新人求助，命令行运行javac提示错误找不到符号

[GISer_Lin]

public class Root {
private double a,b,c,x1,x2;
private boolean hasRealRoots;

public Root(double a, double b, double c) {
super();
this.a = a;
this.b = b;
this.c = c;
}


public Root() {
super();
}

public double getA() {
return a;
}

public void setA(double a) {
this.a = a;
}

public double getB() {
return b;
}

public void setB(double b) {
this.b = b;
}

public double getC() {
return c;
}

public void setC(double c) {
this.c = c;
}

public double getX1() {
return x1;
}

public double getX2() {
return x2;
}

public boolean isHasRealRoots() {
return hasRealRoots;
}


public void computeRoots()
{
double delta = Math.pow(b, 2) - 4*a*c;
if(delta < 0)
{
this.hasRealRoots = false;
}
else
{
this.hasRealRoots = true;
this.x1 = (-b + Math.sqrt(delta)) / (2*a);
this.x2 = (-b - Math.sqrt(delta)) / (2*a);
}
}
}
public class app {

public app() {
// TODO Auto-generated constructor stub
}

public static void main(String[] args) {
// TODO Auto-generated method stub
double a,b,c;
a = Double.parseDouble(args[0]);
b = Double.parseDouble(args[1]);
c = Double.parseDouble(args[2]);

Root eg =new Root(a,b,c);
eg.computeRoots();
if(eg.isHasRealRoots())
{
System.out.println("x1 = " + eg.getX1() +
"\nx2 = " + eg.getX2());
}
else
{
System.out.println(a + "x^2 + " + b + "x" + " + " + c + "has no root!");
}
}
}

网上的方法都试过了，变量配置没问题，另一个类的编译也没问题，就是这个，到底是什么问题啊求助

[小公牛123]

把你这个 Double.parseDouble()去掉 我刚刚运行了下 这个位置会报个 数组下标越界异常

[GISer_Lin]

谢谢。已解决，在编译时声明引用类就可以了

[景天的景景天的天]

怎么声明类？？