java小白学习2

java中的for、数组定义、集合定义

import com.sun.xml.internal.bind.v2.schemagen.xmlschema.Any;

import java.util.HashMap;
import java.util.Map.Entry;

public class Day2 {
    public Day2(Number[] arr) {
        //#java中单引号与双引号的区别
        {
            char c = '2';
            //char c2 = '22'; //error 只能一个字符
            //char c3 = "2"; //error 不能使用双引号
        }
        System.out.println("================================================================================");
        //#java for...
        {
            /*第1种*/
            for (int i = 0; i < arr.length; i++) {// i为局部变量，此处与JavaScript不同
                System.out.println(arr[i]);
            }
            //System.out.println(i);//build错误，java: 找不到符号
            /*第2种，直接取值*/
            for (Number v : arr) {// 同样v为局部变量，与JavaScript不同
                System.out.println(v);
            }
            //System.out.println(v);//build错误，java: 找不到符号
        }
        //#集合 JavaScript中的Object??
        System.out.println("================================================================================");
        {
            //HashMap<String, String>应该只是帮助阅读，主要由new HashMap<String, String>控制
            HashMap<String, String> setting = new HashMap<String, String>() {{
                put("a", "1");
                put("b", "2");
            }};
            for (String i : setting.keySet()) {
                System.out.println(i + "->" + setting.get(i));
            }
            //任何类型 <Object>
            HashMap<Object, Object> ObjectObj = new HashMap<Object, Object>() {{
                put("a", 11);
                put("b", "22");
                put(33, "ccc");
            }};
            for (Object i : ObjectObj.keySet()) {
                System.out.println(i + "->" + ObjectObj.get(i));
            }
            //任何类型 <Any>
            HashMap<Any, Any> anyObj = new HashMap() {{
                //任何类型, HashMap<Any, Any>帮助阅读，new HashMap时，不限制类型即可
                put("a", 11);
                put("b", "22");
                put(33, "ccc");
            }};
            System.out.println(anyObj.get("a"));
            System.out.println(anyObj.get("b"));
            System.out.println(anyObj.get(33));
            /** 报错咯
             for (Any i : anyObj.keySet()) {
             System.out.println(i + "->" + anyObj.get(i));
             }
             */
            //但是HashMap有4种遍历方法，可以选择这种
            for (Entry<Any, Any> entry : anyObj.entrySet()) {
                System.out.println(entry.getValue());
            }
        }

    }

    public static void main(String[] args) {
        new Day2(new Number[]{1, 2, 2.2, 3, .3});
    }
}

console

================================================================================
1
2
2.2
3
0.3
1
2
2.2
3
0.3
================================================================================
a->1
b->2
a->11
33->ccc
b->22
11
22
ccc
11
ccc
22