CODEWITHSUNDEEP
javaconstructorfinalsupertype
Amr Bekhit
Amr Bekhit

Reputation: 4813

Cannot reference "X" before supertype constructor has been called, where x is a final variable

Consider the following Java class declaration:

public class Test {

    private final int defaultValue = 10;
    private int var;

    public Test() {
        this(defaultValue);    // <-- Compiler error: cannot reference defaultValue before supertype constructor has been called.
    }

    public Test(int i) {
        var = i;
    }
}

The code will not compile, with the compiler complaining about the line I've highlighted above. Why is this error happening and what's the best workaround?

Upvotes: 92

Views: 85284

Answers (6)

Sherif Eldeeb
Sherif Eldeeb

Reputation: 229

You are referencing to a variable that doesn't exist yet, if it was static it would exist even before the constructor itself.

But you will face another problem, as defaultValue became static, so all other instances may share the same value which you may don't like it to be:

public class Test {

    private final int defaultValue = 10; //this will exist only after calling the constructor
    private final static int value2= 10; //this exists before the constructor has been called
    private int var;

    public Test() {
       // this(defaultValue);    // this method will not work as defaultValue doesn't exist yet
    this(value2); //will work
    //this(10); will work
    }

    public Test(int i) {
        var = i;
    }
}

Upvotes: 6

Hardik Mehta
Hardik Mehta

Reputation: 887

constructor is called at the time of object creation so no reference to varable is recognized by compiler as compiler has no knowledge regarding instance variable as object is not created yet.

Upvotes: 0

Amr Bekhit
Amr Bekhit

Reputation: 4813

The reason why the code would not initially compile is because defaultValue is an instance variable of the class Test, meaning that when an object of type Test is created, a unique instance of defaultValue is also created and attached to that particular object. Because of this, it is not possible to reference defaultValue in the constructor, as neither it, nor the object have been created yet.

The solution is to make the final variable static:

public class Test {

    private static final int defaultValue = 10;
    private int var;

    public Test() {
        this(defaultValue);
    }

    public Test(int i) {
        var = i;
    }
}

By making the variable static, it becomes associated with the class itself, rather than instances of that class and is shared amongst all instances of Test. Static variables are created when the JVM first loads the class. Since the class is already loaded when you use it to create an instance, the static variable is ready to use and so can be used in the class, including the constructor.

References:

Upvotes: 117

amicngh
amicngh

Reputation: 7899

Until your object is not been constructed default values to the variables won't be set hence if you want their default values to be set at the time of construction make them static or explicitly set them before.

Upvotes: 1

Himanshu Mohta
Himanshu Mohta

Reputation: 1043

Rule : Every constructor must execute super class's constructor before executing itself.

So the first line of every constructor is super() or may be this() and you are send the defaultValue to the this class constructor which(defaultValue) is not existed yet hence there is compile time error.

You can made defaultValue as static and since static variable is created as class is loaded to memory so defaultValue is available at the line this(defaultValue).

Upvotes: 4

Jigar Joshi
Jigar Joshi

Reputation: 240928

It is because the defaultValue is an member of Test 's instance which is under construction (not yet created)

If you had it static it were loaded when your class loads by classloaders

Upvotes: 9

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