这里为啥会报错以及弹出的那个--usrage是啥？？

package com.itheima.operator;

public class OperatorDemo4 {
//目标：学会自增自减运算符++--
int a = 10;
a++;

}

public class OperatorDemo {
    public static void main(String[] args) {
        int a = 10;
        a++;
        System.out.print(a);
    }
}