Array常用操作

Array常用操作

• 将数组a的一部分复制到数组b中,java.lang.System的方法arraycopy

  • System.arraycopy(Object src,int srcPos,Object dest,int destPos,int length);

• 将数组具体内容输出，直接syso数组变量会得到[Ltype;@xxxx的结果，要输出具体元素可以使用Arrays.toString()方法

• 复制一个新的数组，Arrays.copyOf(T[] original, int newLength),newLength小于原数组截断，长于原数组补null或0

• list转数组，Object[] toArray()与 T[] toArray(T[] a)方法，两个方法有一定区别

  • Object[] toArray()：返回一个新的Object[]数组，其实就是把list内部的数组，如ArrayList的elementData[]复制出来

  • T[] toArray(T[] a)：若a的长度小于list长度，则新建并复制一个数组再进行类型转换，若a的长度大等于list大小，则在a上直接复制，a长度大于list的size，a[zize]会被置为null，doc上说是为了识别list的长度（原list元素都是非空的情况下），这种情况比较少见，一般常见的用法是构造一个与原来list相同长度的数组：E[] arr = list.toArray(new E[list.size()])

    public static void main(String[] args) 
    {
        String[] src = {"1","2","3","4","5"};
        String[] dst = {"a","b","c","d"};
        int[] nums = {1,2,3,4,5};
    
        System.arraycopy(src,1,dst,1,2);    //dst被修改
        System.out.println(Arrays.toString(dst));   //[a, 2, 3, d]
        System.out.println(dst.getClass()); //class [Ljava.lang.String;
    
        System.out.println("a".getClass()); //class java.lang.String
    
        System.out.println(Arrays.toString(Arrays.copyOf(nums, 7)));    //[1, 2, 3, 4, 5, 0, 0]
    
        ArrayList<String> list = Lists.newArrayList("1","2","3","4","5");
        System.out.println(Arrays.toString(list.toArray(new String[list.size()]))); //[1, 2, 3, 4, 5]
    
        String[] a = {"a","b","c","d","e","f","g"};
        System.out.println(Arrays.toString(list.toArray(a)));   //[1, 2, 3, 4, 5, null, g]
    
    }