【Java】Java 常用类

public class Demo01 {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

public class Demo02 extends Object {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

Object s = new Object();
System.out.println(s.hashCode());
System.out.println(s.getClass());
System.out.println(s.toString());
System.out.println(s.equals(s));

public boolean equals(Object obj) {
    return (this == obj);
}

String s1 = "abc";
String s2 = new String("abc");

String s1 = "abcabc";
String s2 = "ABCABC";
String s3 = "      abc      ";
String s4 = new String();
System.out.println(s1.equals(s2));  //false
System.out.println(s1.equalsIgnoreCase(s2));    //true
System.out.println(s1.indexOf('b'));    //1
System.out.println(s1.lastIndexOf('b'));    //4
System.out.println(s1.length());    //6
System.out.println(s1.replace('a','A'));    //AbcAbc
System.out.println(s1.substring(2));    //cabc
System.out.println(s1.substring(2,3));  //c
System.out.println(s3.trim());  //abc
System.out.println(s4.isEmpty());   //true

Integer i = 10;

Integer i2 = new Integer(20);

Integer i3 = Integer.valueOf(30);

Integer i = 1000;
Integer i2 = new Integer(20);
Integer i3 = Integer.valueOf(30);
//以byte类型返回该Integer的值
System.out.println(i.byteValue());  //-24
//在数字上比较两个Integer对象。如果相同返回0；调用对象的数值小于anotherInteger返回负值，反之，返回正值。
System.out.println(i2.compareTo(i));    //-1
//比较此对象与指定对象是否相等
System.out.println(i.equals(i2));// false
//以int型返回此Integer对象
System.out.println(i.intValue());   //1000
//以short型返回此Integer对象   
System.out.println(i.shortValue()); //1000
//返回一个表示该Integer值的String对象
System.out.println(Integer.toString(i));    //1000
//返回指定Sting值的Integer对象
System.out.println(Integer.valueOf("20"));  //20
//返回包含在由str指定的字符串中的数字的等价整数值
System.out.println(Integer.parseInt("123"));    //123

//表示int型可取的最大值
System.out.println(Integer.MAX_VALUE);  //2147483647
//表示int型可取的最小值
System.out.println(Integer.MIN_VALUE);  //-2147483648
//表示以2进制补码形式表示int值的位数
System.out.println(Integer.SIZE);   //32

Boolean b = true;
Boolean b2 = Boolean.TRUE;
Boolean b3 = new Boolean(true);
Boolean b4 = new Boolean("true");
Boolean b5 = new Boolean("TRUE");

Byte b = 20;
byte b1 = 45;
Byte b2 = b;

byte b = 45;
Byte b2 = b;
System.out.println(b2.byteValue()); //45
System.out.println(b2.compareTo((byte) 100));   //-55
System.out.println(b2.doubleValue());   //45.0
System.out.println(b2.equals(b));   //true
System.out.println(b2.intValue());  //45
System.out.println(b2.toString());  //45
System.out.println(Byte.valueOf("40")); //40
System.out.println(Byte.parseByte("50"));   //50

System.out.println(Byte.MAX_VALUE);	//127
System.out.println(Byte.MIN_VALUE);	//-128

Character c = '1';
Character c1 = new Character('2');

Character c = '1';
Character c1 = new Character('2');
System.out.println(c.charValue());      //1
System.out.println(c.compareTo('1'));       //0
System.out.println(c.equals(c1));       //false
System.out.println(c.toString());       //1
System.out.println(Character.toUpperCase('a'));     //A
System.out.println(Character.toLowerCase('A'));     //a
System.out.println(Character.isUpperCase('a'));     //false
System.out.println(Character.isLowerCase('a'));     //true

Double d = 10.0;
Double d1 = new Double(10);

Double d = 10.0;
Double d1 = new Double(10);
System.out.println(d.byteValue());      //10
System.out.println(d.compareTo(d1));        //0
System.out.println(d.equals(d1));       //true
System.out.println(d.intValue());       //10
System.out.println(d.isNaN());      //false
System.out.println(Double.valueOf("100"));      //100.0
System.out.println(d.doubleValue());        //10.0
System.out.println(d.longValue());      //10

Number n = 1000;
System.out.println(n.byteValue());      //-24
System.out.println(n.intValue());       //1000
System.out.println(n.floatValue());     //1000.0
System.out.println(n.shortValue());     //1000
System.out.println(n.longValue());      //1000
System.out.println(n.doubleValue());        //1000.0

StringBuffer buffer = new StringBuffer();

long startTime = System.currentTimeMillis(); 
String s = new String();
for (int i = 0; i < 100000; i++) {
    s = s+"a";
}
long endTime = System.currentTimeMillis(); 
System.out.println("程序运行时间：" + (endTime - startTime) + "ms");//程序运行时间：3754ms

long startTime = System.currentTimeMillis(); 
StringBuilder builder = new StringBuilder();
for (int i = 0; i < 100000000; i++) {
    builder.append('a');
}
long endTime = System.currentTimeMillis(); 
System.out.println("程序运行时间：" + (endTime - startTime) + "ms");//程序运行时间：469ms

long startTime = System.currentTimeMillis(); 
StringBuffer s = new StringBuffer();
for (int i = 0; i < 100000000; i++) {
    s.append('a');
}
long endTime = System.currentTimeMillis(); 
System.out.println("程序运行时间：" + (endTime - startTime) + "ms");//程序运行时间：1680ms

StringBuffer buffer = new StringBuffer("Hello World");
System.out.println(buffer);     //Hello World
buffer.append("Hello Java");
System.out.println(buffer);     //Hello WorldHello Java
buffer.insert(10,",");
System.out.println(buffer);     //Hello World,Hello Java
buffer.setCharAt(10,'!');
System.out.println(buffer);     //Hello Worl!dHello Java
buffer.reverse();
System.out.println(buffer);     //avaJ olleHd!lroW olleH

ArrayList list = new ArrayList<>();
ArrayList<String> stringArrayList = new ArrayList<>();

ArrayList<String> games = new ArrayList<String>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
System.out.println(games);  //[Counter-Strike: Global Offensive, Dota 2, PUBG: BATTLEGROUNDS, Apex Legends, Grand Theft Auto V]

ArrayList<String> games = new ArrayList<String>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
System.out.println(games.get(4));   //Grand Theft Auto V

ArrayList<String> games = new ArrayList<String>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
games.set(0,"CS:GO");//修改下标为0的游戏名称为CS:GO
System.out.println(games);  //[CS:GO, Dota 2, PUBG: BATTLEGROUNDS, Apex Legends, Grand Theft Auto V]

ArrayList<String> games = new ArrayList<String>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
games.remove(2);
System.out.println(games);  //[Counter-Strike: Global Offensive, Dota 2, Apex Legends, Grand Theft Auto V]

ArrayList<String> games = new ArrayList<String>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
System.out.println(games.size());   //5

ArrayList<String> games = new ArrayList<String>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
for (int i = 0; i < games.size(); i++) {
    System.out.println(games.get(i));
}

ArrayList<String> games = new ArrayList<String>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
for (String game : games) {
    System.out.println(game);
}

ArrayList<String> games = new ArrayList<String>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
Iterator<String> iterator = games.iterator();
while (iterator.hasNext()){
    System.out.println(iterator.next());
}

// 在遍历过程中遇到PUBG时将其从ArrayList中移除
ArrayList<String> games = new ArrayList<>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
Iterator<String> iterator = games.iterator();
while (iterator.hasNext()){
    String next = iterator.next();
    if ("PUBG: BATTLEGROUNDS".equals(next)){
        games.remove(next);
    }
    System.out.println(next);
}

final void checkForComodification() {
    if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
}

public boolean remove(Object o) {
    final Object[] es = elementData;
    final int size = this.size;
    int i = 0;
    found: {
        if (o == null) {
            for (; i < size; i++)
                if (es[i] == null)
                    break found;
        } else {
            for (; i < size; i++)
                if (o.equals(es[i]))
                    break found;
        }
        return false;
    }
    fastRemove(es, i);
    return true;
}

private void fastRemove(Object[] es, int i) {
    modCount++;
    final int newSize;
    if ((newSize = size - 1) > i)
        System.arraycopy(es, i + 1, es, i, newSize - i);
    es[size = newSize] = null;
}

int expectedModCount = modCount;

// 在遍历过程中遇到PUBG时将其从ArrayList中移除
ArrayList<String> games = new ArrayList<>();
ArrayList<String> remove = new ArrayList<>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
for (int i = games.size() - 1; i >= 0; i--) {
    if ("PUBG: BATTLEGROUNDS".equals(games.get(i))){
        games.remove(i);
    }
}
System.out.println(games);	//[Counter-Strike: Global Offensive, Dota 2, Apex Legends, Grand Theft Auto V]

// 在遍历过程中遇到PUBG时将其从ArrayList中移除
ArrayList<String> games = new ArrayList<>();
ArrayList<String> remove = new ArrayList<>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
Iterator<String> iterator = games.iterator();
while (iterator.hasNext()){
    String next = iterator.next();
    if ("PUBG: BATTLEGROUNDS".equals(next)){
        remove.add(next);
    }
}
games.removeAll(remove);
System.out.println(games);	//[Counter-Strike: Global Offensive, Dota 2, Apex Legends, Grand Theft Auto V]

for(String game: games){
    if ("PUBG: BATTLEGROUNDS".equals(game)){
        games.remove(game);
    }
}

ArrayList<String> games = new ArrayList<>();
games.add("Counter-Strike: Global Offensive");
games.add("Dota 2");
games.add("PUBG: BATTLEGROUNDS");
games.add("Apex Legends");
games.add("Grand Theft Auto V");
Object[] array = games.toArray();
for (int i = 0; i < array.length; i++) {
    System.out.println(array[i]);
}

LinkedList linkedList = new LinkedList();
LinkedList<String> strings = new LinkedList<>();

LinkedList linkedList = new LinkedList();
linkedList.add("Counter-Strike: Global Offensive");
linkedList.add(1,"Dota 2");
linkedList.add("Apex Legends");
linkedList.addFirst("Grand Theft Auto V");
System.out.println(linkedList);		//[Grand Theft Auto V, Counter-Strike: Global Offensive, Dota 2, Apex Legends]
System.out.println(linkedList.get(0));      //Grand Theft Auto V
System.out.println(linkedList.getFirst());      //Grand Theft Auto V
System.out.println(linkedList.getLast());       //Apex Legends
linkedList.set(1,"CSGO");
System.out.println(linkedList);     //[Grand Theft Auto V, CSGO, Dota 2, Apex Legends]
linkedList.removeFirst();
System.out.println(linkedList);     //[CSGO, Dota 2, Apex Legends]
linkedList.removeLast();
System.out.println(linkedList);     //[CSGO, Dota 2]
linkedList.remove("CSGO");
System.out.println(linkedList);     //[Dota 2]
linkedList.clear();
System.out.println(linkedList);     //[]
System.out.println(linkedList.contains("CSGO"));        //false

LinkedList linkedList = new LinkedList();
linkedList.add("Counter-Strike: Global Offensive");
linkedList.add(1,"Dota 2");
linkedList.add("Apex Legends");
linkedList.addFirst("Grand Theft Auto V");
System.out.println("--------------方法一------------------------");
for (int i = 0; i < linkedList.size(); i++) {
    System.out.println(linkedList.get(i));
}
System.out.println("--------------方法二------------------------");
Iterator iterator = linkedList.iterator();
while (iterator.hasNext()){
    System.out.println(iterator.next());
}
System.out.println("--------------方法三------------------------");
for (Object o : linkedList) {
    System.out.println(o);
}

HashSet hashSet = new HashSet();
HashSet<String> strings = new HashSet<>();

HashSet<String> strings = new HashSet<>();
strings.add("Counter-Strike: Global Offensive");
strings.add("Dota 2");
strings.add("PUBG: BATTLEGROUNDS");
strings.add("Lost Ark");
strings.add("Apex Legends");
System.out.println(strings);
strings.remove("PUBG: BATTLEGROUNDS");
System.out.println(strings);
if (!strings.contains("PUBG")){
    strings.add("PUBG");
}
System.out.println(strings);

HashSet<String> strings = new HashSet<>();
strings.add("Counter-Strike: Global Offensive");
strings.add("Dota 2");
strings.add("PUBG: BATTLEGROUNDS");
strings.add("Lost Ark");
strings.add("Apex Legends");
System.out.println("------方法一-------");
for (String s: strings){
    System.out.println(s);
}
System.out.println("------方法二-------");
Iterator<String> stringIterator = strings.iterator();
while (stringIterator.hasNext()){
    System.out.println(stringIterator.next());
}

public class Test<T>{
}

public class Box<E> {
    private E e;

    public void add(E e){
        this.e = e;
    }
}

Box<String> stringBox = new Box<>();
stringBox.add("Java");
Box<Integer> integerBox = new Box<>();
integerBox.add(1);

public class DoubleBox extends Box<Double>{
}

DoubleBox doubleBox = new DoubleBox();
doubleBox.add(10.0);

public interface Test<T> {
    T test(T t);
}

public class TestImpl implements Test<String>{
    @Override
    public String test(String s) {
        return s;
    }
}

Test<String> stringTest = new TestImpl();
String java = stringTest.test("Java");

public static <T> T test(T t){
    return t;
}
public static <T> void test2(T t){
    System.out.println(t);
}

public static void main(String[] args) {
    System.out.println(test(new String("Java")));
    test2(new String("Java"));
}
public static <T> T test(T t){
    return t;
}
public static <T> void test2(T t){
    System.out.println(t);
}

<? extends 具体的类型>

public class Person {
    private String name;
    private int age;
}

public class Student extends Person{

}

    ArrayList<Person> people = new ArrayList<>();
    ArrayList<Student> students = new ArrayList<>();
    people.add(new Person());
    students.add(new Student());
    test(people);
    test(students);
    test2(people);
    test2(students);
    
}
public static void test(List<? extends Person> list){
    System.out.println(list.toString());
}
public static void test2(List<? super Student> list){
    System.out.println(list.toString());
}

public interface Map<K, V>

Map<String,String> stringMap = new HashMap<>();

Map<String,String> stringMap = new HashMap<>();
System.out.println(stringMap.put("a", "A"));        //null
System.out.println(stringMap.put("a", "Apple"));    //A
System.out.println(stringMap.get("a"));     //Apple
System.out.println(stringMap.remove("a"));      //Apple
System.out.println(stringMap.containsKey("a"));     //false
stringMap.put("b","B");
System.out.println(stringMap.size());       //1
System.out.println(stringMap.toString());       //{b=B}
stringMap.clear();
System.out.println(stringMap.toString());       //{}

HashMap<Integer,String> gameMap = new HashMap<>();
gameMap.put(1,"CSGO");
gameMap.put(2,"Dota 2");
gameMap.put(3,"PUBG");
gameMap.put(4,"Apex");
gameMap.put(5,"Lost Ark");
Set<Map.Entry<Integer, String>> entries = gameMap.entrySet();
for (Map.Entry<Integer, String> entry : entries) {
    System.out.println(entry.getKey()+":"+entry.getValue());
}

HashMap<Integer,String> gameMap = new HashMap<>();
gameMap.put(1,"CSGO");
gameMap.put(2,"Dota 2");
gameMap.put(3,"PUBG");
gameMap.put(4,"Apex");
gameMap.put(5,"Lost Ark");
Set<Integer> integers = gameMap.keySet();
for (Integer key : integers) {
    System.out.println(key+":"+gameMap.get(key));
}

File file = new File("C:\\Users\\haiba\\Desktop\\Java\\IO\\file.txt");
File file1 = new File("C:\\Users\\haiba\\Desktop\\Java\\IO", "file.txt");
File file2 = new File("C:\\Users\\haiba\\Desktop\\Java\\IO");
File file3 = new File(file2, "file.txt");

File file = new File("C:\\Users\\haiba\\Desktop\\Java\\IO\\test.txt");
boolean newFile = file.createNewFile();     //创建成功返回TRUE，创建失败返回false
File dir = new File("C:\\Users\\haiba\\Desktop\\Java\\IO\\test");
boolean mkdir = dir.mkdir();        //创建文件夹，只能创建直系子目录或者文件
File dirs = new File("C:\\Users\\haiba\\Desktop\\Java\\IO\\a\\b");
boolean mkdirs = dirs.mkdirs();     //创建多级文件夹，返回是否创建成功
boolean exists = dirs.exists();     //判断文件夹或者文件是否存在
boolean isFile = file.isFile();     //判断是否是一个文件
boolean isDirectory = dir.isDirectory();        //判断是否是文件夹
boolean delete = dir.delete();      //删除一个空文件夹，这个文件夹必须是空文件夹
long length = file.length();        //获取文件的大小，对文件夹无意义
String name = file.getName();       //获取文件的文件名
File parentFile = file.getParentFile();     //获取父级目录的对象
File absoluteFile = file.getAbsoluteFile();     //获取文件的绝对路径

File file = new File("C:\\Users\\haiba\\Desktop\\Java\\IO");
File[] files = file.listFiles();
for (File file1 : files) {
    System.out.println(file1);
}

private static String getExtensionName(File file){
    String fileName = file.getName();
    if (fileName.lastIndexOf('.') != -1 && fileName.lastIndexOf('.')!=0){
        return fileName.substring(fileName.lastIndexOf(".")+1);
    }
    return "";
}

private static long getSize(File file){
    if (file.isFile()){
        return file.length();
    }
    File[] children = file.listFiles();
    long size = 0;
    if (children!=null){
        for (File child : children) {
            size += getSize(child);
        }
    }
    return size;
}

public static void main(String[] args) {
    System.out.println(getFactorial(5));
}
public static long getFactorial(int f){
    if (f==1){
        return 1;
    }
    return f * getFactorial(f-1);
}

public static void main(String[] args) {
    System.out.println(fibonacciSequence(20));
}
private static long fibonacciSequence(int f){
    if (f == 0) return 0;
    if (f == 1) return 1;
    return fibonacciSequence(f - 1) + fibonacciSequence(f - 2);
}