java基础知识积累总结

class AutoUnboxingTest {
 
    public static void main(String[] args) {
        Integer a = new Integer(3);
        Integer b = 3;                  // 将3自动装箱成Integer类型
        int c = 3;
        System.out.println(a == b);     // false 两个引用没有引用同一对象
        System.out.println(a == c);     // true a自动拆箱成int类型再和c比较
    }
}

public class Test03 {
 
    public static void main(String[] args) {
        Integer f1 = 100, f2 = 100, f3 = 150, f4 = 150;
 
        System.out.println(f1 == f2);  // true
        System.out.println(f3 == f4);  // false
    }
}

public static Integer valueOf(int i) {
    if (i >= IntegerCache.low && i <= IntegerCache.high)
        return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
}

/**
     * Cache to support the object identity semantics of autoboxing for values between
     * -128 and 127 (inclusive) as required by JLS.
     *
     * The cache is initialized on first usage.  The size of the cache
     * may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option.
     * During VM initialization, java.lang.Integer.IntegerCache.high property
     * may be set and saved in the private system properties in the
     * sun.misc.VM class.
     */
 
    private static class IntegerCache {
        static final int low = -128;
        static final int high;
        static final Integer cache[];
 
        static {
            // high value may be configured by property
            int h = 127;
            String integerCacheHighPropValue =
                sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
            if (integerCacheHighPropValue != null) {
                try {
                    int i = parseInt(integerCacheHighPropValue);
                    i = Math.max(i, 127);
                    // Maximum array size is Integer.MAX_VALUE
                    h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
                } catch( NumberFormatException nfe) {
                    // If the property cannot be parsed into an int, ignore it.
                }
            }
            high = h;
 
            cache = new Integer[(high - low) + 1];
            int j = low;
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);
 
            // range [-128, 127] must be interned (JLS7 5.1.7)
            assert IntegerCache.high >= 127;
        }
 
        private IntegerCache() {}
    }

 public static void main(String[] args) {
        String s01 = "Programming";
        String s02 = new String("Programming");
        String s03 = "Program" + "ming";
        System.out.println(s01 == s02);    //false
        System.out.println(s01 == s03);    // true
        System.out.println(s01 == s01.intern()); //true

		System.out.println("intern method:");

        　　　　 String s0="kvill";
		String s1=new String("kvill");
		String s2=new String("kvill");
		System.out.println(s0==s1);       //false
		s1.intern();
		s2=s2.intern();
		System.out.println(s0==s1);          //false
		System.out.println(s0==s1.intern()); // true
		System.out.println(s0==s2);　　　　　　// true
    }

/**
 * 扑克类（一副扑克）
 * @author 骆昊
 *
 */
public class Poker {
    private static String[] suites = {"黑桃", "红桃", "草花", "方块"};
    private static int[] faces = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13};
 
    private Card[] cards;
 
    /**
     * 构造器
     * 
     */
    public Poker() {
        cards = new Card[52];
        for(int i = 0; i < suites.length; i++) {
            for(int j = 0; j < faces.length; j++) {
                cards[i * 13 + j] = new Card(suites[i], faces[j]);
            }
        }
    }
 
    /**
     * 洗牌 （随机乱序）
     * 
     */
    public void shuffle() {
        for(int i = 0, len = cards.length; i < len; i++) {
            int index = (int) (Math.random() * len);
            Card temp = cards[index];
            cards[index] = cards[i];
            cards[i] = temp;
        }
    }
 
    /**
     * 发牌
     * @param index 发牌的位置
     * 
     */
    public Card deal(int index) {
        return cards[index];
    }
 
    /**
     * 卡片类（一张扑克）
     * [内部类]
     * @author 骆昊
     *
     */
    public class Card {
        private String suite;   // 花色
        private int face;       // 点数
 
        public Card(String suite, int face) {
            this.suite = suite;
            this.face = face;
        }
 
        @Override
        public String toString() {
            String faceStr = "";
            switch(face) {
            case 1: faceStr = "A"; break;
            case 11: faceStr = "J"; break;
            case 12: faceStr = "Q"; break;
            case 13: faceStr = "K"; break;
            default: faceStr = String.valueOf(face);
            }
            return suite + faceStr;
        }
    }
}

class PokerTest {
 
    public static void main(String[] args) {
        Poker poker = new Poker();
        poker.shuffle();                // 洗牌
        Poker.Card c1 = poker.deal(0);  // 发第一张牌
        // 对于非静态内部类Card
        // 只有通过其外部类Poker对象才能创建Card对象
        Poker.Card c2 = poker.new Card("红心", 1);    // 自己创建一张牌
 
        System.out.println(c1);     // 洗牌后的第一张
        System.out.println(c2);     // 打印: 红心A
    }
}

class Outer {
 
    class Inner {}
 
    public static void foo() { new Inner(); }
 
    public void bar() { new Inner(); }
 
    public static void main(String[] args) {
        new Inner();
    }
}

new Outer().new Inner();

import java.util.Arrays;
import java.util.EmptyStackException;
 
public class MyStack<T> {
    private T[] elements;
    private int size = 0;
 
    private static final int INIT_CAPACITY = 16;
 
    public MyStack() {
        elements = (T[]) new Object[INIT_CAPACITY];
    }
 
    public void push(T elem) {
        ensureCapacity();
        elements[size++] = elem;
    }
 
    public T pop() {
        if(size == 0) 
            throw new EmptyStackException();
        return elements[--size];
    }
 
    private void ensureCapacity() {
        if(elements.length == size) {
            elements = Arrays.copyOf(elements, 2 * size + 1);
        }
    }
}

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
 
public class MyUtil {
 
    private MyUtil() {
        throw new AssertionError();
    }
 
    public static <T> T clone(T obj) throws Exception {
        ByteArrayOutputStream bout = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(bout);
        oos.writeObject(obj);
 
        ByteArrayInputStream bin = new ByteArrayInputStream(bout.toByteArray());
        ObjectInputStream ois = new ObjectInputStream(bin);
        return (T) ois.readObject();
 
        // 说明：调用ByteArrayInputStream或ByteArrayOutputStream对象的close方法没有任何意义
        // 这两个基于内存的流只要垃圾回收器清理对象就能够释放资源，这一点不同于对外部资源（如文件流）的释放
    }
}

class A {
 
    static {
        System.out.print("1");
    }
 
    public A() {
        System.out.print("2");
    }
}
 
class B extends A{
 
    static {
        System.out.print("a");
    }
 
    public B() {
        System.out.print("b");
    }
}
 
public class Hello {
 
    public static void main(String[] args) {
        A ab = new B();
        ab = new B();
    }
 
}

public static String reverse(String originStr) {
      if(originStr == null || originStr.length() <= 1) 
          return originStr;
      return reverse(originStr.substring(1)) + originStr.charAt(0);
  }

String s1 = "你好";
String s2 = new String(s1.getBytes("GB2312"), "ISO-8859-1");

public class DateTimeTest {
    public static void main(String[] args) {
        Calendar cal = Calendar.getInstance();
        System.out.println(cal.get(Calendar.YEAR));
        System.out.println(cal.get(Calendar.MONTH));    // 0 - 11
        System.out.println(cal.get(Calendar.DATE));
        System.out.println(cal.get(Calendar.HOUR_OF_DAY));
        System.out.println(cal.get(Calendar.MINUTE));
        System.out.println(cal.get(Calendar.SECOND));
 
        // Java 8
        LocalDateTime dt = LocalDateTime.now();
        System.out.println(dt.getYear());
        System.out.println(dt.getMonthValue());     // 1 - 12
        System.out.println(dt.getDayOfMonth());
        System.out.println(dt.getHour());
        System.out.println(dt.getMinute());
        System.out.println(dt.getSecond());
    }
}

Calendar.getInstance().getTimeInMillis();
System.currentTimeMillis();
Clock.systemDefaultZone().millis(); // Java 8

Calendar time = Calendar.getInstance();
time.getActualMaximum(Calendar.DAY_OF_MONTH);

import java.text.SimpleDateFormat;
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.util.Date;
 
class DateFormatTest {
 
    public static void main(String[] args) {
        SimpleDateFormat oldFormatter = new SimpleDateFormat("yyyy/MM/dd");
        Date date1 = new Date();
        System.out.println(oldFormatter.format(date1));
 
        // Java 8
        DateTimeFormatter newFormatter = DateTimeFormatter.ofPattern("yyyy/MM/dd");
        LocalDate date2 = LocalDate.now();
        System.out.println(date2.format(newFormatter));
    }
}

import java.util.Calendar;
 
class YesterdayCurrent {
    public static void main(String[] args){
        Calendar cal = Calendar.getInstance();
        cal.add(Calendar.DATE, -1);
        System.out.println(cal.getTime());
    }
}

import java.time.LocalDateTime;
 
class YesterdayCurrent {
 
    public static void main(String[] args) {
        LocalDateTime today = LocalDateTime.now();
        LocalDateTime yesterday = today.minusDays(1);
 
        System.out.println(yesterday);
    }
}

/** 
     * 字符串转换成日期 
     * @param str 
     * @return date 
     */
    public Date StrToDate(String str) {
        if(str.length()<=10){
            str = str.trim()+" 00:00:00";
        }
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        Date date = null;
        try {
            date = format.parse(str);
        }
        catch (ParseException e) {
            e.printStackTrace();
        }
        return date;
    }

try {
    throw new ExampleB("b")
} catch（ExampleA e）{
    System.out.println("ExampleA");
} catch（Exception e）{
    System.out.println("Exception");
}

class Annoyance extends Exception {}
class Sneeze extends Annoyance {}
 
class Human {
 
    public static void main(String[] args) 
        throws Exception {
        try {
            try {
                throw new Sneeze();
            } 
            catch ( Annoyance a ) {
                System.out.println("Caught Annoyance");
                throw a;
            }
        } 
        catch ( Sneeze s ) {
            System.out.println("Caught Sneeze");
            return ;
        }
        finally {
            System.out.println("Hello World!");
        }
    }
}

public class Student implements Comparable<Student> {
    private String name;        // 姓名
    private int age;            // 年龄
 
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
 
    @Override
    public String toString() {
        return "Student [name=" + name + ", age=" + age + "]";
    }
 
    @Override
    public int compareTo(Student o) {
        return this.age - o.age; // 比较年龄(年龄的升序)
    }
 
}

import java.util.Set;
import java.util.TreeSet;
 
class Test01 {
 
    public static void main(String[] args) {
        Set<Student> set = new TreeSet<>();     // Java 7的钻石语法(构造器后面的尖括号中不需要写类型)
        set.add(new Student("Hao LUO", 33));
        set.add(new Student("XJ WANG", 32));
        set.add(new Student("Bruce LEE", 60));
        set.add(new Student("Bob YANG", 22));
 
        for(Student stu : set) {
            System.out.println(stu);
        }
//      输出结果: 
//      Student [name=Bob YANG, age=22]
//      Student [name=XJ WANG, age=32]
//      Student [name=Hao LUO, age=33]
//      Student [name=Bruce LEE, age=60]
    }
}

public class Student {
    private String name;    // 姓名
    private int age;        // 年龄
 
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
 
    /**
     * 获取学生姓名
     */
    public String getName() {
        return name;
    }
 
    /**
     * 获取学生年龄
     */
    public int getAge() {
        return age;
    }
 
    @Override
    public String toString() {
        return "Student [name=" + name + ", age=" + age + "]";
    }
 
}

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
 
class Test02 {
 
    public static void main(String[] args) {
        List<Student> list = new ArrayList<>();     // Java 7的钻石语法(构造器后面的尖括号中不需要写类型)
        list.add(new Student("Hao LUO", 33));
        list.add(new Student("XJ WANG", 32));
        list.add(new Student("Bruce LEE", 60));
        list.add(new Student("Bob YANG", 22));
 
        // 通过sort方法的第二个参数传入一个Comparator接口对象
        // 相当于是传入一个比较对象大小的算法到sort方法中
        // 由于Java中没有函数指针、仿函数、委托这样的概念
        // 因此要将一个算法传入一个方法中唯一的选择就是通过接口回调
        Collections.sort(list, new Comparator<Student> () {
 
            @Override
            public int compare(Student o1, Student o2) {
                return o1.getName().compareTo(o2.getName());    // 比较学生姓名
            }
        });
 
        for(Student stu : list) {
            System.out.println(stu);
        }
//      输出结果: 
//      Student [name=Bob YANG, age=22]
//      Student [name=Bruce LEE, age=60]
//      Student [name=Hao LUO, age=33]
//      Student [name=XJ WANG, age=32]
    }
}

/**
 * 银行账户
 * @author 骆昊
 *
 */
public class Account {
    private double balance;     // 账户余额
 
    /**
     * 存款
     * @param money 存入金额
     */
    public void deposit(double money) {
        double newBalance = balance + money;
        try {
            Thread.sleep(10);   // 模拟此业务需要一段处理时间
        }
        catch(InterruptedException ex) {
            ex.printStackTrace();
        }
        balance = newBalance;
    }
 
    /**
     * 获得账户余额
     */
    public double getBalance() {
        return balance;
    }
}

/**
 * 存钱线程
 * @author 骆昊
 *
 */
public class AddMoneyThread implements Runnable {
    private Account account;    // 存入账户
    private double money;       // 存入金额
 
    public AddMoneyThread(Account account, double money) {
        this.account = account;
        this.money = money;
    }
 
    @Override
    public void run() {
        account.deposit(money);
    }
 
}

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
 
public class Test01 {
 
    public static void main(String[] args) {
        Account account = new Account();
        ExecutorService service = Executors.newFixedThreadPool(100);
 
        for(int i = 1; i <= 100; i++) {
            service.execute(new AddMoneyThread(account, 1));
        }
 
        service.shutdown();
 
        while(!service.isTerminated()) {}
 
        System.out.println("账户余额: " + account.getBalance());
    }
}

/**
 * 银行账户
 * @author 骆昊
 *
 */
public class Account {
    private double balance;     // 账户余额
 
    /**
     * 存款
     * @param money 存入金额
     */
    public synchronized void deposit(double money) {
        double newBalance = balance + money;
        try {
            Thread.sleep(10);   // 模拟此业务需要一段处理时间
        }
        catch(InterruptedException ex) {
            ex.printStackTrace();
        }
        balance = newBalance;
    }
 
    /**
     * 获得账户余额
     */
    public double getBalance() {
        return balance;
    }
}

/**
 * 存钱线程
 * @author 骆昊
 *
 */
public class AddMoneyThread implements Runnable {
    private Account account;    // 存入账户
    private double money;       // 存入金额
 
    public AddMoneyThread(Account account, double money) {
        this.account = account;
        this.money = money;
    }
 
    @Override
    public void run() {
        synchronized (account) {
            account.deposit(money); 
        }
    }
 
}

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
 
/**
 * 银行账户
 * 
 * @author 骆昊
 *
 */
public class Account {
    private Lock accountLock = new ReentrantLock();
    private double balance; // 账户余额
 
    /**
     * 存款
     * 
     * @param money
     *            存入金额
     */
    public void deposit(double money) {
        accountLock.lock();
        try {
            double newBalance = balance + money;
            try {
                Thread.sleep(10); // 模拟此业务需要一段处理时间
            }
            catch (InterruptedException ex) {
                ex.printStackTrace();
            }
            balance = newBalance;
        }
        finally {
            accountLock.unlock();
        }
    }
 
    /**
     * 获得账户余额
     */
    public double getBalance() {
        return balance;
    }
}