java数据类型

数值类型

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public static void main(String[] args){
        int i1 = 10;//十进制
        int i2 = 010;//八进制
        int i3 = 0x10;//十六进制
 
        System.out.println(i1);
        System.out.println(i2);
        System.out.println(i3);
 
        //浮点数扩展：使用BigDecimal
        //float 有限 离散 舍入误差 大约
 
        float f = 0.1f;//0.1
        double d = 1.0/10;//0.1
        float d1 = 2323232323232323f;
        float d2 = d1 + 1;
 
        System.out.println(f==d);
        System.out.println(d1==d2);
 
        //字符扩展
        char c1 = 'a';
        char c2 = '中';
        System.out.println(c1);
        System.out.println((int)c1);
        System.out.println(c2);
        System.out.println((int)c2);
 
        //unicode
        char c3 = '\u0061';
        System.out.println(c3);
 
        String sa = new String("haha");
        String sb = new String("haha");
        System.out.println(sa==sb);//false
 
        String sc = "haha";
        String sd = "haha";
        System.out.println(sc==sd);//true
}

类型转换

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public static void main(String[] args){
    //byte short char int long float double由低到高
    //不能对布尔值转换
    int i = 128;
    byte b = (byte)i;//强制类型转换
    System.out.println(i);//128
    System.out.println(b);//内存溢出-128
 
    System.out.println((int)2.2);//2
    System.out.println((int)-2.2f);//-2
 
    //操作大数注意溢出
    int money = 10_0000_0000;
    int years = 20;
    int total = money * years;//-1474836480 溢出
    long total2 = money * years;//默认是int, 转换之前已经出错了
 
    long total3 = money*((long)years);//先把一个转为long
}

变量

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变量类型

public class Animal {
    //类变量 static 从属类
    static double weight = 2222;
    static final double K = 22;//修饰符不区分先后顺序
    
    //实例变量：从属于对象；不初始化时为默认值
    //除了基本类型默认初始值都是null
    String name;
    int age;
    
    public static void main(String[] args){
        //局部变量：必须声明和初始化值
        int i = 1;
        System.out.println(i);
        
        Animal animal = new Animal();
        System.out.println(animal.age);
        
        //类变量
        System.out.println(weight);
    }
    
}

变量命名规范

类成员变量、局部变量、方法名：首字母小写加驼峰 monthSalary

类名：首字母大写加驼峰 GoodMan

常量：大写字母和下划线MAX_VALUE

基本运算符

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public static void main(String[] args){
    long a = 111111111111L;
    int b  = 111;
    short c = 10;
    byte d = 8;
 
    //只要有long，结果就是long类型
    System.out.println(a+b+c+d);//long
    System.out.println(b+c+d);//int
    System.out.println(c+d);//int
 
    //短路运算
    int aa = 8;
    boolean bb = (aa < 7) && (aa++>8);
    System.out.println(aa);
    
}