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So when is deserialized, first a instance is allocated and then the constructor of its first non-serializable superclass is called (which in turn calls 's constructor). It is unsafe for the JVM to run anything on it until it has been completely constructed."Īctually, it is possible to construct objects in Java without calling every constructor in the hierarchy, although not with the new keyword.įor example, when Java's serialization constructs an object during deserialization, it calls the constructor of the first non-serializable class in the hierarchy. "My guess is that, until a constructor has been called for every level of the heierarchy, the object is in an invalid state.
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#Stackoverflow why use webpack code#
You can gain access to arguments and local variables if you allocated the constructors' stack frames in reverse order, from derived classes to base classes, but this would require all frames to be active at the same time, wasting memory for every object construction to allow for the rare case of code that wants to touch local variables before base classes are constructed.
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Not to mention that the restriction applies to both super() and this() when presumably you know how to not break your own encapsulation when calling this(). And given that you can call super() or this() with all kinds of expressions in the parameter list, it seems like an artificial restriction that you can't compute any expressions before calling the other constructor. In both cases the programmer has to know how the variables are accessed during construction. Then that's just as much their problem as if they make class B above. In some cases this is easy for example, the constructor that takes two longs is trivial: public BigFraction(long numerator, long denominator) For all the other constructors, I just convert the input parameters to BigIntegers, and call the "canonical" constructor, because I don't want to duplicate all the work. The "canonical" constructor is the BigFraction(BigInteger numerator, BigInteger denominator) form. In that code, I have a BigFraction class, which has a BigInteger numerator and a BigInteger denominator. Are there any plans to allow this in future Java releases? (Or has Sun definitively said this is not going to happen?)įor an example of what I'm talking about, consider some code I wrote which I gave in this StackOverflow answer.Is there a good reason for this limitation?.But why can't you have statements before the constructor delegation, in order to compute the complex value to the other function? I can't think of any good reason, and I have hit some real cases where I have written some ugly code to get around this limitation. I assume this is because you shouldn't be allowed to modify any instance variables before the other constructor runs.
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In a constructor in Java, if you want to call another constructor (or a super constructor), it has to be the first line in the constructor. Why must delegation to a different constructor happen first in a Java constructor?