4

I have a pretty general question about java. I want to know if there is a simple way to recreate this c++ code in java:

class A 
{

public:
  int first;
  int second;

  A(const A& other) {
    *this = other;
  }

...
}

So basically a copy constructor where you can pass an existing object of A a new object of a in the constructor, and it will copy the contents and build an exact copy of the existing object of A.

trying

class A {
  int first;
  int second;

  public A(A other){        
    this = other;
  }
 ...
}

sadly doesn't work, since eclipse tells me "this" is not allowed on the left handside of an assignment since it's not a variable.

I know I would achieve the same results doing: 

class A {
      int first;
      int second;

      public A(A other){        
        this.first = other.first;
        this.second = other.second;

      }
     ...
    }

But I would like to know if there is an easier way, since sometimes you have a few more class variables.

Thanks in advance!

kalu
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    You could create a private constructor that takes in all of your variables and have all other constructors call that in some fashion. – Charles Jul 11 '17 at 19:07
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    You mean a constructor that takes in an instance of that object and then copies its data. Thats probably as simple as it gets. – Daedric Jul 11 '17 at 19:08
  • @Daedric excactly - but without having to copy its variables individually. (i was wondering if i had misused the word copy constructor after i submitted the question - Did this lead to confusion?) – kalu Jul 11 '17 at 19:11

3 Answers3

2

There is no easier way defined by the Java language, however there are some tricky techniques that may allow you to do so:

  1. Clone object via serialization: http://www.avajava.com/tutorials/lessons/how-do-i-perform-a-deep-clone-using-serializable.html : precondition - all properties of the class in structure has to be either primitive or of a classes marked as Serializable
  2. toString() -> fromString(String s) - corresponding methods has to be implemented
  3. POJO and beans can be easily reconstructed using intermediate XML / JSON representation with available libraries like Jackson etc.

To my knowledge the most efficient way after direct mapping is through serialization mechanism.

Vladimir L.
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1

What you have in the third version of that class is legal java that does the same thing as your C++ class, however I do not think there is an easier way than what you have written.

ja08prat
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1

Best way to recycle code:

class A {

   int first;
   int second;

   public A(int f, int s){        
      this.first = f;
      this.second = s;
   }

   public A(A a){
      this(a.first, a.second); // id use getters instead ofc.
   }
}
Daedric
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