Java基本语法

System.out.print("sys_num:" + sys_num + "\n");

String str = "abcde";
String str1 = str.substring(0,3);  //abc

String str = "abcde";
int length = str.length()-1;
String str2 = str.substring(0,length);  //abcd

String str = "abcde";
int startIndex = str.length()-3;
int endIndex = str.length();
String str1 = str.substring(startIndex,endIndex);  //cde

String str = "abcde";
String str1 = str.substring(0, str.indexOf("c")); //ab

String str = "abcde";
String str1 = str.substring(0, str.indexOf("c"));
String str2 = str.substring(str1.length()+1, str.length()); //de

float f = 123.456f;
double d = 123.456;
int i1 = Math.round(f);  //四舍五入取整
long l1 = Math.round(d); //四舍五入取整
int i2 = (int)d;         //遇小数忽略

double d1 = Math.ceil(d);  //遇小数进1,返回double
double d2 = Math.floor(d); //遇小数忽略,返回double

double d1 = 123.45678;
double d2 = (double)(Math.round(d1*100))/100;

double d = 1234.34;           
String s = String.valueOf(d);

String s = "abcd";
int i = Integer.parseInt(s);

String s = "abcd";
double d = Double.parseDouble(s);

Map<String, Object> res = new HashMap<>();
res.put("sys_num", sys_num);           //写入 
String sysNum= String.valueOf(res.get("sys_num"));  //读取value转为String

Map<String, String> res = new HashMap<>();
res.put("sys_num", sys_num);    //写入 
String sysNum = res.get("sys_num");     //读取value
List<String> sys_num = new ArrayList<>(res.KeySet()); //读取key组成list

Map<String, Object> res = new LinkedHashMap<>();

HashMap<String, String > res  = new HashMap<String, String>(){{  
      put("sys_num",sys_num);  
      put("sys_name",sys_name);       
}};

sys-num:
  mymap: "{'a':'aaa','b':'bbb'}"

public class learnMap {
    @Value("#{${sys-num.mymap}}")
    private Map<String, String> numMap;
}