Java基礎(chǔ)

簡(jiǎn)單Java程序

Hello World

public class test {
    public static void main(String args[]) {
        System.out.println("Hello World!");
    }

注釋

java有三種注釋:
1.單行注釋

System.out.println("Hello World!");//這是一條單行注釋

2.多行注釋

/*這是一條多行注釋
    第二行
    第三行*/

3.自動(dòng)生成文檔型注釋

/**
  * 文檔型注釋
 * @author 418096798
 *
 */
public class test {
    public static void main(String args[]) {
        System.out.println("Hello World!");//帶換行打印
        System.out.print("Hello World!");/*不帶換行打印*/
    }

}

數(shù)據(jù)類型

整形

浮點(diǎn)型

char型

布爾型

變量

變量初始化

int age;

初始化常量后可賦值

int age = 17;
//一年后...
int age - 18;

常量

利用final聲明常量，聲明后不可更改

final int ID = 001;//ID不可更改

運(yùn)算符

運(yùn)算符優(yōu)先級(jí)

字符串

子串

String hello = "Hello wrold";
String string = hello.substring(1, 4);//ell

拼接

String hello = "Hello";
String world = "world";
String message = hello + world;//HelloWorld

檢測(cè)字符串是否相等

String hello = "Hello";
String world = "world";
System.out.println(hello.equals(world));//false

字符串長(zhǎng)度

String hello = "Hello";
int length = hello.length();
System.out.println(length);//5

大小寫

String hello = "Hello";
String upper = hello.toUpperCase();
String lower = hello.toLowerCase();
System.out.println(upper);
System.out.println(lower);

索引

String hello = "Hello";
int index = hello.indexOf("l");//2
/*從第4個(gè)開始索引*/
int index = hello.indexOf("l", 4);//-1

替代

String hello = "Hello";
String replace = hello.replace("el", "");

刪除字符串前后空格

String hello = "   Hello ";
String trim = hello.trim();

構(gòu)建字符串

String hello = "Hello";
String world = "World";
StringBuilder builder = new StringBuilder();
builder.append(hello);
builder.append(world);
String massae = builder.toString();
System.out.print(massae);//HelloWorld

輸入輸出

讀取輸入

import java.util.Scanner;

public class test {

    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.println("what is your name?");
        String name = in.nextLine();//讀取輸入的下一行內(nèi)容
        System.out.print("my name is ");
        System.out.println(name);
    }
}

格式化輸出

寬度為8、保留2位小數(shù)點(diǎn)

public static void main(String args[]) {
    double x = 1.0 / 3;
    System.out.printf("%8.2f", x);
}

printf中的轉(zhuǎn)換符

printf中的標(biāo)志

日期和時(shí)間的轉(zhuǎn)換符

import java.util.Date;

public class test {

    public static void main(String args[]) {
        System.out.printf("%tc", new Date());
    }
}