Java Basic

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

[root@Sugar j1]🐳 java Hello.java
Hello, world!

int n = 100;
n = 200; // 变量n赋值为200
int i2 = -2147483648;
int i3 = 2_000_000_000; // 加下划线更容易识别
int i4 = 0xff0000; // 十六进制表示的16711680
int i5 = 0b1000000000; // 二进制表示的512

long n1 = 9000000000000000000L; // long型的结尾需要加L
long n2 = 900; // 没有加L，此处900为int，但int类型可以赋值给long
int i6 = 900L; // 错误：不能把long型赋值给int

float f1 = 3.14f;
float f2 = 3.14e38f; // 科学计数法表示的3.14x10^38
float f3 = 1.0; // 错误：不带f结尾的是double类型，不能赋值给float

double d = 1.79e308;
double d2 = -1.79e308;
double d3 = 4.9e-324; // 科学计数法表示的4.9x10^-324

boolean b1 = true;
boolean b2 = false;
boolean isGreater = 5 > 3; // 计算结果为true
int age = 12;
boolean isAdult = age >= 18; // 计算结果为false

final double PI = 3.14; // PI是一个常量
double r = 5.0;
double area = PI * r * r;

var sb = new StringBuilder();

public class Main {
    public static void main(String[] args) {
        int n = -100;
        int x = n >= 0 ? n : -n;
        System.out.println(x);
    }
}

public class Main {
    public static void main(String[] args) {
        // 5位同学的成绩:
        int[] ns;
        ns = new int[] { 68, 79, 91, 85, 62 };
        System.out.println(ns.length); // 5
        ns = new int[] { 1, 2, 3 };
        System.out.println(ns.length); // 3
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.print("A,");
        System.out.print("B,");
        System.out.print("C.");
        System.out.println();
        System.out.println("END");
    }
}


// 格式化输出
public class Main {
    public static void main(String[] args) {
        double d = 3.1415926;
        System.out.printf("%.2f\n", d); // 显示两位小数3.14
        System.out.printf("%.4f\n", d); // 显示4位小数3.1416
    }
}

// 输入
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in); // 创建Scanner对象
        System.out.print("Input your name: "); // 打印提示
        String name = scanner.nextLine(); // 读取一行输入并获取字符串
        System.out.print("Input your age: "); // 打印提示
        int age = scanner.nextInt(); // 读取一行输入并获取整数
        System.out.printf("Hi, %s, you are %d\n", name, age); // 格式化输出
    }
}

// if
public class Main {
    public static void main(String[] args) {
        int n = 70;
        if (n >= 60) {
            System.out.println("及格了");
        } else {
            System.out.println("挂科了");
        }
        System.out.println("END");
    }
}


// switch
// switch
public class Main {
    public static void main(String[] args) {
        int option = 1;
        switch (option) {
        case 1:
            System.out.println("Selected 1");
            break;
        case 2:
            System.out.println("Selected 2");
            break;
        case 3:
            System.out.println("Selected 3");
            break;
        }
    }
}


// while
public class Main {
	public static void main(String[] args) {
		int sum = 0;
		int m = 20;
		int n = 100;
		// 使用while计算M+...+N:
		while (false) {
		}
		System.out.println(sum);
	}
}

// do-while
public class Main {
    public static void main(String[] args) {
        int sum = 0;
        int n = 1;
        do {
            sum = sum + n;
            n ++;
        } while (n <= 100);
        System.out.println(sum);
    }
}

// for
public class Main {
    public static void main(String[] args) {
        int sum = 0;
        for (int i=1; i<=100; i++) {
            sum = sum + i;
        }
        System.out.println(sum);
    }
}

// 不设置结束条件:
for (int i=0; ; i++) {
    ...
}
// 不设置结束条件和更新语句:
for (int i=0; ;) {
    ...
}
// 什么都不设置:
for (;;) {
    ...
}

// for each
public class Main {
    public static void main(String[] args) {
        int[] ns = { 1, 4, 9, 16, 25 };
        for (int n : ns) {
            System.out.println(n);
        }
    }
}

int[] ns = { 1, 1, 2, 3, 5, 8 };
for (int n : ns) {
    System.out.print(n + ", ");
}
// 遍历数组
import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] ns = { 1, 1, 2, 3, 5, 8 };
        System.out.println(Arrays.toString(ns));
    }
}

// 构造方法
public class Main {
    public static void main(String[] args) {
        Person p = new Person("Xiao Ming", 15);
        System.out.println(p.getName());
        System.out.println(p.getAge());
    }
}

class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public String getName() {
        return this.name;
    }

    public int getAge() {
        return this.age;
    }
}

// 构造方法，与类名相同
class Person {
    public Person() {
    }
}


// 多个构造方法
class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public Person(String name) {
        this(name, 18); // 调用另一个构造方法Person(String, int)
    }

    public Person() {
        this("Unnamed"); // 调用另一个构造方法Person(String)
    }
}

// 方法重载 Overload
class Hello {
    public void hello() {
        System.out.println("Hello, world!");
    }

    public void hello(String name) {
        System.out.println("Hello, " + name + "!");
    }

    public void hello(String name, int age) {
        if (age < 18) {
            System.out.println("Hi, " + name + "!");
        } else {
            System.out.println("Hello, " + name + "!");
        }
    }
}



// 继承
class Person {
    private String name;
    private int age;

    public String getName() {...}
    public void setName(String name) {...}
    public int getAge() {...}
    public void setAge(int age) {...}
}

class Student extends Person {
    // 不要重复name和age字段/方法,
    // 只需要定义新增score字段/方法:
    private int score;

    public int getScore() { … }
    public void setScore(int score) { … }
}

// 重写
class Student extends Person {
    @Override
    public void run() {
        System.out.println("Student.run");
    }
}

// String
String

// StringBuilder
// 链式操作
public class Main {
    public static void main(String[] args) {
        var sb = new StringBuilder(1024);
        sb.append("Mr ")
          .append("Bob")
          .append("!")
          .insert(0, "Hello, ");
        System.out.println(sb.toString());
    }
}

// StringJoiner
import java.util.StringJoiner;
public class Main {
    public static void main(String[] args) {
        String[] names = {"Bob", "Alice", "Grace"};
        var sj = new StringJoiner(", ");
        for (String name : names) {
            sj.add(name);
        }
        System.out.println(sj.toString());
    }
}

public class Weekday {
    public static final int SUN = 0;
    public static final int MON = 1;
    public static final int TUE = 2;
    public static final int WED = 3;
    public static final int THU = 4;
    public static final int FRI = 5;
    public static final int SAT = 6;
}

public class Color {
    public static final String RED = "r";
    public static final String GREEN = "g";
    public static final String BLUE = "b";
}

// exception
public class Main {
    public static void main(String[] args) {
        try {
            process1();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    static void process1() {
        process2();
    }

    static void process2() {
        Integer.parseInt(null); // 会抛出NumberFormatException
    }
}