java学习笔记（1）

1.初体验

编写HelloWorld程序

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

}

2.变量类型

public class VariableDemo {
    public static void main(String[] args) {
        // 定义byte类型,一个字节，范围-128到127
        byte b = -128;
        System.out.println(b);
        // 定义short类型
        short s = 100;
        System.out.println(s);
        // 定义int
        int i = 1000;
        System.out.println(i); 
        // 定义float
        float c = 3.14f;
        System.out.println(c);
        // 定义double
        double d = 3.14;
        System.out.println(d);
        // 定义boolean
        boolean x = true;
        System.out.println(x);
    }
}

3.算数运算符

package daigua.zhuwa;

public class OperatorDemo {
    public static void main(String[] args) {
        int a = 3;
        int b = 4;
        System.out.println(a + b);
        System.out.println(a - b);
        System.out.println(a * b);
        System.out.println(a / b);
        
        // 有浮点数参与才会显示小数
        float c = 3.0f;
        System.out.println(c / b);
        
        int d = 10;
        int e = 3;
        System.out.println(d % e); // 取余
        
        // 字符参与加法运算
        char f = 'A';
        System.out.println(f);
        System.out.println(a + f);
        // 字符串跟其他数据做加法 是做字符串的拼接
        System.out.println("hello" + a);
        System.out.println(a + b + "hello");
        
        // ++ -- 自增自减，单独使用时，放在前面或者后面效果一样
        int g = 10;
        System.out.println("g:" + g);
        g++;
        System.out.println("g:" + g);
        ++g;
        System.out.println("g:" + g);
        
//      int h = a;
//      int h = a++;
        int h = ++ a;
        System.out.println("a:" + a);
        System.out.println("h:" + h);

    }
}

4.赋值运算符

package daigua.zhuwa;
/*
 * 赋值运算符：
 * 基本： = 
 * 扩展： += -= *= /=
 * */

public class OperatorDemo2 {
    public static void main(String[] args) {
        int a = 10;
        System.out.println("a:" + a);
        
        // +=
        a += 20;
        System.out.println("a:" + a);
        
        // 注意，扩展的赋值运算符隐含了强制类型转换
        short s = 1;
        s += 1;
        System.out.println("s:" + s);
    }
}

5.关系运算符

package daigua.zhuwa;
/*
 * 关系运算符
 *      == != > >= < <=
 * */
public class OperatorDemo3 {
    public static void main(String[] args) {
        int a = 10;
        int b = 20;
        int c = 10;
        System.out.println(a == c);
        System.out.println(a == b);
        System.out.println(a < b);
        System.out.println(a < c);
        System.out.println(a <= c);
        System.out.println(a != b);
    }
}

6.逻辑运算符

package daigua.zhuwa;
/*
 * 逻辑运算符
 * & | ^ !
 * && ||
 * */
public class OperatorDemo4 {
    public static void main(String[] args) {
        int a = 3;
        int b = 4;
        int c = 5;
        
        // & 就是and，同时满足
        System.out.println((a>b) & (a>c));
        System.out.println((a>b) & (a<c));
        System.out.println((a<b) & (a<c));
        System.out.println((a<b) & (a>c));
        
        // | 就是or，一个满足
        System.out.println((a>b) | (a>c));
        System.out.println((a>b) | (a<c));
        System.out.println((a<b) | (a<c));
        System.out.println((a<b) | (a>c));
        
        // ^ 就是异或，不同为真
        System.out.println((a>b) ^ (a>c));
        System.out.println((a>b) ^ (a<c));
        System.out.println((a<b) ^ (a<c));
        System.out.println((a<b) ^ (a>c));
        
        // ! 非
        System.out.println(!(a>b));
        System.out.println(!(a<b));
        
        // && 跟&结果一样，区别在于有短路效应
        System.out.println((a>b) && (a>c));
        System.out.println((a>b) && (a<c));
        System.out.println((a<b) && (a<c));
        System.out.println((a<b) && (a>c));
        
        // || 跟|结果一样，区别在于有短路效应
        System.out.println((a>b) || (a>c));
        System.out.println((a>b) || (a<c));
        System.out.println((a<b) || (a<c));
        System.out.println((a<b) || (a>c));

    }
}

7.三元运算符

package daigua.zhuwa;
/*
 * 三元运算符
 * 关系表达式?表达式1:表达式2
 * */

public class OperatorDemo5 {
    public static void main(String[] args) {
        int a = 1;
        int b = 2;
        int c = (a > b)? a:b;
        System.out.println(c);
    }
}

8.键盘输入

package daigua.zhuwa;
import java.util.Scanner;
/*
 * 实现键盘录入，使用Scanner
 * */
public class MyScanner {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        
        System.out.print("请输入：");
        int i = sc.nextInt();
        
        System.out.println("结果："+ i);
    }
}