缺少返回 语句，
[code=Java][/code]
import java.io.*;
public class ComplexNumber{
public double real;

  public double image;

  public ComplexNumber(double a, double b)
  {

  this.real=a;

  this.image=b;

  }

  public static String readString()
  {
  //从键盘读取字符串
  BufferedReader br=new BufferedReader(new InputStreamReader(System.in),1);
  String s1="";
  try{
  s1=br.readLine();
  }catch(IOException ex){
  System.out.println(ex);
   
  return s1;
  }
  }
  public static float readFloat(){
  //字符串转换为浮点数
  return Float.parseFloat(readString());
  }
   
   
   
  public ComplexNumber()
  {
  real=0.0;
  image=0.0;
  }
  
  public ComplexNumber add(ComplexNumber c)
  {
  ComplexNumber result=new ComplexNumber(this.real+c.real,this.image+c.image);

  return result;
  }
  public ComplexNumber sub(ComplexNumber c)
  {
  ComplexNumber result=new ComplexNumber(this.real-c.real,this.image-c.image);
  return result;
  }
  public ComplexNumber mul(ComplexNumber c)
  {
  ComplexNumber result=new ComplexNumber(this.real*c.real-this.image*c.image,

  this.real*c.image+this.image*c.real);

  return result;
  }
  public ComplexNumber div(ComplexNumber c)
  {
  ComplexNumber result=new ComplexNumber((this.real*c.real+this.image*c.image)/(c.real*c.real+c.image*c.image),

  (this.image*c.real+this.real*c.image)/(c.real*c.real+c.image*c.image));
  return result;
  }
  public String toString(){

  String ComplexNunber=new String(this.real+"+"+this.image+"i");

  return ComplexNunber;

  }
   
  
public static void main(String[] args )
  {
  ComplexNumber a=new ComplexNumber();
  ComplexNumber b=new ComplexNumber();
  System.out.println("Enter the first ComplexNumber");
 
  a.real=ComplexNumber.readFloat();
  a.image=ComplexNumber.readFloat();
  System.out.println("Enter the second ComplexNumber");
 
  b.real=ComplexNumber.readFloat();
  b.image=ComplexNumber.readFloat();
  System.out.println((a.add(b)).toString());

  System.out.println((a.sub(b)).toString());

  System.out.println((a.mul(b)).toString());

  System.out.println((a.div(b)).toString());
  }
 
  }


------解决方案--------------------
把return s1;后面的 } 移动到return s1;的前面