Primitive types

  • Primitive types: for storing simple values
  • Refrences: for storing complex objects
Type Bytes Range
byte 1 [-128, 127]
short 2 [-32k, 32k]
int 4 [-2B, 2B]
long 8
float 4
double 8
char 2 A, B, C, …
boolean 1 true / false

Reference types

  • Primitive types: numbers, characters, booleans
  • Refrences: data, mail message
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public class Main {

    public static void main(String[] args) {
        byte age = 21;
        Date now = new Date();
        System.out.println(now);
    }
}

  • primitive types 存储直接存储值

  • reference types 存储地址

Casting and Parse

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public class Main {

    public static void main(String[] args) {
        // Implicit casting
        // byte > short > int > long > float > double
        short x = 1;
        int y = x + 2;

        // Explicit casting
        double a = 1.1;
        int b = (int)a + 2;

        // Parse
        String m = "1";
        int y = Integer.parseInt(m) + 2;
    }
}

Math

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public class Main {

    public static void main(String[] args) {
        // 四舍五入
        int round = Math.round(1.1F);

        // 取上取下
        int ceil = (int)Math.ceil(1.1F);
        int floor = (int)Math.floor(1.1F);

        // 取大取小
        int max = Math.max(1, 2);
        int min = Math.min(1, 2);

        // 随机
        double random = Math.random();
    }
}

Formating numbers

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public class Main {

    public static void main(String[] args) {
        NumberFormat currency = NumberFormat.getCurrencyInstance();
        String result = currency.format(12345);
        System.out.println(result);
    }
}

Reading input

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public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Nom: ");
        String name = scanner.next().trim();
        String name = scanner.nextLine().trim();
        System.out.print("Âge: ");
        byte age = scanner.nextByte();
        System.out.println("Tu es " + name);
        System.out.println("Tu as " + age);
    }
}