Does Java have a default copy constructor (like in

问题:

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• Why doesn't Java have a copy constructor? 9 answers

Does Java has a default copy constructor as C++? If it has one - does it remain usable if I declare another constructor (not a copy constructor) explicitly?

回答1:

Java does not have bulit-in copy constructors.

But you can write your own such constructors. See an example below:

class C{
    private String field;
    private int anotherField;
    private D d;

    public C(){}

    public C(C other){
        this.field = other.field;
        this.anotherField = other.anotherField;
        this.d = new D(other.d); //watch out when copying mutable objects; they should provide copy constructors, as well. Otherwise, a deep copy may not be possible
    }

    //getters and setters
}

class D{//mutable class

    //fields
    public D(D other){
        //this is a copy constructor, like the one for C class
    }
}

回答2:

Java does not have a default copy constructor. You'll need to define it yourself.

回答3:

There is a copy constructor (But not default one), but it should be called explicitly (In C++ it'll be implicitly called when needed):

public MyClass(MyClass toCopy) { 
   someField = toCopy.someField; 
}

回答4:

No, it doesn't have a default copy constructor. A default constructor.

You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn't have a no-argument constructor so you must verify that it does. If your class has no explicit superclass, then it has an implicit superclass of Object, which does have a no-argument constructor.

Usually I provide a one like,

public class CopyConEx {

      /**
      * Regular constructor.
      */
      public CopyConEx(type  field1, type field2) {
        this.field1 = field1;
        this.field2 = field2;
      }

      /**
      * Copy constructor.
      */
      public CopyConEx(CopyConEx aCopyConEx) {
        this(aCopyConEx.getField1(), aCopyConEx.getField2());   

      }

回答5:

Java support cloning but not using copy constructor. Please find below url on Java cloning.

http://adtmag.com/articles/2000/01/18/effective-javaeffective-cloning.aspx

回答6:

In Java the provision of an automatic copy constructor would be pointless.

Java doesn't need one since you can only have references to objects. (In C++ you can pass objects round by value so the grammar needs to allow for an automatic object copy).

If you need to take deep copies of object in Java, then implement Cloneable.

回答7:

Like C++, Java also supports copy constructor. But, unlike C++, Java doesn’t create a default copy constructor if you don’t write your own.

Also see write a Copy Constructor using Deep copy.