java 错误: 在类 Example 中找不到 main 方法, 请将 main 方法定义为

Question

源码：
public class Example{
String str;
char[] ch;
public static void main(String[] args){
Example ex=new Example();
ex.change(ex.str,ex.ch);
System.out.print(ex.str+" and ");
System.out.print(ex.ch);
}
public void change(String str,char ch[]){
str="test ok";
ch[0]= 'g';

}
}

Answer 1

main 函数没有问题报的是空指针异常

 public class Example{
String str="ds";
char[] ch={'a','b'};
public static void main(String[] args){
Example ex=new Example();
ex.change(ex.str,ex.ch);
System.out.print(ex.str+" and ");
System.out.print(ex.ch);
}
public void change(String str,char ch[]){
this.str="test ok";
this.ch[0]= 'g';

}
}