Java编程思想 -- 数组相关

数组拷贝的比较

System.arraycopy()

Java标准类库提供的方法，用它复制数组比for循环快得多。
（1）对基本类型采用值复制。
（2）对对象采用引用复制。所以是‘浅拷贝’
（3）没有自动装箱和自动拆箱功能

        int[] a = {1, 2, 3, 4, 5};
        int[] b = new int[5];
        Integer[] c =  new Integer[5];

        System.out.println(Arrays.toString(b));
        System.arraycopy(a, 0, b, 0, a.length);
        System.out.println(Arrays.toString(b));

        System.out.println("----");
        //System.arraycopy（）不会执行自动包装盒自动拆包，两个数组必须要有相同的确切类型。
        //所以int[]数组拷贝到Integer[]数组会报错，java.lang.ArrayStoreException  
        System.out.println(Arrays.toString(c));
        System.arraycopy(a, 0, c, 0, a.length);
        System.out.println(Arrays.toString(c));

输出

[0, 0, 0, 0, 0]
Exception in thread "main" [1, 2, 3, 4, 5]
----
[null, null, null, null, null]
java.lang.ArrayStoreException
    at java.lang.System.arraycopy(Native Method)
    at Arrays测试.System.arraycogy.Test.main(Test.java:17)

Arrays.fill()

源码

    public static void fill(int[] a, int val) {
        for (int i = 0, len = a.length; i < len; i++)
            a[i] = val;
    }

作用：用同一个值填充各个位置，而对于对象而言，都是复制都一个应用进行填充

测试代码

public class Test {
    public static void main(String[] args) {
        int i = 100;
        int[] intArr = new int[10];
        Arrays.fill(intArr, i);
        System.out.println("用变量i填充int数组");
        System.out.println(Arrays.toString(intArr));        //[100, 100, 100, 100, 100, 100, 100, 100, 100, 100]
        System.out.println("修改变量i的值");
        i = 111;
        System.out.println(Arrays.toString(intArr));        //[100, 100, 100, 100, 100, 100, 100, 100, 100, 100]



        People p = new People("chuqin");
        People[] pArr = new People[10];
        Arrays.fill(pArr, p);
        System.out.println("用变量p填充People数组");
        System.out.println(Arrays.toString(pArr));          //[People [name=chuqin], People [name=chuqin], People [name=chuqin], People [name=chuqin], People [name=chuqin], People [name=chuqin], People [name=chuqin], People [name=chuqin], People [name=chuqin], People [name=chuqin]]
        System.out.println("修改变量p的值");
        p.setName("afang");
        System.out.println(Arrays.toString(pArr));          //[People [name=afang], People [name=afang], People [name=afang], People [name=afang], People [name=afang], People [name=afang], People [name=afang], People [name=afang], People [name=afang], People [name=afang]]

    }
}


class People{
    private String name;

    @Override
    public String toString() {
        return "People [name=" + name + "]";
    }

    public People(String name) {
        super();
        this.name = name;
    }

    public People() {
        super();
        // TODO Auto-generated constructor stub
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}