how to copy an Arrays

Array跟C语言差不多；后续需要更深入的应用实例。

//input
class ArrayDemo {
    public static void main(String[] args) {

        char[] copyFrom = { 'd', 'e', 'c', 'a', 'f', 'f', 'e',
                'i', 'n', 'a', 't', 'e', 'd' };
        char[] copyTo = new char[7];

        System.arraycopy(copyFrom, 2, copyTo, 0, 7);
        System.out.println(new String(copyTo));
    }
}
//output 
caffein

self-adding 自加自减

主要是先后问题，把它翻译成一个句子就不会错了。

//input 
class ConcatDemo {
    public static void main(String[] args) {
        int i = 3; 
        i++;
        System.out.println(i);  // i = 4 
        ++i;
        System.out.println(i); //i=5
        System.out.println(++i); //first i = i+1 = 6, then print i =6
        System.out.println(i++); //first print i = 6, then i = i +1 =7
        System.out.println(i); //i = 7
    }
}

//output 
4
5
6
6
7

Instances of and relations with extends and implements

主要是看extends ， implements ，和其关联的class 的关系。也是会写会懂..

//input 
class InstanceofDemo {
    public static void main(String[] args) {

        Parent obj1 = new Parent();
        Parent obj2 = new Child();

        System.out.println("obj1 instanceof Parent: "
                + (obj1 instanceof Parent));
        System.out.println("obj1 instanceof Child: "
                + (obj1 instanceof Child));
        System.out.println("obj1 instanceof MyInterface: "
                + (obj1 instanceof MyInterface));
        System.out.println("obj2 instanceof Parent: "
                + (obj2 instanceof Parent));
        System.out.println("obj2 instanceof Child: "
                + (obj2 instanceof Child));
        System.out.println("obj2 instanceof MyInterface: "
                + (obj2 instanceof MyInterface));
    }
}

class Parent {}
class Child extends Parent implements MyInterface {}
interface MyInterface {}

//output 
obj1 instanceof Parent: true
obj1 instanceof Child: false
obj1 instanceof MyInterface: false
obj2 instanceof Parent: true
obj2 instanceof Child: true
obj2 instanceof MyInterface: true

2018.6.20

源代码依旧来自java8官方文档，这里做个调试和存档