System.arraycopy()方法详解

参考：https://blog.****.net/qq_32440951/article/details/78357325?utm_source=blogxgwz0

System中提供了一个native静态方法arraycopy(),可以使用这个方法来实现数组之间的复制

import java.util.Arrays;

import static java.lang.System.arraycopy;

public class Solution4 {
    public static void main(String[] args) {
        int[] arr1 ={1,2,3,4,5};
        int[] arr2= {11,12,13,14,15};

        arraycopy(arr1, 3, arr2, 2, 2);
        System.out.println(Arrays.toString(arr2));
    }
}
//[11, 12, 4, 5, 15]

关于次方法的示例代码1：

public class Solution3 {
    private static String method(String s) {
// TODO Auto-generated method stub
        if (s == null || s.length() == 0) {
            return null;
        }
        int count = 0;
        for (int i = 0; i < s.length(); i++) {//计算空格数目
            if (s.charAt(i) == ' ') {
                count++;
            }
        }
        int oldLen = s.length();
//扩展数组的长度
        int newLen = s.length() + 2 * count;
        char[] newc = new char[newLen];
        /**
         * arraycopy(s.toCharArray(), 0, newc, 0, s.length())
         *String.toCharArray 方法,作用：将字符串转换为字符数组。
         *
         */
        System.arraycopy(s.toCharArray(), 0, newc, 0, s.length());//index=0,相当于数组copy

//定义两个游标，i指向old组中的当前值，j指向newc数组中初始化的值，然后从后往前初始化newc数组，直到i指向0或者j追上i。
        int i = oldLen - 1, j = newLen - 1;
        while (i >= 0 && i < j) {
            if (newc[i] == ' ') {
                newc[j--] = '0';
                newc[j--] = '2';
                newc[j--] = '%';


            } else {
                newc[j--] = newc[i];
            }
            i--;
        }
        String newStr = new String(newc);//转换成字符串
        return newStr;
    }
    public static void main(String[] args) {
        String s = "1 2 3 4 5 6 7 8 9 0";
        s = method(s);
        System.out.println(s);

    }

}

2

/*System中提供了一个native方法arraycopy()*/
public class SsytemArrayCopy {
    public static void main(String[] args) {
        User [] users=new User[]{new User(1,"admin","[email protected]"),new User(2,"maco","[email protected],com"),new User(3,"kitty","[email protected],com")};//初始化对象数组
        User [] target=new User[users.length];//新建一个目标对象数组
        System.arraycopy(users, 0, target, 0, users.length);//实现复制
        System.out.println("源对象与目标对象的物理地址是否一样："+(users[0] == target[0]?"浅复制":"深复制"));
        target[0].setEmail("[email protected]");
        System.out.println("修改目标对象的属性值后源对象users：");
        for (User user : users){//User user   相当于User user=new User();会调用构造函数User{}的方法
            System.out.println(user);
        }
    }
}
class User{
    private Integer id;//实例变量
    private String username;
    private String email;
    //无参构造函数
    public User() {	}
    //有参的构造函数
    public User(Integer id, String username, String email) {
        super();//调用父类的构造函数
        this.id = id;//注意this()与this的区别，
        this.username = username;
        this.email = email;
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username = username;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    @Override
    public String toString() {//此处才为方法
        return "User [id=" + id + ", username=" + username + ", email=" + email
                + "]";
    }
}