1、Java课件范例程序运行过程:javac 编译,java 运行例1-1 FirstProgram.javaclass Test public static void main() System.out.println(Hello,my first java program!); 编译:javac FirstProgram.java错误:FirstProgram.java:1: class Test is public, should be declared in a file named Test.java原因:如果文件中只有一个类,则文件名必须与类名相同。修改:Test改为FirstProg
2、ram编译:javac FirstProgram.java运行:java FirstProgram错误:Exception in thread main java.lang.NoSuchMethodError: main原因:程序的入口函数声明必须为public static void main(String args)。修改:第二行改为public static void main(String args)编译:javac FirstProgram.java运行:java FirstProgram结果:Hello,my first java program!例1-2 Another.java
3、修改FirstProgram.java的代码为:class FirstProgram public static void main(String args) Another another=new Another(); another.say(); public class Another public void say() System.out.println(Another one.); 编译:javac FirstProgram.java错误:FirstProgram.java:7: class Another is public, should be declared in a fi
4、le named Another.javapublic class Another 1 error原因:文件中如果有两个以上的类,则类名必须与其中的public类的类名一致。在没有public类时,则必须与其中任意一个类名一致。修改:文件名改为Another.java(或删除public)编译:javac Another.java运行:java Another错误:Exception in thread main java.lang.NoSuchMethodError: main原因:java Another的含义为,程序从Another类的main方法开始运行,但程序的Another类中并没
5、有main方法。运行:java FirstProgram结果:Another one.例1-3 FirstAppliet.java FirstApplet.htmlpublic class FirstApplet public void paint(Graphics g) g.setColor(Color.blue); g.drawString(Java是一门很优秀的语言,12,30); g.setColor(Color.red); g.drawString(我一定认真学习Java,22,56); 编译:javac FirstApplet.java错误:FirstApplet.java:2:
6、cannot find symbolsymbol : class Graphicslocation: class FirstAppletpublic void paint(Graphics g) 原因:无法识别Graphics类,没有引入相关的类包。修改:添加 import java.applet.*; import java.awt.*;编译:javac FirstApplet.java运行: appletviewer FirstApplet.html错误:java.lang.ClassCastException: FirstApplet at sun.applet.AppletPanel.
7、createApplet(AppletPanel.java:710) at sun.applet.AppletPanel.runLoader(AppletPanel.java:639) at sun.applet.AppletPanel.run(AppletPanel.java:319) at java.lang.Thread.run(Thread.java:595)原因:该类不是一个Applet,没有声明为Applet的子类。修改:将类声明修改为public class FirstApplet extends Applet编译:javac FirstApplet.java运行:appletv
8、iewer FirstApplet.html结果:关键字:abstract boolean break byte case catch char class continue default do double else enum extends final finally float for if implements import instanceof int interface long native new package private protected public return short static super switch synchronized this throw
9、throws transient try void volatile while 例2-1 InputTest.javaimport java.util.*;class InputTest public static void main(String args) Scanner reader=new Scanner(System.in); int n; n=reader.nextInt(); System.out.println(n=+n); 编译:javac InputTest.java运行:java InputTest输入:455k错误:Exception in thread main j
10、ava.util.InputMismatchException at java.util.Scanner.throwFor(Scanner.java:909) 原因:输入了不正确的数据,程序不够健壮修改:在语句n=reader.nextInt();前添加代码 while (!reader.hasNextInt() 修改后的代码:import java.util.*;class InputTest public static void main(String args) Scanner reader=new Scanner(System.in); int n; System.out.printl
11、n(请输入一个整数:); while (!reader.hasNextInt() reader.nextLine(); System.out.println(输入格式错误,请重新输入:); n=reader.nextInt(); System.out.println(输入的整数为:+n); 例2-2 Division.javaimport java.util.*; class Division public static void main(String args) Scanner reader=new Scanner(System.in); int a,b; double q; System
12、.out.println(请输入被除数:); if (reader.hasNextInt() a=reader.nextInt(); System.out.println(请输入除数:); if (reader.hasNextInt() b=reader.nextInt(); q=a/b; System.out.printf(除法算式及运算结果:n%10d / %d = %10.2f,a,b,q); 编译:javac Division.java错误:Division.java:13: 错误: 可能尚未初始化变量a q=a/b; 原因:a、b只在分支语句的一个分支中获得初值,所以有可能并未被初始
13、化修改:在定义变量a、b时便给出初值,或者在使用a、b变量之前的所有分支中都给a、b赋值修改后的代码;import java.util.*; class Division public static void main(String args) int a,b; double q; System.out.println(请输入被除数:); a=input(); System.out.println(请输入除数:); b=input(); if (b=0) System.out.println(被0除,非法运算!); return; q=a/b; System.out.printf(除法算式即运
14、算结果:n%10d / %d = %10.2f,a,b,q); private static int input() Scanner reader=new Scanner(System.in); while (!reader.hasNextInt() reader.nextLine(); System.out.println(输入格式错误,请重新输入:); return reader.nextInt(); 依然存在的问题:小数部分始终为0原因:整数的除法实现的是整除修改:将被除数a或除数b进行强制类型转换 (q=(double)a/b;)例2-3 ArrayTest.javaclass Arr
15、ayTest public static void main(String args) System.out.println(b); for (int i=0;i0 & i0 & j6) tablei-1j-1=true; else break; System.out.println(); System.out.println( 周一 周二 周三 周四 周五 周六 周日 ); System.out.println(); for (i=0;i10;i+) System.out.printf(%2d,(i+1); for (j=0;j7;j+) if (tableji/2) System.out.
16、print( Java); else System.out.print( ); System.out.println(); if (i%4=3) System.out.println(); private static int input() Scanner reader=new Scanner(System.in); while (!reader.hasNextInt() reader.nextLine(); System.out.println(输入格式错误,请重新输入:); return reader.nextInt(); 优先级:postfixexpr+ expr-unary+expr
17、 -expr +expr -expr !multiplicative* / %additive+ -shift relational = instanceofequality= !=bitwise AND&bitwise exclusive ORbitwise inclusive OR|logical AND&logical OR|ternary? :assignment= += -= *= /= %= &= = |= = =例 3-1 OperatorTest.javaclass OperatorTest public static void main(String args) int a=
18、3,b=4,c=5,d=6,m=7,n=8; int x=64,arr=new int8; long y=-0x1000000000L,z=4; boolean flag; flag = a3 & b-4; flag = +c5 | +d6; flag = m=7 | +n=8; x=33; y=33; z=65; System.out.printf(a=%d,b=%d;c=%d,d=%d,m=%d,n=%dn,a,b,c,d,m,n); System.out.printf(x=%d,y=%d,z=%d,xz=%dn,x,y,z,xz); System.out.println(arr inst
19、anceof int); 例 3-2 ControlTest.java import java.util.*;public class ControlTest public static void main(String args) System.out.println(请输入一个整数:); int n=input(); for (int i=0;in;i+) if (i%2=0) continue; if (i%n=6) /(替换6为7) break; System.out.println(i=+i); private static int input() Scanner reader=ne
20、w Scanner(System.in); while (!reader.hasNextInt() reader.nextLine(); System.out.println(输入格式错误,请重新输入:); return reader.nextInt(); 例4-1 Circle.javafinal static double PI;class Circle PI=3.14; double r; double circumference() return 2*PI*r; /实例方法可以访问类变量 double area() return PI*r*r; 编译错误:Circle.java:1:
21、需要为 class、interface 或 enumfinal static double PI; Circle.java:3: 需要 PI=3.14; 原因:类定义之外不能有任何语句;类体内,方法体外不允许有除成员变量定义外的任何语句;修改:class Circle final static double PI=3.14; 例4-2 CircleTest.javaclass CircleTest public static void main(String args) Circle c=new Circle(); System.out.printf(Area of the Circle is
22、 : %8.4f,c.area(); 编译运行正确:原因: 类的成员变量可不初始化Circle类的r变量 类有默认的构造方法(不带任何参数)Circle c=new Circle(); 为Circle添加构造方法: Circle(double r) this.r=r; /形参r与成员变量r重名,必须使用this访问 运行错误:Exception in thread main java.lang.NoSuchMethodError: Circle: method ()V not found at CircleTest.main(CircleTest.java:3)原因:添加新的构造方法后,则默认
23、的构造方法失效修改:Circle c=new Circle(3); 在CircleTest类的main方法中添加以下代码:Circle c1=null;System.out.printf(PI is : %8.4f,c1.PI); /或Circle.PI编译运行正确:原因:类变量的初始化不需要该类的任何实例存在 在Circle类中添加方法:static void setPI(double pi) PI=pi;编译错误: Circle.java:19: 无法为最终变量 PI 指定值 PI=pi; Circle.java:23: 无法从静态上下文中引用非静态变量 this this.r=r; 原因
24、:final常量的值无法更改,静态方法不能访问实例变量。修改:删除PI的final定义以及setPI方法的static定义 修改CircleTest类的main方法:Circle c1=null; System.out.printf(PI is : %8.4fn,c1.PI); c1.setPI(3.1415926); Circle c=new Circle(3); System.out.printf(Area of the Circle is : %8.4fn,c.area();运行结果: PI is : 3.1400Area of the Circle is : 28.2743原因:Cir
25、cle类的类变量PI已经发生改变,改变对所有Circle实例均有效例4-3 SomeThing.javaclass SomeThing int x; int y; void move(int x,int y) this.x=x; this.y=y; 以下那些形式的重载有效?boolean move(int x,int y) boolean flag=(this.x=x)&(this.y=y); this.x=x; this.y=y; return flag; /错误,仅返回值不同 void move(int width,int height) x+=width; y+=height; /错误,仅变量名不同 void move(int distance,boolean direction) if (direction) x+=distance; else y+=distance; /正确,参数类型不同例 4-4 Card.javaclass Card int id; double balance; Card(int id,double balance) this.id=id; this.balance
copyright@ 2008-2022 冰豆网网站版权所有
经营许可证编号:鄂ICP备2022015515号-1