为什么Java的数组复制方法会这样？

Question

以下代码以各种方式使用Java的数组复制方法：

public static void main(String[] args) {
    char[] copyFrom={'s','n','a','d','g'};
    char[] copyto=new char[7];
    System.arraycopy(copyFrom, 1, copyto, 0, 3);
    System.out.println(new String (copyto));
    System.arraycopy(copyFrom, 1, copyto, 1, 3);
    System.out.println(new String (copyto));
    System.arraycopy(copyFrom, 1, copyto, 2, 3);
    System.out.println(new String (copyto));
    System.arraycopy(copyFrom, 1, copyto, 4, 3);
    System.out.println(new String (copyto));
}

这是输出：

nad
nnad
nnnad
nnnanad

为什么"n"会被重复

Answer 1

因为您使用了一个copyto数组，所以我在注释中向您展示

   char[] copyFrom={'s','n','a','d','g'};
    char[] copyto=new char[7]; // copyto = {'','','','','','',''}
    System.arraycopy(copyFrom, 1, copyto, 0, 3); // copyto = {'n','a','d','','','',''}
    System.out.println(new String (copyto));
    System.arraycopy(copyFrom, 1, copyto, 1, 3); // copyto = {'n','n','a','d','','',''}, 
// because first element 'n' stay from previous step

   System.out.println(new String (copyto));  
   System.arraycopy(copyFrom, 1, copyto, 2, 3); // from previous step you have {'n', 'n'} and replace only 2, 3 and 4 elements

//  {'n','n','n','a','d','',''}, 
    System.out.println(new String (copyto));
    System.arraycopy(copyFrom, 1, copyto, 4, 3); // from previous step you have {'n', 'n', 'n'} and replace only 4, 5 and 6 elements
    System.out.println(new String (copyto));
}

您应该每次创建新的copyto阵列：

public static void main(String[] args) {
    char[] copyFrom={'s','n','a','d','g'};
    char[] copyto=new char[7];
    System.arraycopy(copyFrom, 1, copyto, 0, 3);
    System.out.println(new String (copyto));
    copyto=new char[7];
    System.arraycopy(copyFrom, 1, copyto, 1, 3);
    System.out.println(new String (copyto));
    copyto=new char[7];
    System.arraycopy(copyFrom, 1, copyto, 2, 3);
    System.out.println(new String (copyto));
    copyto=new char[7];
    System.arraycopy(copyFrom, 1, copyto, 4, 3);
    System.out.println(new String (copyto));
}