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

问题

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?

回答1:

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

回答2:

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

回答3:

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).

回答4:

you are referencing to a variable that dosen't exists yet , if it was static so it will be exists even before the constructor itself

but you will face another problem , as defaultValue became static , so all the other instances may share the same value whitch you may dont like it to be ,

public class Test {

    private final int defaultValue = 10; //this will be exists only after calling the contractor
    private final static int vakue2= 10; //this is exists before the contractor has been called
    private int var;

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

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

回答5:

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.

回答6:

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.

回答7:

Actually this is not correct answer, as during the creation of object, fields initialization instructions execute before the constructor. You can debug the object creation process and see that on your own. I myself am confused about this problem as well.. for example, if you will change a bit and first constructor will be:

public Test(int i) {
   this(i, 0);
}
public Test (int a, int k) {
}

This will work.. so, when the first/null constructor calls another, it is not working for a strange reason, even if I explicitly call super(); before.

The most relevant explanation would be, that JVM loads declarations in the memory, but NO CONSTRUCTOR IS ABLE TO REACH ANY INSTANCE VARIABLE/FIELD before it gets executed entirely.

来源：https://stackoverflow.com/questions/10963775/cannot-reference-x-before-supertype-constructor-has-been-called-where-x-is-a

标签

java

constructor

final

supertype