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Static method cannot access members of class instance

In the 9th edition, Liang's Introduction to Java Programming states, "A static method cannot access the members of an instance of a class" (p. 312). I can see why a member of an instance of a class would need to access a method (which could be static), but why would a method need to access an instance member? For me, "access" means "access via the dot operator". In other words:

 Class myClass = new Class();
 myClass.someStaticMethod();

makes sense, whereas:

 someNonStaticMethod.myClass

or

 someStaticMethod.myClass

not. Is the syntax someNonStaticMethod.myClass allowed? I don't believe I have ever seen such formatting. If this is not allowed, why mention that static methods cannot access members of an instance of a class?

Please help raise my confusion.

-DI

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5 answers


Accessing an instance member means accessing a field or attribute of an instance, not the instance itself, as this will not compile. Dot does not literally mean "access" exactly as you think, and I assume you have a source of confusion. The dot is used to call a method on a specific object (see link ) or to access an object's field (or class if the field is static).

For example, assuming the class is defined like this:

class MyClass {

   private int x;

   static void foo() {
      ... // foo cannot access member x
   }
}


So, in a method, foo

you cannot reference x

, because it is an MyClass

instance-bound member field MyClass

.

Also see Understanding Class Members to understand the difference between static members and instance members.

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You cannot access instance variables from static methods.

public class Example {
    private Object instanceVariable;
    public static void staticMethod() {
        // Cannot use this in a static context
        this.instanceVariable = null;
    }
}

You can access instance variables from instance methods.

public class Example {
    private Object instanceVariable;
    public void instanceMethod() {
        this.instanceVariable = null;
    }
}


You should not access static variables from instance methods with this

.

public class Example {
    private static Object staticVariable;
    public void instanceMethod() {
        // The static field Example.staticVariable should be accessed in a static way
        this.staticVariable = null;
    }
}

You can always access static variables. You must use the class name.

public class Example {
    private static Object staticVariable;
    public void instanceMethod() {
        Example.staticVariable = null;
    }
}
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A static method cannot refer to a field of a non-static class instance.

If you want to understand why: A static method can be called without an instance of the class, so a non-static method will not exist anyway when the method is called.

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This suggests it:

public MyClass
{
   private String test;

   public static String getTest()
   {
       return this.test; //this is not possible because a static method cannot access an instance variable or method
   }

}

The reason a static method cannot access an instance variable is because static refers to a class and not a specific instance of the class, so there is no instance to access the variable. The test will only exist when used new MyClass

, the test will now exist. But if I call a static method MyClass.getTest()

then no instance variable is created test

.

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you can access instance variables in static methods by creating objects

public class Test {
int x =10;

public static void main(String[]args)
{
    Test t1 = new Test();
    System.out.println(t1.x);
}

}

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