Java数组列表和对象传递

Question

I have a list of string in my code. I am passing that list of string to a function and modifying it.( I am adding/deleting few elements from the list).But i dont want these changes to get reflected in the caller function. The changes should be reflected only in the callee function. But because objects are passed by reference, I think the changes are getting reflected in both functions. How can I avoid this. Please help

Answer 1

Inside the method, you can explicitly make a copy of the list:

private void method(ArrayList<String> list) {
    ArrayList<String> copy = new ArrayList<String>(list);
    // Rest of the method
}

Answer 2

您必须先复制该对象，然后再将其传递给函数。

Answer 3

在sub方法中使用原始数组列表的新ArrayList。

Answer 4

You can use Clone method.

caller(ArrayList<String> a)
{
    callee((List<String>)a.clone());
}

callee(List<String> aCloned)
{

}

Answer 5

Typically the clone method is not recommended. See this clone(): ArrayList.clone() I thought does a shallow copy

However you can try this method

public void doOperation(List<String> list){
    List<String> duplicateList = new ArrayList(list);
    // add, or delete stuff on duplicateList
}

Here, you use the constructor of the ArrayList to give you a new copy of the list that is passed in.

Answer 6

You have to create new list from old list manually using looping and need to pass to that method. ie

   List<String> list=new ArrayList<>();
    list.add("first");
    list.add("last");
    List<String>list2 =new ArrayList<String>();
    for(String value:list)
    {
    list2.add(new String(value));
    }
    System.out.println("list2-->"+list2);

Because new ArrayList<String>(list); is give a new reference of list but the object still have a same reference.

Edit:

public class ArrayCopy {


    public static void main(String[] args) {

        ArrayCopy arrayCopy=new ArrayCopy();
        arrayCopy.copyData();

    }
    public void copyData()
    {
        List<Test> oldList=new ArrayList<Test>();
        oldList.add(new Test("1",10));
        oldList.add(new Test("2",45));
        List<Test> newList =new ArrayList<Test>(oldList);
        System.out.println("newList-->"+newList);

        /**
         * New Copy of Data
         */
        List<Test> newList1 =new ArrayList<Test>();
        for(Test test:newList)
        {
            newList1.add(copyProperty(test));
        }
        System.out.println("newList-->"+newList1);
    }
    private Test copyProperty(Test test)
    {
        Test newTest=new Test(test.getId(), test.getNumber());
        return newTest;
    }
    class Test{
        String id;
        int number;
        public Test(String id,int number) {
            this.id=id;
            this.number=number;
        }
        public String getId() {
            return id;
        }
        public void setId(String id) {
            this.id = id;
        }
        public int getNumber() {
            return number;
        }
        public void setNumber(int number) {
            this.number = number;
        }
    }
}