Java数组克隆奇怪的行为

Question

I'm trying to clone an instance of a custom class I made called CSP. I have one instance called csp and I want to make a clone of csp called cspclone. Here is what I'm using to do that:

CSP cspclone = new CSP((csp.x).clone(), (csp.d).clone(), (csp.c).clone());

For some reason though when I pass cspclone to a method that modifies it csp gets modified also as if I forgot the .clone() functions but I didn't! Why is this happening?!

Answer 1

Override the clone method in CSP :

public class CSP {
    private String aField;
    private int[] array;
    private int[][] twoDArr;
    private List<ALContent> list; //here ALContent also needs to override clone properly

    @Override
    public Object clone() {
        CSP clone = new CSP();
        clone.aField = this.aField;
        clone.array = new int[this.array.length];
        System.arraycopy(this.array, 0, clone.array, 0, this.array.length);

        clone.list = new ArrayList<ALContent>();
        for(ALContent content : this.list) {
            clone.list.add(content.clone()); //make sure you add the clone of the content
        }

        clone.twoDArr = new int[this.twoDArr.length][];
        for(int i=0; i<this.twoDArr.length; i++) {
            clone.twoDArr[i] = new int[this.twoDArr[i].length];
            System.arraycopy(this.twoDArr[i], 0, clone.twoDArr[i], 0, this.twoDArr[i].length);
        }

        return clone;
    }
}

Then you can do:

CSP csp = new CSP();
CSP cspClone = (CSP) csp.clone();

Answer 2

如果您的数组类型属性使用System.arraycopy

Answer 3

According to http://download.oracle.com/javase/1.3/docs/api/java/lang/Object.html#clone%28%29

this method creates a new instance of the class of this object and initializes all its fields with exactly the contents of the corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned. Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation.

You might have to override the clone method and clone() a reference type attribute within the object (ie, perform deep copy operation).

Answer 4

To solve your problem you need deep cloning. The default clone method does a shallow copy. See Object.clone() .

Here are some approaches. All have advantages and drawbacks.

• Override: Override the clone() method.
• Serialization: Write the object to a stream and a read it back from the stream.
• Reflection: Here is a tutorial on reflection .
• Use a deep close library, such as Java Deep-Cloning library .

Here are several other stackoverflow discussions of cloning.

• Java: recommended solution for deep cloning/copying an instance
• Deep clone utility recomendation
• How to clone ArrayList and also clone its contents?