Method is static, yet I get compiler error saying it cannot be accessed from static context

[Sandra Bachan]

Sierra/Bates chapter 1 code:

class Zoo { 
   public String coolMethod() {
      return "Wow baby";
   }
} 
class Moo extends Zoo {
   public void useMyCoolMethod() { 
      System.out.println("Moo says, " + this.coolMethod());  
      System.out.println("Zoo says, " + z.coolMethod()); 
   }
}

I changed it to the following:

class Zoo { 
	public static String coolMethod() {
		return "Wow baby";
	}
}

class Moo extends Zoo {

	public static void useMyCoolMethod() { 
		System.out.println("Moo says, " + this.coolMethod());
		Zoo z = new Zoo(); 
		// If the preceding line compiles Moo has access 
		// to the Zoo class 
		// But... does it have access to the coolMethod()? 
		System.out.println("A Zoo says, " + z.coolMethod()); 
		// The preceding line works because Moo can access the 
		// public method
	}
	
	public static void main (String[] args){
		useMyCoolMethod();
	}
}

I get compiler error:

Moo.java:10: non-static variable this cannot be referenced from a static context
System.out.println("Moo says, " + this.coolMethod());
^
1 error


But I am not referencing a variable...please explain

[Henry Wong]

The non-static variable being referred to here is the "this" variable.

Henry

[mohammad shaid]

this & super cannot be used to invoke on nonstatic stuff(non-static methods fo eg)

[pete stein]

mohammad shaid wrote:this & super cannot be used to invoke on nonstatic stuff(non-static methods fo eg)

Please clarify what you mean here as the statement above confuses me. Thanks.

[mohammad shaid]

coolMethod in the above program is static and static methods cannot be invoked on this as its nonstatic (like super).. try create two classes(one inheriting another) and create static methods in both classes with same name.. try call the overridden method using super from the overriding method.. you will get to know..

[pete stein]

mohammad shaid wrote:....static methods cannot be invoked on this as its nonstatic (like super)

Are you sure about this?

public class StaticFun {
  public static void staticMethod() {
    System.out.println("from static method");
  }
  
  public void nonStaticMethod() {
    System.out.print("from non-static method: ");
    
    this.staticMethod(); // the compiler complains, but this still works
  }
  
  public static void main(String[] args) {
    StaticFun.staticMethod();
    
    StaticFun staticFun = new StaticFun();
    staticFun.nonStaticMethod();
  }
}

[mohammad shaid]

ya well.. thanks for correcting me.. i took it wrong.. thought we cannot invoke static methods on this... but this is non static and it cannot be invoked from static method.. am i right???

[pete stein]

mohammad shaid wrote:ya well.. thanks for correcting me.. i took it wrong.. thought we cannot invoke static methods on this... but this is non static and it cannot be invoked from static method.. am i right???

Yes. As far as I understand it, the main problem for the OP is that they tried to use "this" from within a static method, and inside a static method, "this" simply doesn't exist.

[Sandra Bachan]

Now I understand that this() and super() are non-static.

According to the text, and invocation for this() from a static context is supposed to work:

class Zoo { 
   public String coolMethod() {
      return "Wow baby";
   }
} 
class Moo extends Zoo {
   public void useMyCoolMethod() { 
      // Does an instance of Moo inherit the coolMethod()? 
      System.out.println("Moo says, " + this.coolMethod()); 
      // The preceding line works because Moo can inherit the 
      // public method 
      // Can an instance of Moo invoke coolMethod() on an 
      // instance of Zoo? 
      Zoo z = new Zoo(); 
      System.out.println("Zoo says, " + z.coolMethod()); 
      // coolMethod() is public, so Moo can invoke it on a Zoo //reference
   }
}

Please clarify "The preceding line works because Moo can inherit the public method"

[Jim Hoglund]

Pete : In your "are-you-sure" code, you note that the compiler complains, but the code still
runs okay. Consider the situation where instance 'aa' is of type AA. Its static methods can be
called in two ways: AA.staticMethod() -or- aa.staticMethod(). Your example behaves the same
way. Reference 'this' has a known type that can be examined for the requested static method.

Jim ... ...

[Jim Hoglund]

Sandra : Your code appears again below with the 'z' object removed, because (unless it's
marked private) coolMethod() can be called directly from anywhere in Moo; it is a member
of Moo. As you demonstrated, creating a Moo object does no harm but the instance is not
needed to access coolMethod().

class Zoo { 
   public String coolMethod() {
      return "Wow baby";
   }
} 
class Moo extends Zoo {
   public void useMyCoolMethod() { 
      // Does an instance of Moo inherit the coolMethod()? 
      System.out.println("Moo says, " + this.coolMethod()); 
      // The preceding line works because Moo can inherit the 
      // public method 
      // Can an instance of Moo invoke coolMethod() on an 
      // instance of Zoo? 
      //Zoo z = new Zoo(); 
      //System.out.println("Zoo says, " + z.coolMethod()); 
      System.out.println("Zoo says, " + coolMethod());
      // coolMethod() is public, so Moo can invoke it on a Zoo //reference
   }
}

Jim ... ...

[Sandra Bachan]

Hi Jim,

Now I understand.

I do have a follow-on question. When I run the code as is (without adding public static void main()), I get the following error:


Exception in thread "main" java.lang.NoSuchMethodError: main


How to fix this?

[pete stein]

Sandra Bachan wrote:
I do have a follow-on question. When I run the code as is (without adding public static void main()), I get the following error:


Exception in thread "main" java.lang.NoSuchMethodError: main

How to fix this?

Hm, perhaps add a main method? ;)

[Sandra Bachan]

Of course!

The following seems to work:

class Zoo { 
   public String coolMethod() {
      return "Wow baby";
   }
}
class Moo extends Zoo {
	public void useMyCoolMethod() { 
		// Does an instance of Moo inherit the coolMethod()? 
		System.out.println("Moo says, " + this.coolMethod()); 
		// The preceding line works because Moo can inherit the 
		// public method 
		// Can an instance of Moo invoke coolMethod() on an 
		// instance of Zoo? 
		//Zoo z = new Zoo(); 
		//System.out.println("Zoo says, " + z.coolMethod()); 
		System.out.println("Zoo says, " + coolMethod());
		// coolMethod() is public, so Moo can invoke it on a Zoo //reference
	}
   
	public static void main (String[] args){
		Moo m = new Moo();
		m.useMyCoolMethod();
   	}
   
}