非法的表达式在此题我不理解

package bibilili.test;

/*
目标：能够说出方法的优点：
       1.可以提高代码的复用性和开发效率
       2.让程序的逻辑更清晰
 */
public class test16 {
    public static void main(String[] args) {

        //张工
       /* int a2 = 10;
        int b2 = 30;
        int c2 = a2 + b2;*/
        int c2=sum(10,30);
        System.out.println(c2);
        //许工
       /* int a1 = 10;
        int b1 = 30;
        int c1 = a1 + b1;*/
        int c1=sum(10,30);
        System.out.println(c1);
        public static int sum(int a,int b){
            int c=a+b;
            return c;
        }

    }
}
//运行后报错：非法的表达式开始
我不懂什么意思

注意sum方法的位置

public class A {

  public static void main(String[] args) {
    //张工
    /* int a2 = 10;
    int b2 = 30;
    int c2 = a2 + b2;*/
    int c2 = sum(10, 30);
    System.out.println(c2);
    //许工
    /* int a1 = 10;
    int b1 = 30;
    int c1 = a1 + b1;*/
    int c1 = sum(10, 30);
    System.out.println(c1);
  }

  public static int sum(int a, int b) {
    int c = a + b;
    return c;
  }
}

package Action;

public class test16 {
    public static void main(String[] args) {
 
        //张工
       /* int a2 = 10;
        int b2 = 30;
        int c2 = a2 + b2;*/
        int c2=sum(10,30);
        System.out.println(c2);
        //许工
       /* int a1 = 10;
        int b1 = 30;
        int c1 = a1 + b1;*/
        int c1=sum(10,30);
        System.out.println(c1);
      
    }
    public static int sum(int a,int b){
        int c=a+b;
        return c;
    }
}

我知道了，我是定义方法时嵌套定义了，定义在main方法里面了，哎呀！尴尬了