JDK 相关知识点

class App148 {
  public static void main(String args[]) throws MyException {
    MyException myecpt = new MyException("Ops!");
    throw myexcpt;
    // System.out.println("Dead code");
  }
}

class MyException extends IOException {
  MyException(String message) {
    super(message);
  }
}

public static void main(String args[]) {
  int num1, num2;
  try {
    num1 = 0;
    num2 = 62 / num1;
    System.out.println(num2);
    System.out.println("Hey I'm at the end of try block");
  } catch (ArithmeticException e) {
    System.out.println("You should not divide a number by zero");
  } catch (Exception e) {
    System.out.println("Exception occurred");
  }
  System.out.println("I'm out of try-catch block in Java.");
}

// 匿名内部类写法
Runnable run01 =
    new Runnable() {
      @Override
      public void run() {
        System.out.println("run01");
      }
    };

// Lambda 表达式写法
Runnable run02 =
    () -> {
      System.out.println("run02");
    };

import java.util.Random;

class Employee {
  private static int nextId;

  private int id;
  private String name;
  private double salary;

  // 静态初始化块: 初始化静态变量
  static {
    System.out.println("Static Initialezation Block");
    Random gen = new Random();
    nextId = gen.nextInt(1000);
  }

  // 对象初始化块: 当对象实例化时调用
  {
    System.out.println("Object Initialezation Block");
    id = nextId;
    nextId++;
  }

  // 默认构造器
  public Employee() {
    System.out.println("Default Constructor");
  }

  public static void main(String[] args) {
    Employee e1 = new Employee();
    Employee e2 = new Employee();
  }
}

////////////////////////////////////////////////////////////////////////////////
//  Static Initialezation Block
//  Object Initialezation Block
//  Default Constructor
//  Object Initialezation Block
//  Default Constructor
////////////////////////////////////////////////////////////////////////////////

class Employee {
  private String firstname;
  private String lastname;

  public void setName(String firstname) {
    this.firstname = firstname;
    this.lastname = null;
  }

  public void setName(String firstname, String lastname) {
    this.firstname = firstname;
    this.lastname = lastname;
  }
}

class Teacher extends Employee {

  private String fullname;

  @Override
  public void setName(String firstname, String lastname) {
    this.firstname = firstname;
    this.lastname = lastname;
    this.fullname = firstname + " " + lastname;
  }
}

$ java -version
java version "1.8.0_221"
Java(TM) SE Runtime Environment (build 1.8.0_221-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.221-b11, mixed mode)
$ javac -version
javac 1.8.0_221

$ ls
Hello.java
$ cat Hello.java
public class Hello {
  public static void main(String args[]) {
    System.out.println("hello");
  }
}
$ javac Hello.java
$ ls
Hello.class  Hello.java
$ java Hello
hello
$

java -jar $JAVA_HOME/lib/javarepl-dev.jar

$ jrepl
[INFO] Unable to bind key for unsupported operation: edit-and-execute-command
Welcome to JavaREPL version dev (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_231)
Type expression to evaluate, :help for more options or press tab to auto-complete.
Connected to local instance at http://localhost:40173
java> "hello world"
java.lang.String res0 = "hello world"
java>

class Apple {

  public int add(int a, int b) {
    return a + b;
  }

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

$ javap Apple
Compiled from "Apple.java"
class Apple {
  Apple();
  public int add(int, int);
  public static void main(java.lang.String[]);
}

$ javap -c Apple
Compiled from "Apple.java"
class Apple {
  Apple();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public int add(int, int);
    Code:
       0: iload_1
       1: iload_2
       2: iadd
       3: ireturn

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #3                  // String Hello World
       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: return
}

$ javap -v Apple
Classfile /tmp/java/Apple.class
  Last modified Mar 17, 2020; size 471 bytes
  MD5 checksum ad20aeca420725caad4423644222e82d
  Compiled from "Apple.java"
class Apple
  minor version: 0
  major version: 52
  flags: ACC_SUPER
Constant pool:
   #1 = Methodref          #6.#17         // java/lang/Object."<init>":()V
   #2 = Fieldref           #18.#19        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = String             #20            // Hello World
   #4 = Methodref          #21.#22        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = Class              #23            // Apple
   #6 = Class              #24            // java/lang/Object
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               add
  #12 = Utf8               (II)I
  #13 = Utf8               main
  #14 = Utf8               ([Ljava/lang/String;)V
  #15 = Utf8               SourceFile
  #16 = Utf8               Apple.java
  #17 = NameAndType        #7:#8          // "<init>":()V
  #18 = Class              #25            // java/lang/System
  #19 = NameAndType        #26:#27        // out:Ljava/io/PrintStream;
  #20 = Utf8               Hello World
  #21 = Class              #28            // java/io/PrintStream
  #22 = NameAndType        #29:#30        // println:(Ljava/lang/String;)V
  #23 = Utf8               Apple
  #24 = Utf8               java/lang/Object
  #25 = Utf8               java/lang/System
  #26 = Utf8               out
  #27 = Utf8               Ljava/io/PrintStream;
  #28 = Utf8               java/io/PrintStream
  #29 = Utf8               println
  #30 = Utf8               (Ljava/lang/String;)V
{
  Apple();
    descriptor: ()V
    flags:
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 1: 0

  public int add(int, int);
    descriptor: (II)I
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=3
         0: iload_1
         1: iload_2
         2: iadd
         3: ireturn
      LineNumberTable:
        line 4: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String Hello World
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 8: 0
        line 9: 8
}
SourceFile: "Apple.java"

// 字符串基本操作
String str1 = "abcdef";
str1.isEmpty();
str1.length();
str1.indexOf("c"); // => 2
str1.substring(0, 3); // => "abc"

// 合并字符串
String str2 = "1,2,3,4";
String[] res1 = str2.split(","); // => ["1", "2", "3", "4"]
// 分割字符串
List<String> res2 = Arrays.asList(res1);
// res2.add("xxx"); // => throw UnsupportedOperationException
List<String> res3 = new LinkedList<>(Arrays.asList(res1));
res3.add("xxx"); // now is OK
String str3 = String.join(":", res3); // => "1:2:3:4:xxx"

// 正则表达式替换
String str6 = "3 +5-4";
str6 = str6.replaceAll("(\\+|-|\\*|/)", " $1 ").replaceAll("  ", " ").trim(); // => "3 + 5 - 4"

if (str != null && !str.isEmpty()) {
  // if str is not empty
}

String.format("Hi, %s", "Jack"); // => "Hi, Jack"
MessageFormat.format("Hi, {0}, I''m {1} years old.", "Tom", 18);
// => "Hi, Tom, I'm 18 years old."

// 单引号表示字面量，MessageFormat 不会替换
MessageFormat.format("'Hi, {0}, I''m {1} years old.'", "Tom", 18);
// => "Hi, {0}, I'm {1} years old."

// 处理数值类型的对象
MessageFormat.format("{0,number,#}", 1889989898989L); // => "1889989898989"
MessageFormat.format("{0}", 1889989898989L); // => "1,889,989,898,989"
MessageFormat.format("{0,number,#000000}", 1889989898989L); // => "1889989898989"
MessageFormat.format("{0,number,#000000}", 12345L); //  => "012345"
MessageFormat.format("{0,number,#}", 12345L); // => "12345"

StringBuilder sb = new StringBuilder(10);
sb.append("Hello.."); // => "Hello.."
sb.append("!"); // => "Hello..!"
sb.insert(8, "Java"); // => "Hello..!Java"
sb.delete(5, 8); // => "HelloJava"
sb.reverse(); // => "avaJolleH"
System.out.println(sb.toString());

// 定长数组
int[] arr1 = new int[10]; // 定义数组
int[] arr2 = {1, 2, 3, 4}; // 定义并初始化数组

Arrays.fill(arr1, 1); // 填充数组的所有元素为 1

// 数组复制
int[] arr3 = Arrays.copyOf(arr2, arr2.length);
int[] arr4 = Arrays.copyOfRange(arr2, 2, 3); // => [3]
// System.out.println(Arrays.toString(arr4));

// 变长数组
ArrayList<Integer> vec1 = new ArrayList<Integer>();
vec1.add(1); // 添加元素到数组最后
vec1.add(3);
vec1.add(4);
vec1.get(0); // 获取第 0 个元素
vec1.size(); // 数组的长度
// System.out.println(vec1);
vec1.add(0, 8); // 添加到数组的第 0 个元素
// System.out.println(vec1);
vec1.set(1, 4); // 修改数组的第 1 个元素
// System.out.println(vec1);
vec1.remove(1); // 删除第 1 个元素
// System.out.println(vec1);

List<Integer> vec2 = vec1.subList(0, 2); // 获取子链表的前 2 个元素
System.out.println(vec2);

// 数组排序
Arrays.sort(arr2);

// 声明 HashMap 数组
Map<Character, Integer>[] mapArray = new HashMap[9];
// 需要初始化元素
for (int i = 0; i < 9; i++) mapArray[i] = new HashMap<>();

// List 转 Array
List<String> list = new ArrayList<>();
String[] arr1 = new String[] {"a", "b", "c"};
for (int i = 0; i < arr1.length; i++) {
  list.add(arr1[i]);
}

// Array 转 List
List<String> names = new LinkedList<String>() {{add("Jack"); add("Tome");}};
String[] arr2 = new String[names.size()];
names.toArray(arr2);

assert(a > 0); // throws an AssertionError if a <= 0

assert Expression1;
assert Expression1 : Expression2;

javac -source 1.4 Test.java

new Date().getTime(); // =>  1583839335691
System.currentTimeMillis(); // => 1583839349221