同一个目录下的两个JAVA文件彼此调用的问题

[fancuixia]

JavaTutorial\java\classes\example 下有两个目录 One & Tow.

One 目录下有两个JAVA文件 Alpha.java, DeltaOne.java。

javac Alpha.java, 没有任何问题；
javac DeltaOne.java，提示错误：
===============================================
DeltaOne.java:5: cannot resolve symbol
symbol : class Alpha
location: class One.DeltaOne
Alpha a = new Alpha();
^
DeltaOne.java:5: cannot resolve symbol
symbol : class Alpha
location: class One.DeltaOne
Alpha a = new Alpha();
^
2 errors
==============================================

以下为源代码：
Alpha.java
====>
package One;

public class Alpha {

//member variables
private int iamprivate = 1;
int iampackage = 2; //package access
protected int iamprotected = 3;
public int iampublic = 4;


//methods
private void privateMethod() {
System.out.println("iamprivate Method");
}
/* */void packageMethod() { //package access
System.out.println("iampackage Method");
}
protected void protectedMethod() {
System.out.println("iamprotected Method");
}
public void publicMethod() {
System.out.println("iampublic Method");
}

public static void main(String[] args) {
Alpha a = new Alpha();
a.privateMethod(); //legal
a.packageMethod(); //legal
a.protectedMethod(); //legal
a.publicMethod(); //legal

System.out.println("iamprivate: " + a.iamprivate); //legal
System.out.println("iampackage: " + a.iampackage); //legal
System.out.println("iamprotected: " + a.iamprotected); //legal
System.out.println("iampublic: " + a.iampublic); //legal
}
}

DeltaOne.java
===============>
package One;

public class DeltaOne {
public static void main(String[] args) {
Alpha a = new Alpha();
//a.privateMethod(); //illegal
a.packageMethod(); //legal
a.protectedMethod(); //legal
a.publicMethod(); //legal

//System.out.println("iamprivate: " + a.iamprivate); //illegal
System.out.println("iampackage: " + a.iampackage); //legal
System.out.println("iamprotected: " + a.iamprotected); //legal
System.out.println("iampublic: " + a.iampublic); //legal
}
}

清不吝指教。