今天我遇到了一个程序编译时的问题
源代码：
public class ch5ex4{
    static void swap(int x, int y){
        System.out.print("Before swap:");
        System.out.println("x = " + x + "y = " +y);
        int temp=x ; x=y ; y=temp;
        System.out.print("After swap:");
        System.out.println("x = " + x + "y =" + y);
    }
    public static void main(String[]args){
        int num1,num2;
        System.out.print("Enter a num1:");
        num1 = Keyboard.readInt();
        System.out.print("Enter a num2:");
        num2 = Keyboard.readInt();
        System.out.print("Before calling:");
        System.out.println("num1 = " + num1 + "num2 =" + num2);
        swap(num1,num2);
        System.out.print("after calling:");
        System.out.println("num1 = " + num1 + "num2 =" + num2);
    }
}
编译时提示
E:\testjava>javac ch5ex4.java
ch5ex4.java:12: 找不到符号
符号： 变量 Keyboard
位置： 类 ch5ex4
                num1 = Keyboard.readInt();
                       ^
ch5ex4.java:14: 找不到符号
符号： 变量 Keyboard
位置： 类 ch5ex4
                num2 = Keyboard.readInt();
                       ^
2 错误
请问是什么原因呢？小弟刚学不久，希望大家知道的指点下谢谢