javac类路径找不到符号？

Question

i have two classes at the same folder:

• First.java
• Second.java

First class implementation:

package mypackage;

public class First {
    public static void main(String[] args) {
        Second.myMethod();
    }

}

Second class implementation:

public class Second {
    public static void myMethod() {
        System.out.println("this is the second class");
    }
}

the Second has been compiled, so it becomes:

• First.java
• Second.java
• Second.class

when i try to compile First.java: javac -cp.;.. -d. First.java

i got an error:

cannot find symbol
                 Second.myMethod();
                        ^
  symbol:   variable Second
  location: class First
1 error

i have tried: "javac -d. *.java" and i got the same result

Answer 1

Starting point

You showed two class definitions..

• Here is First, which belongs to package "mypackage":

   package mypackage; public class First { public static void main(String[] args) { Second.myMethod(); } }

• Here is Second, which has no package statement:

   public class Second { public static void myMethod() { System.out.println("this is the second class"); } }

If you attempt to compile those together, you'll get a compilation error like below (OpenJDK 18.0.1.1):

% javac *.java
First.java:5: error: cannot find symbol
        Second.myMethod();
        ^
  symbol:   variable Second
  location: class First
1 error

---

Why it doesn't work

From 7.4.2. Unnamed Packages :

A compilation unit that has no package declaration is part of an unnamed package.

Unnamed packages are provided by the Java SE platform principally for convenience when developing small or temporary applications or when just beginning development.

This describes "Second" – a compilation unit which has no package declaration. JLS 7.4.2 tells us that this class would belong to an unnamed package. Further, we can see that unnamed packages are not intended to be used for much of anything.

From 7.5. Import Declarations :

Without the use of an appropriate import declaration, the only way to refer to a type declared in another package, or a static member of another type, is to use a fully qualified name ( §6.7 ).

This tells us that in First.java, if we wish to refer to a type defined in another package, we would need to use the fully qualified name of the type we wish to reference.

This is what you tried to do – First is in named package "mypackage", and Second is in an unnamed package. For First to see Second, First would need to use a fully-qualified name for Second. But there is no fully-qualified name for Second – it belongs to an unnamed package; it doesn't have a name, so it isn't possible to reference it.

---

Solution 1

Edit Second – move it from the unnamed package to the named package "mypackage" by putting this at the top: package mypackage; . By doing this, both First and Second would belong to the same named package, and compilation will succeed.

---

Solution 2

Edit First – move it from the named package "mypackage" to the unnamed package by commenting out or deleting this line: package mypackage; . By doing this, you are removing the named package altogether, leaving First and Second both in the unnamed package, and compilation will succeed.