Objective 5, Unassigned variables

State the effect of using a variable or array element of any kind when no explicit assignment has been made to it.

Variables

You could learn to program in Java without really understanding the agenda behind this objective, but it does represent valuable real world knowledge. Essentially a class level variable will always be assigned a default value and a member variable (one contained within a method) will not be assigned any default value. If you attempt to access an unassigned variable an error will be generated. For example

class MyClass{
        public static void main(String argv[]){
                int p;
                int j = 10;
                j=p;
        }
}

This code will result in an error along the lines

"error variable p might not have been assigned"

This can be viewed as a welcome change from the tendency of C/C++ to give you enough rope by leaving an arbitrary value in p. If p had been defined at class level it would have been assigned its default value and no error would be generated.

class MyClass{
static int p;
        public static void main(String argv[]){ 
                int j = 10;
                j=p;System.out.println(j);
                }
}

The default value for an integer is 0, so this will print out a value of 0.

The default values for numeric types is zero, a boolean is false and an object reference is the only type that defaults to a null.

Arrays

Learning this part of the objective requires understanding a simple rule. The value of the elements of an array of any base type will always be initialised to a default value, wherever the array is defined. It does not matter if the array is defined at class or method level, the values will always be set to default values. You may get questions that ask you what value is contained in a particular element of an array. Unless it is an array of objects the answer will not be null (or if they are being particularly tricky NULL).

Questions

Answer 1)

Given the following code what will element b[5] contain?

public class MyVal{
public static void main(String argv[]){
        MyVal m = new MyVal();
        m.amethod();
}


public void amethod(){
boolean b[] = new boolean[5];
        }

}

1) 0
2) null
3) ""
4) none of these options

Question 2)

Given the following constructor what will element 1 of mycon contain?

MyCon(){
        int[] mycon= new int[5];
}

1) 0
2) null
3) ""
4) None of these options

Question 3)

What will happen when you attempt to compile and run the following code?

public class MyField{
int i=99;
public static void main(String argv[]){
        MyField m = new MyField();
        m.amethod();
        }

void amethod(){
        int i;
        System.out.println(i);
        }

}

1) The value 99 will be output
2) The value 0 will be output
3) Compile time error
4) Run time error

Question 4)

What will happen when you attempt to compile and run the following code?

public class MyField{
String s;
public static void main(String argv[]){
        MyField m = new MyField();
        m.amethod();
        }
void amethod(){
        System.out.println(s);
        }
}

1) Compile time error s has not been initialised
2) Runtime error s has not been initialised
3) Blank output
4) Output of null

Answers

Answer 1)

4) none of these options

Sneaky one here. Array element numbering starts at 0, therefore there is no element 5 for this array. If you were to attempt to perform

System.out.println(b[5])

You would get an exception.

Answer 2)

1) 0

A constructor acts no different to any other method for this purpose and an array of integers will be initialised to contain zeros wherever it is created.

Answer 3)

3) Compile time error

You will get a compile time error indicating that variable i may not have been initialised. The classs level variable i is a red herring, as it will be shadowed by the method level version. Method level variables do not get any default initialisation.

Answer 4)

4) Output of null

A variable created at class level will always be given a default value. The default value of an object reference is null and the toString method implicitly called via System.out.println will output null

Other sources on this topic

The Java Language Specification
java.sun.com/docs/books/jls/second_edition/html/typesValues.doc.html#10946

Jyothi Krishnan on this topic at
www.geocities.com/SiliconValley/Network/3693/obj_sec4.html#obj12