java复习笔记4

阅读目录

• 　　java常用类库
• 　　StringBuffer
• 　　Runtime
• 　　System
• 　　Calendar类
• 　　Math类
• 　　Random类
• 　　BigInteger
• 　　Cloneable类
• 　　java.util.Arrays
• 　　java.util.Comparable
• 　　java的观察者Observer和Observable类
• 　　TimerTask和Timer定时调度的使用

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　　java常用类库

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　　StringBuffer

 　　StringBuffer适合经常对同一个string进行修改的情况， StringBuffer是java.lang.StringBuffer的实现， 默认引用了, StringBuffer拥有很多js的常见方法;

运行下面代码

public class StringBufferDemo {
    public static void run( StringBuffer sb ) {
        sb.append(" world");
    }
    public static void main(String args[]) {
        StringBuffer sb = new StringBuffer();
        //append
        sb.append("hello");
        run(sb);
        System.out.println(sb.toString());

        //insert
        sb.insert(0, "start ");
        sb.insert(sb.length(), " end");
        System.out.println(sb.toString());

        //reverse
        System.out.println(sb.reverse());

        //replace
        System.out.println(sb.replace(0, 4, "????"));

        //substring
        System.out.println(sb.substring(0,4));

        System.out.println(sb.delete(0,4));

        System.out.println(sb.indexOf( "olleh"));
    }
}

 

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　　Runtime

运行下面代码

public class RuntimeDemo {
    public static void main(String args[]) {
        Runtime run = Runtime.getRuntime();
        System.out.println( run.maxMemory() );
        System.out.println( run.freeMemory() );
        String str = "";
        for(int i=0; i<10; i++) {
            str+=i;
            str+=str;
        };
        System.out.println( str + " " + run.freeMemory() );
        run.gc();
        System.out.println( run.freeMemory() );

        //runtime.exec();
        try{
            run.exec("calc");
        }catch(Exception e) {
            e.printStackTrace();
        }

        //process
        //Thread
        //Process.destroy();
        Process p = null;
        try{
            p = run.exec("notepad");
        }catch(Exception e) {
            e.printStackTrace();
        }
        try{
            Thread.currentThread().sleep(2000);
        }catch(Exception e) {
            e.printStackTrace();
        }
        p.destroy();
    }
}

 

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　　System

运行下面代码

public class SystemDemo {
    public static void main(String args[]) {
        System.out.println(System.getProperties());
        System.out.println(System.currentTimeMillis());
        //list(System.out)
        System.getProperties().list(System.out);
        /*** error
        int arr[] = {1,2,3,4,5,6};
        arr.list(System.out);
        */
    }
}

 

　　System.gc()

运行下面代码

class FinalizeTest {
    public String name = "name";
    public String age = "17";
    public String toString() {
        return this.name+"____"+this.age;
    }
    public void finalize() throws Throwable{
        System.out.print(this);
    } 
}

public class FinalizeDemo {
    public static void main( String args[] ) {
        FinalizeTest fin = new FinalizeTest();
        fin = null;
        System.gc();
    }
}

 

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　　Calendar类

运行下面代码

import java.util.*;
public class CalendarDemo {
    public static void main(String args[]) {
         Calendar calendar = new GregorianCalendar();
         System.out.println("YEAR: " + calendar.get(Calendar.YEAR));
         System.out.println("MONTH: " + calendar.get(Calendar.MONTH));
         System.out.println("DATE: " + calendar.get(Calendar.DATE));
         System.out.println("DAY_OF_MONTH: " + calendar.get(Calendar.DAY_OF_MONTH));
         System.out.println("HOUR: " + calendar.get(Calendar.HOUR));
         System.out.println("MINUTE: " + calendar.get(Calendar.MINUTE));
         System.out.println("SECOND: " + calendar.get(Calendar.SECOND));
         System.out.println("MILLISECOND: " + calendar.get(Calendar.MILLISECOND));
    }
}

 

　　

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　　Math类

运行下面代码

public class MathDemo {
    public static void main(String args[]) {
        System.out.println(Math.sqrt(9));
        System.out.println(Math.min(10,100));
        System.out.println(Math.max(10,100));
        System.out.println(Math.pow(3,3));
        System.out.println(Math.round(3.3));
    }
}

 

 

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　　Random类

运行下面代码

import java.util.Random;
public class RandomDemo{
    public static void main(String args[]) {
        Random ran = new Random();
        for(int i=0; i<4; i++) {
            System.out.println( ran.nextInt(40) );
        }
    }
}

 

 

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　　BigInteger

运行下面代码

import java.math.BigInteger;
public class BigIntegerDemo {
    public static void main(String args[]) {
        BigInteger bi = new BigInteger("1111111111111111");
        BigInteger bi1 = new BigInteger("1111111111111110");
        System.out.println(bi.add(bi1));
        System.out.println(bi.multiply(bi1));
        System.out.println(bi.divide(bi1));
        System.out.println(bi.max(bi1));
        System.out.println(bi.min(bi1));
    }
}

 

 

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　　Cloneable类

运行下面代码

class Person implements Cloneable{
    public String name = "nono";
    public void setName(String name) {
        this.name = name;
    };
    public String toString() {
        return this.name;
    }
    public Object clone() throws CloneNotSupportedException{
        return super.clone();
    }
}
public class CloneDemo {
    public static void main(String args[]) throws Exception{
        Person per = new Person();
        Person per1 = (Person)per.clone();
        per1.setName("heheda");
        System.out.println(per);
        System.out.println(per);
    }
}

 

 

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　　java.util.Arrays

运行下面代码

 import java.util.*;
 public class ArraysDemo {
     public static void main(String args[]) {
         int temp[] = {5,1,2,3,4};
         Arrays.sort(temp);
         System.out.println("sort");
         System.out.println(Arrays.toString(temp));
         int point = Arrays.binarySearch(temp,3);
         System.out.println(point);
         Arrays.fill(temp, 1);
         System.out.println(Arrays.toString(temp));
     }
 }

 

 

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　　java.util.Comparable

运行下面代码

import java.util.*;
class Student implements Comparable<Student>{
    public String name;
    public float score;
    public Student(String name, float score) {
        this.name = name;
        this.score = score;
    }
    public String toString() {
        return "name :"+this.name+"  score:"+this.score;
    }
    public int compareTo(Student stu) {
        if(this.score<stu.score) {
            return -1;
        }else if(this.score==stu.score){
            return 0;
        }else{
            return 1;
        }
    }
}
public class ComparableDemo {
    public static void main(String args[]) {
        Student stus[] = {
            new Student("dada",10),
            new Student("meme",100),
            new Student("hehe",9),
            new Student("6666",60)
        };
        java.util.Arrays.sort(stus);
        System.out.println(Arrays.toString(stus));
    }
}

 

 

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　　java的观察者Observer和Observable类

运行下面代码

import java.util.*;
class House extends Observable{
    public float price;
    public void setPrice(float p) {
        super.setChanged();
        super.notifyObservers(p);
        this.price = p;
    };
    public float getPrice() {
        return this.price;
    };
    public House(float p) {
        this.price = p;
    };
    public String toString() {
        return "house: "+this.price;
    }
}
class HouseObserver implements Observer{
    public String name = "";
    public HouseObserver(String name) {
        this.name = name;
    }
    public void update(Observable o, Object arg) {
        System.out.println(o);
        System.out.println("change to : " + arg);
    }
}
public class ObservableDemo {
    public static void main(String args[]) {
        House h = new House(10000);
        //System.out.println(new HouseObserver("nono").name);
        h.addObserver( new HouseObserver("nono") );
        h.addObserver( new HouseObserver("hehe") );
        h.addObserver( new HouseObserver("dada") );
        h.setPrice(10);
    }
}

 

 

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　　TimerTask和Timer定时调度的使用

运行下面代码

import java.util.Timer;
import java.util.TimerTask;
import java.util.Date;
import java.text.SimpleDateFormat;

class MyTask extends TimerTask{
    public void run() {
        SimpleDateFormat sdf = null;
        sdf = new SimpleDateFormat("yyyy-mm-dd hh:mm:ss");
        System.out.println(sdf.format(new Date()));
    }
}

public class TimerDemo {
    public static void main(String args[]) {
        Timer t = new Timer();
        MyTask task = new MyTask();
        t.schedule(task, 1000, 2000);
    }
}

 

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