脚踏实地

public static void main(String[] args) {
    int i = 1;
    System.out.println(i++);// 1
    System.out.println(++i);// 3
}

public static void main(String[] args) {
    int j = 0, i = 1;
    // 编译报错
    System.out.println((j+i)++);
}

byte a = 10;
a += 5; // 合法，等同于 a = (byte)(a + 5);

byte b = 127;
b += 1; // b 现在是 -128，发生了溢出

public static void main(String[] args) {
    int i = 1, j = 2;
    boolean flag = (i++ == 2) && (++j == 3);
    // flag的值: false,i的值:2,j的值:2
    System.out.printf("flag的值: %s,i的值:%s,j的值:%s", flag, i, j);
}

    public static void main(String[] args) {
        int var1 = 1;
        int var2 = 0;
        var1 = var2 > 1 ?  1: 2;
        
        // 等价于if...else以下代码
        if (var2 > 1) {
            var1 = 1;
        }else {
            var1 = 2;
        }
    }

public static void main(String[] args) {
    int var1 = 1;
    int var2 = 0;
    // 如果var2大于1 则var1等于1 否则等于2
    var1 = var2 > 1 ?  1: 2;
}

  // ...
{
 // ...
  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: iconst_1 // 向栈里添加int类型 1
         1: istore_1 // 将int 1 存储到局部变量
         2: iconst_0 // 向栈里添加int类型 0
         3: istore_2 // 将int 2 存储到局部变量
         4: iload_2  // 从局部变量加载int 2
         5: iconst_1
         6: if_icmple     13  // 将1,3步骤栈中的变量弹出 进行比较
         9: iconst_1 // 比较成功 存储该值到栈中
        10: goto          14 // 改变地址
        13: iconst_2
        14: istore_1
        15: return
      LineNumberTable:
        line 6: 0
        line 7: 2
        line 8: 4
        line 9: 15
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      16     0  args   [Ljava/lang/String;
            2      14     1  var1   I
            4      12     2  var2   I
      StackMapTable: number_of_entries = 2
        frame_type = 253 /* append */
          offset_delta = 13
          locals = [ int, int ]
        frame_type = 64 /* same_locals_1_stack_item */
          stack = [ int ]
}
// ...

  public static void main(String[] args) {
      int var1 = 1;
      int var2 = 0;
      if (var2 > 1){
          var1 = 1;
      } else {
          var1 = 2;
      }
  }

  // ...
  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: iconst_1
         1: istore_1
         2: iconst_0
         3: istore_2
         4: iload_2
         5: iconst_1
         6: if_icmple     14
         9: iconst_1
        10: istore_1
        11: goto          16
        14: iconst_2
        15: istore_1
        16: return
      LineNumberTable:
        line 6: 0
        line 7: 2
        line 8: 4
        line 9: 9
        line 11: 14
        line 13: 16
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      17     0  args   [Ljava/lang/String;
            2      15     1  var1   I
            4      13     2  var2   I
      StackMapTable: number_of_entries = 2
        frame_type = 253 /* append */
          offset_delta = 14
          locals = [ int, int ]
        frame_type = 1 /* same */
}
// ...

class Animal { }
class Dog extends Animal { }

public class Main {
    public static void main(String[] args) {
        Animal animal = new Dog();

        // 判断 animal 是否是 Animal 类的实例
        boolean isAnimal = animal instanceof Animal;
        System.out.println("Is animal an instance of Animal? " + isAnimal); // 输出：true

        // 判断 animal 是否是 Dog 类的实例
        boolean isDog = animal instanceof Dog;
        System.out.println("Is animal an instance of Dog? " + isDog); // 输出：true

        // 判断 animal 是否是 Object 类的实例
        boolean isObject = animal instanceof Object;
        System.out.println("Is animal an instance of Object? " + isObject); // 输出：true
    }
}

int sum = a + b * c;
int length = str.length();
boolean result = (x > 0) && (y < 10);
int x = 10;
int max = (a > b) ? a : b;

public static void main(String[] args) {
    // 表示匹配以字母a为开头的单词
    String regx = "\\ba\\w*\\b";
    // 将给定的正则表达式编译到具有给定标志的模式中
    Pattern pattern = Pattern.compile(regx);
    // 创建匹配给定输入与此模式的匹配器
    Matcher matcher = pattern.matcher("abcdab cccabcd aaacd");
    int index = 0;
    // 循环 查找与上边正则匹配的字符序列
    while (matcher.find()) {
        // 返回 由以前匹配操作 所匹配的 输入子序列
        String res = matcher.group();
        System.out.println(index + ":" + res);
        index++;
    }
}

Consumer<String> consumer = (s) -> System.out.println(s);

// 使用匿名类
Comparator<String> comparator = new Comparator<String>() {
  @Override
  public int compare(String s1, String s2) {
    return s1.compareTo(s2);
  }
};

// 使用Lambda表达式
Comparator<String> comparatorLambda = (s1, s2) -> s1.compareTo(s2);

List<String> list = Arrays.asList("a", "b", "c");

// 使用forEach方法
list.forEach(item -> System.out.println(item));

// 使用stream API进行过滤和排序
List<String> filteredList = list.stream()
                                .filter(s -> s.startsWith("a"))
                                .sorted((s1, s2) -> s1.compareTo(s2))
                                .collect(Collectors.toList());

public static void main(String[] args) {
    boolean var1 = true;
    if (var1){
        System.out.println("hello world ...");
    }
}

public static void main(String[] args) {
    boolean var1 = false;
    if (var1){
        System.out.println("if ...");
    }else{
        System.out.println("else ...");
    }
}

public class HelloWorld {
    public static void main(String[] args) {
        String var = "123";
        if ("123".equals(var)){
            System.out.println("123 ...");
        }else if("234".equals(var)){
            System.out.println("234 ...");
        }else if ("345".equals(var)){
            System.out.println("345 ...");
        }else{
            System.out.println("...");
        }
    }
}

public static void main(String[] args) {
    String s = "123";
    switch (s) {
        case "123":
            System.out.println("123");
            break;
        case "456":
            System.out.println("456");
            break;
         default :
            System.out.println("over ...");
    }
}   

public static void main(String[] args) {
      int i = 100;
      switch (i){
          case 100:
              System.out.println("first case ...");
          case 200:
              System.out.println("second case ...");
              break;
      }
}

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

for(;;){
    // to do ...
}

public static void main(String[] args) {
    String[] arr = {"1", "2", "3"};
    ArrayList<String> list = new ArrayList<>(Arrays.asList(arr));
    for (String s : list) {
        System.out.println(s);
    }
    for (String s : arr) {
        System.out.println(s);
    }
}

public static void main(String[] args) {
    String[] arr = {"1", "2", "3"};
    for (String s : arr) {
        // 如果s等于1,则结束本次循环 执行下次循环
        if ("1".equals(s)) {
            continue;
        }
        System.out.println("...");
    }
}

public class HelloWorld {
    public static void main(String[] args) {
        String[] arr = {"1", "2", "3"};
        for (String s : arr) {
            // s等于2,结束for循环
            if ("2".equals(s)) {
                break;
            }
            System.out.println("...");
        }
    }

}

public static void main(String[] args) {
    String[] arr = {"1", "2", "3"};
    for (int i = 0; i < 10; i++) {
        for (String s : arr) {
            if ("2".equals(s)) {
                break;
            }
            System.out.println("...");
        }
        System.out.println("外层 for ...");
    }
}

public static void main(String[] args) {
    String[] arr = {"1", "2", "3"};
    
    test:
    for (int i = 0; i < 10; i++) {
        for (String s : arr) {
            if ("2".equals(s)) {
                break test;
            }
            System.out.println("...");
        }
        System.out.println("外层 for ...");
    }
}

public static void main(String[] args) {
    String[] arr = {"1", "2", "3"};

    test:
    for (int i = 0; i < 10; i++) {
        for (String s : arr) {
            if ("1".equals(s)) {
                continue test;
            }
        }
        System.out.println("外层 for ...");
    }
}

public static void main(String[] args) {
    String[] arr = {"1", "2", "3"};

    for (int i = 0; i < 10; i++) {
        for (String s : arr) {
            if ("1".equals(s)) {
                return;
            }
        }
        System.out.println("外层 for ...");
    }
}

public static void main(String[] args) {
    int i = 0;
   while(++i >= 1){
       i--;
    }
}

public static void main(String[] args) {
    int i = 0;
    do{
        i++;
        System.out.println(" ... ");
    }while (--i == 1);
}

public class HelloWorld {
    public static void main(String[] args) {
        // 实参 
        test("world");
    }
    
    // 形参
    public static void test(String param){
        System.out.println("hello " + param);
    }
}

class PassByValueExample {
    public static void main(String[] args) {
        Dog dog = new Dog("A");
        func(dog);
        System.out.println(dog.getName());          // B
    }

    private static void func(Dog dog) {
        dog.setName("B");//dog.name="B";
    }
}

class MyObject {
  int value;
}

public class PassByValueExample {
  public static void main(String[] args) {
    MyObject obj = new MyObject();
    obj.value = 10;
    modifyObject(obj);
    System.out.println("After method call: " + obj.value);  // 输出: 20

    resetObject(obj);
    System.out.println("After resetObject call: " + obj.value);  // 输出: 20
  }

  public static void modifyObject(MyObject obj) {
    obj.value = 20;  // 修改对象的属性，会影响原对象
  }

  public static void resetObject(MyObject obj) {
    obj = new MyObject();  // 修改的是引用的副本，不影响原引用
    obj.value = 30;
  }
}

public static void main(String[] args) {
    // 0.20000005
    System.out.println(1.2f - 1);
    // 0.19999999999999996
    System.out.println(1.2d - 1);
}

0.1 x 2 = 0.2  取整数位 0
0.2 x 2 = 0.4  取整数位 0
0.4 x 2 = 0.8  取整数位 0
0.8 x 2 = 1.6  取整数位 1
0.6 x 2 = 1.2  取整数位 1
0.2 x 2 = 0.4  取整数位 0
0.4 x 2 = 0.8  取整数位 0
0.8 x 2 = 1.6  取整数位 1
0.6 x 2 = 1.2  取整数位 1
........无限循环

public static void main(String[] args) {
    // 打印结果: 0.200000000000000011102230246251565404236316680908203125
    System.out.println(new BigDecimal(0.2));
}

public static void main(String[] args) {
    // 0.2
    System.out.println(new BigDecimal("0.2"));
}

public static void main(String[] args) {
    BigDecimal num1 = new BigDecimal("2");
    BigDecimal num2 = new BigDecimal("1");
    BigDecimal num3;

    num3 = num1.add(num2);
    System.out.printf("num1 + num2 = %s\n",num3);

    num3 = num1.subtract(num2);
    System.out.printf("num1 - num2 = %s\n",num3);

    num3 = num1.multiply(num2);
    System.out.printf("num1 * num2 = %s\n",num3);

    num3 = num1.divide(num2);
    System.out.printf("num1 / num2 = %s\n",num3);
    
    // 绝对值
    System.out.printf("|num1 / num2| = %s\n",num3.abs());
}

public static void main(String[] args) {
    BigDecimal bigDecimal = new BigDecimal("1.2");
    System.out.println(bigDecimal.byteValue());
    System.out.println(bigDecimal.shortValue());
    System.out.println(bigDecimal.intValue());
    System.out.println(bigDecimal.floatValue());
    System.out.println(bigDecimal.doubleValue());
    System.out.println(bigDecimal.longValue());
}

public static void main(String[] args) {
    BigDecimal num1 = new BigDecimal("1");
    BigDecimal num2 = new BigDecimal("3");
    BigDecimal num3;

    num3 = num1.divide(num2);
    System.out.printf("num1 / num2 = %s\n",num3);
}

Exception in thread "main" java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result.

public static void main(String[] args) {
    BigDecimal num1 = new BigDecimal("1");
    BigDecimal num2 = new BigDecimal("3");
    BigDecimal num3;

    num3 = num1.divide(num2,3, BigDecimal.ROUND_UP);
    // 打印结果: num1 / num2 = 0.334
    System.out.printf("num1 / num2 = %s\n",num3);
}

// divisor: 除数; scale: 精确小数位; roundingMode: 舍入规则
public BigDecimal divide(BigDecimal divisor, int scale, int roundingMode);

public class BigDecimalSetScaleExample {
  public static void main(String[] args) {
    BigDecimal num = new BigDecimal("123.4567");

    // 增加 scale
    BigDecimal increasedScale = num.setScale(6, RoundingMode.UNNECESSARY);
    System.out.println("Increased scale: " + increasedScale); // 123.456700

    // 减少 scale，并使用四舍五入模式
    BigDecimal decreasedScale = num.setScale(2, RoundingMode.HALF_UP);
    System.out.println("Decreased scale: " + decreasedScale); // 123.46

    // 减少 scale，并使用向下舍入模式
    BigDecimal decreasedScaleDown = num.setScale(2, RoundingMode.DOWN);
    System.out.println("Decreased scale (down): " + decreasedScaleDown); // 123.45
  }
}

public static void main(String[] args) {
    BigDecimal num1 = new BigDecimal("1.12345");
    num1.setScale(4);
    System.out.println(num1);
}

java.lang.ArithmeticException: Rounding necessary

public static void main(String[] args) {
    BigDecimal num1 = new BigDecimal("1.10");
    BigDecimal num2 = new BigDecimal("1.1");
    BigDecimal num3 = new BigDecimal("1.1");
    // false
    System.out.println(num1.equals(num2));
    // true
    System.out.println(num2.equals(num3));
}

/**
 * Compares this {@code BigDecimal} with the specified
 * {@code Object} for equality.  Unlike {@link
 * #compareTo(BigDecimal) compareTo}, this method considers two
 * {@code BigDecimal} objects equal only if they are equal in
 * value and scale (thus 2.0 is not equal to 2.00 when compared by
 * this method).
 *
 * @param  x {@code Object} to which this {@code BigDecimal} is
 *         to be compared.
 * @return {@code true} if and only if the specified {@code Object} is a
 *         {@code BigDecimal} whose value and scale are equal to this
 *         {@code BigDecimal}'s.
 * @see    #compareTo(java.math.BigDecimal)
 * @see    #hashCode
 */
@Override
public boolean equals(Object x) {
    if (!(x instanceof BigDecimal))
        return false;
    BigDecimal xDec = (BigDecimal) x;
    if (x == this)
        return true;
    if (scale != xDec.scale)
        return false;
    long s = this.intCompact;
    long xs = xDec.intCompact;
    if (s != INFLATED) {
        if (xs == INFLATED)
            xs = compactValFor(xDec.intVal);
        return xs == s;
    } else if (xs != INFLATED)
        return xs == compactValFor(this.intVal);

    return this.inflated().equals(xDec.inflated());
}

public static void main(String[] args) {
    BigDecimal a = new BigDecimal("1.10");
    BigDecimal b = new BigDecimal("1.1");

    if(a.compareTo(b) == -1){
        System.out.println("a小于b");
    }

    if(a.compareTo(b) == 0){
        System.out.println("a等于b");
    }

    if(a.compareTo(b) == 1){
        System.out.println("a大于b");
    }

    if(a.compareTo(b) > -1){
        System.out.println("a大于等于b");
    }

    if(a.compareTo(b) < 1){
        System.out.println("a小于等于b");
    }
}

public static void main(String[] args) {
  // 计算平方根
  System.out.printf("4的平方根: %s\n",Math.sqrt(4));

  //计算立方根
  System.out.printf("8的立方根: %s\n",Math.cbrt(8));

  // 计算n的m次方
  System.out.printf("2的3次方: %s\n",Math.pow(2,3));

  // 计算最大值
  System.out.printf("1和2中最大值: %s\n",Math.max(1,2));

  // 计算最小值
  System.out.printf("1和2中最小值: %s\n",Math.min(1,2));

  // 求绝对值
  System.out.printf("-1的绝对值: %s\n",Math.abs(-1));

  // 向上取整
  System.out.printf("1.2向上取整: %s\n",Math.ceil(1.2));

  // 向下取整
  System.out.printf("1.2向下取整: %s\n",Math.floor(1.2));

  // [0,1)区间的随机数
  System.out.printf("[0,1)区间的随机数: %s\n",Math.random());

  // 返回与参数值最接近的 double值
  double rint = Math.rint(1.5);
  System.out.printf("rint方法: %s\n",rint);

  // 四舍五入 float时返回int值，double时返回long值
  long round = Math.round(1.5);
  int round1 = Math.round(1.5f);
  System.out.printf("round方法: 1.5四舍五入: %s\n",round);
}

// 向上取整
System.out.printf("1.2向上取整: %s\n",Math.ceil(1.2));

// 向下取整
System.out.printf("1.2向下取整: %s\n",Math.floor(1.2));

Math.ceil(1.2) = 2.0
-Math.floor(-1.2) = 2.0

public class RandomExample {
  public static void main(String[] args) {
    // 生成一个 0.0 到 1.0 之间的随机数（不包括 1.0）
    double randomValue = Math.random();
    System.out.printf("随机数: %s\n", randomValue);
  }
}