如果所有属性的值不为 null 或为空，如何将所有属性的值从对象的一个​​实例复制到另一个实例

Question

Given a class Person:

String name;
String surname;
String id;
String address;

I have an object obj1 with the following values:

name="Name"
surname=null
id="ABC123"
address="Here"

Through an api call I get the following json:

{
   "name":"John",
   "surname":"Doe",
   "id":"A1B2C3"
}

which gets mapped into an object obj2 like this:

name="John"
surname="Doe"
id="A1B2C3"
address=null

I want to copy all non-null (or empty string) values of obj2 into obj1 so the final result is this:

name="John"
surname="Doe"
id="A1B2C3"
address="Here"

I have two problems. The first one is that I don't want to have to manually type the get/set call for each attribute of the object. The second problem, is that I want the method to work for any type of object with no or minimal changes.

At the very least, I need the first problem solved. The second one is optional, but would be great to learn a solution too.

Answer 1

You can use reflection to get all the instance fields and use Field#set to copy over non-null values.

try {
    for (Field field : Person.class.getDeclaredFields()) {
        if (!Modifier.isStatic(field.getModifiers())) {
            field.setAccessible(true);
            Object val = field.get(obj2);
            if (val != null) {
                field.set(obj1, val);
            }
        }
    }
    System.out.println(obj1);
} catch (IllegalAccessException e) {
    // Handle exception
}

Demo

Answer 2

public static void main(String[] args) {
    MyClass obj1 = new MyClass(1,"1");
    MyClass obj2 = new MyClass(2,"2");
    System.out.println(obj2);
    copy(obj1, obj2);
    System.out.println(obj2);
}

public static MyClass copy(MyClass obj1, MyClass obj2){
    Arrays.stream(obj1.getClass().getDeclaredFields()).forEach(f -> {
        try {
             f.set(obj2, f.get(obj1));
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    });
    return obj2;
}

static class MyClass {
    int myInt;
    String myString;
    String emptyString = null;

    public MyClass(int myInt, String myString) {
        this.myInt = myInt;
        this.myString = myString;
    }

    @Override
    public String toString() {
        return "MyClass{" +
                "myInt=" + myInt +
                ", myString='" + myString + '\'' +
                ", emptyString='" + emptyString + '\'' +
                '}';
    }
}

in case you have private fields you can use

f.setAccessible(true);

but I don't suggest that.

Answer 3

Reading your question, seems like, you have two classes, in two distinct projects, which, exchange information in json.

So, It is easy if you can use Jackson(default in spring)

You could annotate the target class with the annotation of jackson @JsonInclude(JsonInclude.Include.NON_NULL)