JAva学習ノートのMath、Random、Integer、Calender類

数字を当てるゲーム:

package com.mcq;

import java.util.Scanner;

public class MathDemo {
	public static void main(String[] args) {
		int num=(int)(Math.random()*100)+1,cnt=0;
		while(true){
			System.out.println("     1-100   ");
			Scanner scanner=new Scanner(System.in);
			String next=scanner.next();
			int guess=-1;
			cnt++;
			try {
				 guess=Integer.valueOf(next);
			} catch (NumberFormatException e) {//      
				// TODO: handle exception
				System.out.println("     !!!");
				continue;
			}
			if(guess<1||guess>100){
				System.out.println("          ");
//				throw new NumberException("         ");
			}
			if(guessnum){
				System.out.println("     ");
			}else{
				System.out.println("        "+cnt+" ");
				break;
			}
		}
	}
}

Randomクラスの使い方:

package com.mcq;

import java.util.Random;

public class RandomDemo {
	public static void main(String[] args) {
		Random random=new Random();
		System.out.println(random.nextInt(100));//    
	}
}

Integer:

package com.mcq;

public class IntegerDemo {
	public static void main(String[] args) {
		Integer i=1;//    Integer i=Integer.valueOf(1);
		int a=i;//    int a=i.intValue();
	}
}

面接問題:Integer num=0;num+=1;この過程でどのような操作をしましたか?
答え:まず自動的に箱を取り外して、それから自動的に箱に入れます.
BigDecimal

package com.mcq;

import java.math.BigDecimal;

public class BigDecimalDemo {
	public static void main(String[] args) {
		double a=0.01,b=0.09;
		BigDecimal aa=new BigDecimal("0.01"),bb=new BigDecimal("0.09");//            
		System.out.println(a+b);
		System.out.println(aa.add(bb));
	}
}

Calender

package com.mcq;

import java.util.Date;
import java.util.Calendar;

public class CalenderDemo {
	public static void main(String[] args) {
		Calendar calender=Calendar.getInstance();
		System.out.println(calender);
		
		System.out.println(new Date().getTime());
		
		System.out.println(calender.get(calender.YEAR));
		System.out.println(calender.get(calender.MONTH)+1);//   1
		System.out.println(calender.get(calender.DATE));
		calender.set(2000,2,30);//    
		
		System.out.println(calender.get(calender.YEAR));
		System.out.println(calender.get(calender.MONTH)+1);//   1
		System.out.println(calender.get(calender.DATE));
	}
}

String

package com.mcq;

public class StringDemo {
	public static void main(String[] args) {
		String str = "12345";
		char[] arr = str.toCharArray();
		for (char c : arr) {
			System.out.println(c);
		}
		String s = String.valueOf(arr);
		String s1 = String.valueOf(true);
		System.out.println(s);
		System.out.println(s1);
		
		String s2="  12 5 36 8 ";
		System.out.println(s2.trim());//       
		String s3=s2.replace("12 ", "233");//   /        /   
		System.out.println(s3);
		System.out.println("--------------------");
		int num = 123;
		String string=String.valueOf(num);//int String
		int a=Integer.valueOf(string);//String int
		
		String string2="hehe";
		String string3="hehe";//String          ，  string3   ==string2   
		String string4=new String("hehe");
		System.out.println(string2==string3);//==     
		System.out.println(string2==string4);
		String string5=string2.concat("");//concat              ，        
		String string6=string2+"";//+        
		System.out.println(string2==string5);
		System.out.println(string2==string6);
		System.out.println(string2.equals(string5));
	}
}

/*
1
2
3
4
5
12345
true
12 5 36 8
  2335 36 8 
--------------------
true
false
true
false
true

*/

StringBuilder

package com.mcq;

public class StringBuilderDemo {
	public static void main(String[] args) {
		StringBuilder stringBuilder=new StringBuilder();//     16
		System.out.println(stringBuilder.capacity());
		StringBuilder stringBuilder2=new StringBuilder(1024);
		//    
		StringBuilder append=stringBuilder2.append("hello").append(" world").append(new Student("  ",18));
		System.out.println(append);//       student toString        
		stringBuilder2.insert(5, "gg");//          
		System.out.println(stringBuilder2);
		
		stringBuilder2.reverse();//  
		System.out.println(stringBuilder2);
		
	}
}

/*
16
hello world   18
hellogg world   18
81   dlrow ggolleh
*/

効率:StringBuilder>StringBuffer>String

package com.mcq;

public class StringBuilderDemo2 {
	public static void main(String[] args) {
		int arr[]={1,2,3,4,5,6,7};
		long start=System.currentTimeMillis();
		for(int i=0;i<1000000;i++){
			
//			stringBuilder(arr);//386 379 389 ms
//			stringBuffer(arr);//953 955 664 ms
			str(arr);//1191 1072 1117 ms
		}
		long end=System.currentTimeMillis();
		System.out.println(end-start);
		
	}
	public static void stringBuilder(int []arr){
		StringBuilder sb=new StringBuilder("[");
		for(int i=0;i