java编程题，急！

java编程题，急！

public static double multiple(double a, double b){return a*b;}
public static double dvd(double a, double b){return a/b;}
public static double op(double a){return a*-1;}
public static double daoshu(double a){return 1/a;}
public static double abs(double a){return Math.abs(a);}

楼上正解
public class test_1_3 extends JFrame{

public static void main(String args[]){
multiple(20, 30);
dvd(20,20);
op(-20);
daoshu(2);
abs(23);
}
public static double multiple(double a, double b){
System.out.println("a*b="+a*b);
return a*b;
}
public static double dvd(double a, double b){
System.out.println("a/b="+a/b);
return a/b;
}
public static double op(double a){
System.out.println("a的相反数"+a*-1);
return a*-1;
}
public static double daoshu(double a){
System.out.println("a的倒数"+1/a);
return 1/a;
}
public static double abs(double a){
System.out.println("a的绝对值"+Math.abs(a));
return Math.abs(a);
}
}