java第二节课 java语法基础动手动脑

发布时间：2020-12-15 07:42:34 所属栏目：Java 来源：网络整理

导读：动手动脑1：枚举变量 ? 运行EnumTest.java package test2; public class EnumTest { ?public static void main(String[] args) { ??Size s=Size.SMALL; ??Size t=Size.LARGE; ??//s和t引用同一个对象？ ??System.out.println(s==t);? // ??//是原始数据类型

动手动脑1：枚举变量 ? 运行EnumTest.java

package test2;

public class EnumTest {

?public static void main(String[] args) {
??Size s=Size.SMALL;
??Size t=Size.LARGE;
??//s和t引用同一个对象？
??System.out.println(s==t);? //
??//是原始数据类型吗？
??System.out.println(s.getClass().isPrimitive());
??//从字符串中转换
??Size u=Size.valueOf("SMALL");
??System.out.println(s==u);? //true
??//列出它的所有值
??for(Size value:Size.values()){
???System.out.println(value);
??}
?}

}
?enum Size{SMALL,MEDIUM,LARGE};

?枚举类型是Java 5中新增特性的一部分，它是一种特殊的数据类型，之所以特殊是因为它既是一种类(class)类型却又比类类型多了些特殊的约束，但是这些约束的存在也造就了枚举类型的简洁性、安全性以及便捷性。这个题目中这个枚举变量中定义 Small middle large三种。Size.SMALL！=Size.MIddle后面是枚举变量的foreach迭代。枚举变量不属于原始数据类型，他的每个具体值都引用一个特定的对象。相同的值引用同一个对象。

动手动脑2：数值的二进制表示。

原码表示法是机器数的一种简单的表示法。其符号位用0表示正号，用：表示负号，数值一般用二进制形式表示。设有一数为x，则原码表示可记作〔x〕原。

机器数的补码可由原码得到。如果机器数是正数，则该机器数的补码与原码一样；如果机器数是负数，则该机器数的补码是对它的原码（除符号位外）各位取反，并在未位加1而得到的。设有一数X，则X的补码表示记作〔X〕补。

机器数的反码可由原码得到。如果机器数是正数，则该机器数的反码与原码一样；如果机器数是负数，则该机器数的反码是对它的原码（符号位除外）各位取反而得到的。设有一数X，则X的反码表示记作〔X〕反。

package test2;

public class CeShiCLSASS {
?public static void main(String[]args)
?{
??int a=15;
??int b=2;
??System.out.println(a&b);
??System.out.println(a|b);
??System.out.println(a^b);
?}

}

?示例1：两数相加

import javax.swing.JOptionPane;? // import class JOptionPane

public class Addition {
?? public static void main( String args[] )
?? {
????? String firstNumber,?? // first string entered by user
???????????? secondNumber;? // second string entered by user
????? int number1,????????? // first number to add
????????? number2,????????? // second number to add
????????? sum;????????????? // sum of number1 and number2

????? // read in first number from user as a string
????? firstNumber =
???????? JOptionPane.showInputDialog( "Enter first integer" );

????? // read in second number from user as a string
????? secondNumber =
???????? JOptionPane.showInputDialog( "Enter second integer" );

????? // convert numbers from type String to type int
????? number1 = Integer.parseInt( firstNumber );
????? number2 = Integer.parseInt( secondNumber );

????? // add the numbers
????? sum = number1 + number2;

????? // display the results
????? JOptionPane.showMessageDialog(
???????? null,"The sum is " + sum,"Results",
???????? JOptionPane.PLAIN_MESSAGE );

????? System.exit( 0 );?? // terminate the program
?? }
}

?动手实验1：运行TestDouble.java

public class TestDouble {

??? public static void main(String args[]) {
??????? System.out.println("0.05 + 0.01 = " + (0.05 + 0.01));
??????? System.out.println("1.0 - 0.42 = " + (1.0 - 0.42));
??????? System.out.println("4.015 * 100 = " + (4.015 * 100));
??????? System.out.println("123.3 / 100 = " + (123.3 / 100));
??? }
}

?使用double类型的数值进行计算是，其结果是不精确的。

解决方案1：使用BigDecimal类 ? BigDecimal.java

注意，在构建BigDecima对象时应该使用字符串而不是double数值，否则，仍有可能引发计算精度问题。字符串转为数字的基本方法 int number=Integer。parseInt（numberString）；

动手动脑：以下代码的输出结果是啥？

package test2;

public class CeShiCLSASS {
?public static void main(String[]args)
?{
??int X=100;
??int Y=200;
??System.out.println("X+Y="+X+Y);
??System.out.println(X+Y+"=X+Y");
?}

}

?测试：InputTest.java

import java.util.*;

public class InputTest
{?
?? public static void main(String[] args)
?? {?
????? Scanner in = new Scanner(System.in);

????? // get first input
????? System.out.print("What is your name? ");
????? String name = in.nextLine();

????? // get second input
????? System.out.print("How old are you? ");
????? int age = in.nextInt();
?????
?????
??? /* int i;
???? String value="100";
???? i=Integer.parseInt(value);
???? i=200;
???? String s=String.valueOf(i);*/
????
????? // display output on console
????? System.out.println("Hello," + name + ". Next year,you‘ll be " + (age + 1));
?
?????
?? }
}

测试：RandomStr

package test2;

public class RandomStr
{
?public static void main(String[] args)
?{
??//定义一个空字符串
??String result = "";
??//进行6次循环
??for(int i = 0 ; i < 6 ; i ++)
??{
???//生成一个97～122的int型的整数
???int intVal = (int)(Math.random() * 26 + 97);
???//将intValue强制转换为char后连接到result后面
???result = result + (char)intVal;
??}
??//输出随机字符串
??System.out.println(result);
??}
}

测试：SwitchTest.java

?// Drawing shapes
import java.awt.Graphics;
import javax.swing.*;

public class SwitchTest extends JApplet {
?? int choice;??

?? public void init()
?? {
????? String input;

????? input = JOptionPane.showInputDialog(
???????????????? "Enter 1 to draw linesn" +
???????????????? "Enter 2 to draw rectanglesn" +
???????????????? "Enter 3 to draw ovalsn" );

????? choice = Integer.parseInt( input );
?? }

?? public void paint( Graphics g )
?? {
????? for ( int i = 0; i < 10; i++ ) {
???????? switch( choice ) {
??????????? case 1:
?????????????? g.drawLine( 10,10,250,10 + i * 10 );
?????????????? break;
??????????? case 2:
?????????????? g.drawRect( 10 + i * 10,10 + i * 10,
?????????????????????????? 50 + i * 10,50 + i * 10 );
?????????????? break;
??????????? case 3:
?????????????? g.drawOval( 10 + i * 10,50 + i * 10 );
?????????????? break;
??????????? default:
?????????????? JOptionPane.showMessageDialog(
????????????????? null,"Invalid value entered" );
???????? } // end switch
????? } // end for
?? } // end paint()
} // end class SwitchTest

/**************************************************************************
* (C) Copyright 1999 by Deitel & Associates,Inc. and Prentice Hall.???? *
* All Rights Reserved.?????????????????????????????????????????????????? *
*??????????????????????????????????????????????????????????????????????? *
* DISCLAIMER: The authors and publisher of this book have used their???? *
* best efforts in preparing the book. These efforts include the????????? *
* development,research,and testing of the theories and programs??????? *
* to determine their effectiveness. The authors and publisher make?????? *
* no warranty of any kind,expressed or implied,with regard to these??? *
* programs or to the documentation contained in these books. The authors *
* and publisher shall not be liable in any event for incidental or?????? *
* consequential damages in connection with,or arising out of,the?????? *
* furnishing,performance,or use of these programs.???????????????????? *
*************************************************************************/

课堂测试：

package sizeyunsuan;
import java.util.Random;
import java.util.Scanner;
public class Sizeyunsuan
{
??
?? public static void main(String[]args)
?? {??

? //a控制前一个数，b控制加减乘除，c控制后一个数。
??? int a=(int)(Math.random()*100);
??? int b=(int)(Math.random()*4);
??? int c=(int)(Math.random()*100);
???
??? System.out.println("请输入您想要多少道题");
??? Scanner in=new Scanner(System.in);
??? int j;
??? j=in.nextInt();
??? System.out.println("请输入您想几个一排：");
??? int k;
??? k=in.nextInt();
???
?for(int i=1;i<=j;i++)
?{
??b=(int)(Math.random()*4);//产生[0,4)的随机数，用此来控制加减乘除，0代表加，依次类推。
??
??
??if(b==0)//实现两数相加不超过100
??{?
??? do
??? {
???? a=(int)(Math.random()*100);
???? c=(int)(Math.random()*100);
???? if(a+c<100)
???? {
????? if(i%k==0)
????? {
????? System.out.println("第"+i+"道题："+a+"+"+c+"="+"???????? ");
????? break;
????? }
????? else
????? {
?????? System.out.print("第"+i+"道题："+a+"+"+c+"="+"???????? ");
?????? break;
????? }
???? }
????
??? }while(a>=0);
??}
??
??
??if(b==1)//实现两数相减大于0
??{
???do
???{
????a=(int)(Math.random()*100);
????c=(int)(Math.random()*100);
????if(a>c)
????{
????? if(i%k==0)
????? {
????? System.out.println("第"+i+"道题："+a+"-"+c+"="+"???????? ");
????? break;
????? }
????? else
????? {
?????? System.out.print("第"+i+"道题："+a+"-"+c+"="+"???????? ");
?????? break;
????? }
????}
???}while(a>=0);
??}
??
??
??
??if(b==2)//实现两数相乘不超过100
??{
???
???do
???{
???a=(int)(Math.random()*100);
???c=(int)(Math.random()*100);
???if(a*c<100)
???{
???? if(i%k==0)
???? {
???? System.out.println("第"+i+"道题："+a+"*"+c+"="+"???????? ");
???? break;
???? }
???? else
???? {
????? System.out.print("第"+i+"道题："+a+"*"+c+"="+"???????? ");
????? break;
???? }
???}
???}while(a>=0);
???
???
??}
??
??
?? if(b==3)//解决整除问题
??{
??? do
??? {
????a=(int)(Math.random()*100);
????c=(int)(Math.random()*100);
????if(c!=0&&a%c==0)
????{
????? if(i%k==0)
????? {
????? System.out.println("第"+i+"道题："+a+"/"+c+"="+"???????? ");
????? break;
????? }
????? else
????? {
?????? System.out.print("第"+i+"道题："+a+"/"+c+"="+"???????? ");
?????? break;
????? }
????}
??? }while(a>=0);
???
???
???
??}
??
??
??
? }
?
?? }

}

（编辑：李大同）

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