Java static non inner classes SCJP 6 Find AlinaIoana Florea Flash

Author: AlinaIoana Florea

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Question 289 / 297: (SCJP 6) Find the errors in:
Choose 3

static class X // line 1
{
    static { a = 3; } // line 2 

    static X() {} // line 1 
    
    int a;
    static int b; 
    
    void doX(){  System.out.println("In X" + a + b);  }  // line 3  
    static void doStuff() {   System.out.println("In X" + /*a*/ + b);   } // line 4
}   

public class Test extends X
{
    public static void main(String[] args) 
    {
        X x = new X();  
        
        doStuff(); 
        x.doStuff(); // line 5
        X.doStuff(); // line 5.2
    }
    
    void doStuff() // line 6
    {
        System.out.println("In Test");
    } 
}

lines 1 - "static" can not mark classes and constructors

line 2 - "static" can not mark an initialization block {}

lines 2 - "b" must be used instead of "a" as in a static context(method/init block) it's possible to access only static variables

lines 3 - a non-static context can access only non-static variables

line 4 - a static method can access non-static variables("a"), but an error will occur because of the way "a" is commented leaving two "+" inside the print()

line 6 - to be a valid override it must not be less accessible, so it must keep its static status + "static" is needed also because it's called directly in the static method main()

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Explanation:

Code marked with "static" belongs only to the class(will not be created per obj). Therefor in a static method you can call only static vars, as the non-static/object ones might not exist yet.

So it's possible to call static members on their class without an obj: line 5.2.

But it's OK to call it on an obj too: line 5 (as all objs can access everything inside their class).

line 1 - a constructor is used only when an object is created with the "new" operator

line 2 - a static initialization block runs to initialize the static vars, so after the class and its static vars are created for the first time in memory by the JVM.

lines 4 - the comments are needed bc in a static method you can NOT use non-static vars. The reverse is valid: non-static methods can use static vars bc they exist already since the class was created.

If the app is corrected it will print:

In Test

In X3

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