我有一個關(guān)于下面發(fā)布的代碼的問題。為什么我需要在 SumArray 對象的聲明前面加上“static”這個詞才能使這段代碼正常運行？為什么不保證（由它自己）一次為一個線程調(diào)用同步方法？事實上，如果我刪除“靜態(tài)”一詞，那么 SumArray 方法將被 2 個線程調(diào)用可互換。盡管如此，兩個線程的總和將是正確的。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);    }}代碼的示例結(jié)果 static SumArray sa = new SumArray();Thread 2 is starting.Thread 1 is starting.Current Sum for Thread 2 equals 1Current Sum for Thread 2 equals 3Current Sum for Thread 2 equals 6Current Sum for Thread 2 equals 10Current Sum for Thread 2 equals 15Current Sum for Thread 1 equals 1Final Sum for Thread 2 equals 15Thread 2 has been finished.Current Sum for Thread 1 equals 3Current Sum for Thread 1 equals 6Current Sum for Thread 1 equals 10Current Sum for Thread 1 equals 15Final Sum for Thread 1 equals 15Thread 1 has been finished.結(jié)果與 SumArray sa = new SumArray();