System class (system class)

The java.lang.System class provides a large number of static methods (no need to guide the package), which can obtain system-related information or system-level operations. In the API documentation of the System class, the commonly used methods are:

public static long currentTimeMillies(): returns the current time in milliseconds

/*
public static long currentTimeMillies()：返回以毫秒为单位的当前时间
用来程序的效率
验证for循环打印数字1-9999需要使用的时间
 */
private static void demo01(){
    long start = System.currentTimeMillis();
    for (int i = 1; i < 10000; i++) {
        System.out.println(i);
    }
    long end = System.currentTimeMillis();
    System.out.println(end-start);
}

public static void arraycopy(Object str,int srcPos,Object dest,int destPos,int length): copy the specified data in the array to another array

/*
public static void arraycopy(Object src,int srcPos,Object dest,int destPos,int length)：
将数组中指定的数据拷贝到另一个数组中
参数：src--源数组
    srcPos--源数组中的起始位置（索引）
    dest--目标数组
    destPos--目标数据中的起始位置
    length--要复制的数组元素的数量
练习：将src数组中的前3个元素，复制到dest数组的前3个位置上，
复制元素前：src数组元素[1,2,3,4,5]，dest数组元素[6,7,8,9,10]
复制元素后：src数组元素[1,2,3,4,5]，dest数组元素[1,2,3,9,10]
 */
private static void demo02(){
    int[] src = {1,2,3,4,5};
    int[] dest = {6,7,8,9,10};
    System.arraycopy(src,0,dest,0,3);
    // 打印数组的方法
    System.out.println(Arrays.toString(src));
    System.out.println(Arrays.toString(dest));

}

Knowledge point recall: the tool class Arrays.toString(Object obj) converts the array into a string in the default format [xx,xx,xx]