Copy object to another object java

Question

I have an object A that contains for example:

class A{
    String elem1;
    Int elem2;
    ...get and set 
}

and i have a class B that contains same element and name of field:

class B{
    String elem1;
    Int elem2;
    ...get and set 
}

i want copy class A value into class B value without modify the class. How can solve? thanks.

Answer 1

There's no "clean" way to do this; you will need to manually copy every entry.

A a = ...;
B copy = new B();
copy.elem1 = a.elem1;
copy.elem2 = a.elem2;

Alternatively, you could use reflection, but it's performance costly and somewhat unclear, and will fail if there is any inconsistencies between the classes' field definitions.

A a = ...;
B copy = new B();
for (Field field : a.getClass().getDeclaredFields()) {
    Field copyField = copy.getClass().getDeclaredField(field.getName());
    field.setAccessible(true);
    copyField.setAccessible(true);
    if (!field.getType().isPrimitive()) {
        Object value = field.get(a);
        field.set(copy, value);
    } else if (field.getType().equals(int.class)) {
        int value = field.getInt(a);
        field.setInt(copy, value);
    } else if { ... } // and so forth
}

Answer 2

try this simple way

 firstObj.setElem1 (secObj.getElem1());

Answer 3

You can create a third class which would act as a factory, with a method taking an instance of A as a parameter, and returning an instance of B.

public final class MyFactory
    {
    public static B createBFromA (A instance)
        {
        B newInstance = new B ();
        newInstance.setXXX (instance.getXXX ());
        return newInstance;
        }
    }

The advantage of using an external factory is to isolate the code that creates your B instances from the rest of your code.

Answer 4

Better late than never but here is the answer I have found. You can use GSON to serialize the Object to String and then deserialize to the class B. Here is the tested example:

package JSON.test.obj_copy;
public class TestClassA {

    private String field1 = "pikachu";
    private Integer field2 = 1;
    private List<String> field3 = new ArrayList<String>(Arrays.asList("A", "B", "C"));
    private Obj field4 = new Obj();

    @Override
    public String toString(){
        return new StringBuilder("field1: " + field1 + "\n")
                .append("field2: " + field2 + "\n")
                .append("field3: " + field3 + "\n")
                .append("field4: " + field4)
                .toString();
    }
}

public class TestClassB {

    private String field1;
    private Integer field2;
    private List<String> field3;
    private Obj field4;

    @Override
    public String toString(){
        return new StringBuilder("field1: " + field1 + "\n")
                .append("field2: " + field2 + "\n")
                .append("field3: " + field3 + "\n")
                .append("field4: " + field4)
                .toString();
    }
}

public class Obj {
}

public class MainObjCopy {

    public static void main(String[] args){

        TestClassA a = new TestClassA();
        TestClassB b;
        Gson g = new Gson();
        String s1 = g.toJson(a);
        b = g.fromJson(s1, TestClassB.class);

        System.out.println(b);
    }
}

Output:

field1: pikachu
field2: 1
field3: [A, B, C]
field4: JSON.test.obj_copy.Obj@22d8cfe0