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

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:

• Forum post asking the same question
• Understanding Instance and Class Members
• Explanation of how classloader loads static variables

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

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;
    }
}