java避免对象通过引用传递

Question

I'm facing problem with object cloning in java.

public class TestClass {

private int count;
private List<Integer> list;
public final int getCount() {
    return count;
}
public final void setCount(int count) {
    this.count = count;
}
public final List<Integer> getList() {
    return list;
}
public final void setList(List<Integer> list) {
    this.list = list;
}

}


public class Main {

public static void main(String[] args) {
TestClass obj = new TestClass();
obj.setCount(5);
List<Integer> temp = new ArrayList<Integer>();
temp.add(1);
temp.add(2);
obj.setList(temp);

TestClass newObj= obj;


System.out.println(newObj.getList().size());
obj.getList().add(3);
System.out.println(newObj.getList().size());


}

}

The output I'm getting here is 2,3. But my desired output is 2,2 but since java assign reference of "obj" to newObj. Is there anyway to avoid this? I have seen that serialize the object and deserialize it will give brand new reference to "newObj". But is there anyother efficient ways?

Answer 1

First, you aren't actually cloning which in java means you use the object.clone() method. clone makes a shallow copy of an object. You just made a 2nd reference to the same object.

In your code, calling

obj.getList().add(3);

Is the same as

newObj.getList().add(3);

Which is why it is printing the way it is.

You need to make a deep copy of the object so not only is the instance of TestClass different, but all the fields are different too.

The easiest way to do this is to make a copy method and/or a copy constructor

public class TestClass{
      public TestClass(){
      }

      //copy constructor
      public TestClass( TestClass copy){
         this.list = new ArrayList<Integer>(copy.list);
         this.count = copy.count;
      }

      ...
 }

Then to use it :

TestClass newObj= new TestClass(obj);