread implements Runnable {
Thread t;
int a[];
int ans;
static SumArray sa = new SumArray();
MyThread(String name, int numbers[]) {
t = new Thread(this, name);
t.start();
a = numbers;
}
public void run() {
System.out.println(t.getName() + " is starting.");
ans = sa.sumArray(a);
System.out.println("Final Sum for " + t.getName() + " equals " + ans);
System.out.println(t.getName() + " has been finished.");
}
}
public class Main {
public static void main(String[] args) {
int a[] = { 1, 2, 3, 4, 5 };
new MyThread("Thread 1", a);
new MyThread("Thread 2", a);
}
}
代码的示例结果 static SumArray sa = new SumArray();
Thread 2 is starting.
Thread 1 is starting.
Current Sum for Thread 2 equals 1
Current Sum for Thread 2 equals 3
Current Sum for Thread 2 equals 6
Current Sum for Thread 2 equals 10
Current Sum for Thread 2 equals 15
Current Sum for Thread 1 equals 1
Final Sum for Thread 2 equals 15
Thread 2 has been finished.
Current Sum for Thread 1 equals 3
Current Sum for Thread 1 equals 6
Current Sum for Thread 1 equals 10
Current Sum for Thread 1 equals 15
Final Sum for Thread 1 equals 15
Thread 1 has been finished.
结果与 SumArray sa = new SumArray();
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