java 中的 clone()

clone() 是 Object 類中的函數

函數原型：

protected Object clone() throws CloneNotSupportedException
           

作用：java 中沒有拷貝構造函數，是以有 clone()

我們可以自己寫一個類，實作 Cloneable 接口

package test_clone;

public class Octagon extends GeometricObject implements Comparable<Octagon>, Cloneable {
    
    private double side;
    
    public Octagon() {
        this(1);
    }

    public Octagon(double side) {
        super();
        this.side = side;
    }
    
    public void setSide(double side) {
        this.side = side;
    }

    public double getArea() {
        return (2 + 4 / Math.sqrt(2)) * this.side * this.side;
    }
    
    public double getPerimeter() {
        return 8 * this.side;
    }

    @Override
    public int compareTo(Octagon o) {
        return (int) (this.side - o.side);
    }
    
    @Override
    public Octagon clone() {
        Octagon octagon = new Octagon();
        octagon.setSide(this.side - 1);
        return octagon;
    }
    
    @Override
    public String toString() {
        return "Octagon [side=" + side + "]";
    }
           

其中 GeometricObject 是自定義的一個類，如下

package test_clone;

public class GeometricObject {
    private String color;
    private boolean filled;
    
    public GeometricObject() {
        this.color = "white";
        this.filled = false;
    }

    public GeometricObject(String color, boolean filled) {
        super();
        this.color = color;
        this.filled = filled;
    }

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }

    public boolean isFilled() {
        return filled;
    }

    public void setFilled(boolean filled) {
        this.filled = filled;
    }

    @Override
    public String toString() {
        return "GeometricObject [color=" + color + ", filled=" + filled + "]";
    }
    
}
           

我們在類 Octagon 中可以不用覆寫 clone() 函數，即不寫：

@Override
    public Octagon clone() {
        Octagon octagon = new Octagon();
        octagon.setSide(this.side - 1);
        return octagon;
    }
           

那麼當調用 clone() 時，是預設的值的複制，我們覆寫後可以讓 side 減1。

測試代碼

package test_clone;

import java.util.Arrays;

public class TestClone {

    public static void main(String[] args) {
    
        Octagon octagon1 = new Octagon(5);
        Octagon octagon2 = octagon1.clone();
        System.out.println(octagon1.toString());
        System.out.println(octagon2.toString());
    }

}
           

運作結果：

補充

數組是預設實作了 Cloneable 接口的。

例如：

int[] a = new int[] {1,2,3,4,5};
        int[] b = a.clone();
        System.out.println(Arrays.toString(b));
           

結果：

說明：

Arrays.toString(b)

能夠将數組 b 中元素如圖輸出。

下面是重點：

那麼對于實作了 Cloneable 接口并覆寫了 clone() 函數的類 Octagon 來講，對 Octagon 的對象數組調用 clone() 會發生什麼？

public static void main(String[] args) {
        
        Octagon octagon1 = new Octagon(5);
        Octagon octagon2 = octagon1.clone();
        System.out.println(octagon1.toString());
        System.out.println(octagon2.toString());
        
        Octagon[] octagons1 = new Octagon[3]; 
        for (int i = 0; i < octagons1.length; i++) {
            octagons1[i] = new Octagon(i + 1);
        }
        Octagon[] octagons2 = octagons1.clone();
        System.out.println(Arrays.toString(octagons1));
        System.out.println(Arrays.toString(octagons2));
    }
           

輸出：

由 octagons1 數組克隆得到的 octagons2 數組中的值沒有對應 減1。

官方文檔解釋是：

Otherwise, 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.

對象數組調用 clone 方法時，用的是淺拷貝。

以上均為個人見解，結果均在 win10：eclipse 上運作所得，如有錯誤，歡迎指正。