学习心得总结
package Demo0126;
public class IntegerDemo01 {
public static void main(String[] args) {
Integer it1 = new Integer(10);
System.out.println("it1 = " + it1);
//将Integer类型的对象，转变为int的数值，叫做手动装箱
int i =it1.intValue();
System.out.println(i);
System.out.println("-------------");
Integer integer = Integer.valueOf(i);
System.out.println(integer);
Integer it2 = new Integer(20);
System.out.println("it2 = " + it2);
System.out.println("============");
Integer i1 = 12;
int i2 = i1;
System.out.println(i1);
System.out.println(i2);
System.out.println("----------------");
String string = "123";
int i3 = Integer.parseInt(string);
System.out.println(i3);
}
}
package Demo0126;
import jdk.internal.dynalink.beans.StaticClass;
import java.math.BigDecimal;
import java.util.concurrent.LinkedTransferQueue;
public class BigDecimalTest01 {
public static void main(String[] args) {
double d1 = 0.1 + 0.2;
System.out.println(d1);
System.out.println("-----------");
BigDecimal bd1 = new BigDecimal("0.1");
BigDecimal bd2 = new BigDecimal("0.23");
System.out.println(bd1.add(bd2));
System.out.println(bd1.subtract(bd2));
System.out.println(bd1.multiply(bd2));
System.out.println(bd1.divide(bd2,BigDecimal.ROUND_HALF_UP));
}
}
package Demo0126;
public class StringDemo01 {
public static void main(String[] args) {
String str1 = "abc";
String str2 = "123";
System.out.println(str1 == str2);
String str3 = new String("abc");
String str4 = new String("abc");
System.out.println(str3 ==str4);
System.out.println("----------");
String str5 = "abc";
System.out.println(str1 == str5);
}
}
package Demo0126;
public class StringDemo02 {
public static void main(String[] args) {
String s = "cheng NB";
String s1 = "NB";
System.out.println(s.length());//返回字符串字符序列的长度
System.out.println(s.charAt(3));//返回字符串指定位置的字符
System.out.println(s.contains("NB"));//判断当前字符串是否包含参数指定的内容
System.out.println(s.toLowerCase());//将所有字母小写
System.out.println(s.toUpperCase());//将所有字母大写
System.out.println(s.trim());//返回去掉签到和后继空白的字符串
System.out.println(s.startsWith("zh"));//判断字符串是否以参数字符串开头
System.out.println(s.endsWith("NB"));//判断字符串是否以参数字符串结尾
System.out.println(s.codePointAt(3));// 方法返回字符串中指定索引处字符的Unicode值。
System.out.println(s.codePointBefore(3));//方法返回字符串中指定索引之前的字符的Unicode值。
System.out.println(s.compareTo(s1));//两字符串长度不一，参与比较的两个字符串如果首字符相同，则比较下一个字符，直到有不同的为止，返回该不同的字符的asc码差值 。
}
}
package Demo0126;
public class StringBuilderDemo01 {
public static void main(String[] args) {
StringBuilder sb = new StringBuilder();
System.out.println("初始容量为: " + sb.capacity());//返回当前容量
System.out.println("长度为：" + sb.length());//返回当前长度
System.out.println("____________");
StringBuilder sb1 = new StringBuilder();
System.out.println("初始容量为: " + sb1.capacity());
System.out.println("长度为：" + sb1.length());
System.out.println("___________________");
System.out.println(sb1.insert(0, "cheng"));//插入字符串insert
System.out.println(sb1.append("大帅逼"));//追加字符串append
System.out.println(sb1.delete(0, 8));//删除字符串delete，前闭后开区间
System.out.println(sb1.replace(0, 5, "haha"));//替换字符串replace
System.out.println(sb1.reverse());//字符串反转reverse
System.out.println(sb1.indexOf("h"));//查找字符串出现位置indexOf
}
}
package Demo0126;
import java.text.SimpleDateFormat;
import java.util.Date;
public class DateTest01 {
public static void main(String[] args) {
Date date = new Date();
/*System.out.println(date);
Date date1 = new Date(10000);
System.out.println(date1);*/
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String format = simpleDateFormat.format(date);
System.out.println(format);
}
}