// 使用BufferedReader从控制台读取字符
import java.io.*;
class BRRead {
public static void main(String args[]) throws IOException {
char c;
BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); // 创建缓冲读取器
System.out.println("Enter characters, 'q' to quit.");
// 读取字符直到输入'q'
do {
c = (char) br.read(); // 读取单个字符
System.out.println(c);
} while (c != 'q');
}
}
// 使用BufferedReader从控制台读取字符串import java.io.*;
class BRReadLines {
public static void main(String args[]) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); // 包装System.in
String str;
System.out.println("Enter lines of text.");
System.out.println("Enter 'stop' to quit.");
do {
str = br.readLine(); // 读取整行字符串
System.out.println(str);
} while (!str.equals("stop"));
}
}
// 简易文本编辑器：读取多行文本并存储
import java.io.*;
class TinyEdit {
public static void main(String args[]) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String str[] = new String[100]; // 存储最多100行
System.out.println("Enter lines of text.");
System.out.println("Enter 'stop' to quit.");
for (int i = 0; i < 100; i++) {
str[i] = br.readLine();
if (str[i].equals("stop")) break; // 输入"stop"时终止 }
System.out.println("
Here is your file:");
// 显示存储的文本行 for (int i = 0; i < 100; i++) {
if (str[i].equals("stop")) break;
System.out.println(str[i]);
}
}
}
// 演示System.out.write()方法：输出单个字节
class WriteDemo {
public static void main(String args[]) {
int b;
b = 'A';
System.out.write(b); // 输出字符'A'的ASCII码
System.out.write('
'); // 输出换行符
System.out.flush(); // 建议添加以确保输出 }
}
// 演示PrintWriter：格式化输出到控制台
import java.io.*;
public class PrintWriterDemo {
public static void main(String args[]) {
PrintWriter pw = new PrintWriter(System.out, true); // autoFlush=true
pw.println("This is a string");
int i = -7;
pw.println(i); // 自动转换为字符串输出 double d = 4.5e-7;
pw.println(d); // 输出科学计数法数值 }
}
// 显示文本文件内容（基本版本）
import java.io.*;
class ShowFile {
public static void main(String args[]) {
int i;
FileInputStream fin;
// 检查命令行参数 if (args.length != 1) {
System.out.println("Usage: ShowFile filename");
return; // 提前退出
}
// 尝试打开文件 try {
fin = new FileInputStream(args[0]); // 创建文件输入流
} catch (FileNotFoundException e) {
System.out.println("Cannot Open File");
return;
}
// 读取文件内容 try {
do {
i = fin.read(); // 读取一个字节
if (i != -1) System.out.print((char) i); // 转换为字符输出
} while (i != -1); // -1表示EOF } catch (IOException e) {
System.out.println("Error Reading File");
}
// 关闭文件
try {
fin.close();
} catch (IOException e) {
System.out.println("Error Closing File");
}
}
}
// 显示文本文件内容（改进版本：使用单个try块和finally）
import java.io.*;
class ShowFile {
public static void main(String args[]) {
int i;
FileInputStream fin = null; // 声明为null if (args.length != 1) {
System.out.println("Usage: ShowFile filename");
return;
}
try {
fin = new FileInputStream(args[0]);
do {
i = fin.read();
if (i != -1) System.out.print((char) i);
} while (i != -1);
} catch (FileNotFoundException e) {
System.out.println("File Not Found.");
} catch (IOException e) {
System.out.println("An I/O Error Occurred");
} finally {
//确保文件被关闭 try {
if (fin != null) fin.close(); // 避免NullPointerException } catch (IOException e) {
System.out.println("Error Closing File");
}
}
}
}
// 复制文本文件
import java.io.*;
class CopyFile {
public static void main(String args[]) throws IOException {
int i;
FileInputStream fin = null;
FileOutputStream fout = null;
if (args.length != 2) {
System.out.println("Usage: CopyFile from to");
return;
}
// 复制文件
try {
fin = new FileInputStream(args[0]); // 源文件 fout = new FileOutputStream(args[1]); // 目标文件 do {
i = fin.read();
if (i != -1) fout.write(i); // 写入目标文件
} while (i != -1);
} catch (IOException e) {
System.out.println("I/O Error: " + e);
} finally {
// 分别关闭两个流
try {
if (fin != null) fin.close();
} catch (IOException e2) {
System.out.println("Error Closing Input File");
}
try {
if (fout != null) fout.close();
} catch (IOException e2) {
System.out.println("Error Closing Output File");
}
}
}
}
// 使用try-with-resources自动关闭文件（Java 7+）
import java.io.*;
class ShowFile {
public static void main(String args[]) {
int i;
if (args.length != 1) {
System.out.println("Usage: ShowFile filename");
return;
}
// try-with-resources自动管理资源
try (FileInputStream fin = new FileInputStream(args[0])) { // 自动关闭
do {
i = fin.read();
if (i != -1) System.out.print((char) i);
} while (i != -1);
} catch (FileNotFoundException e) {
System.out.println("File Not Found.");
} catch (IOException e) {
System.out.println("An I/O Error Occurred");
}
}
}
// 使用try-with-resources复制文件（管理多个资源）
import java.io.*;
class CopyFile {
public static void main(String args[]) throws IOException {
int i;
if (args.length != 2) {
System.out.println("Usage: CopyFile from to");
return;
}
// 单个try语句管理两个资源
try (FileInputStream fin = new FileInputStream(args[0]);
FileOutputStream fout = new FileOutputStream(args[1])) {
do {
i = fin.read();
if (i != -1) fout.write(i);
} while (i != -1);
} catch (IOException e) {
System.out.println("I/O Error: " + e);
}
}
}
//演示instanceof运算符：类型检查
class A { int i, j; }
class B { int i, j; }
class C extends A { int k; }
class D extends A { int k; }
class InstanceOf {
public static void main(String args[]) {
A a = new A();
B b = new B();
C c = new C();
D d = new D();
// 基本类型检查
if (a instanceof A) System.out.println("a is instance of A");
if (c instanceof A) System.out.println("c can be cast to A"); // 子类实例 if (a instanceof C) System.out.println("a can be cast to C"); // 不会执行
// 多态情况下的类型检查
A ob;
ob = d;
System.out.println("ob now refers to d");
if (ob instanceof D) System.out.println("ob is instance of D"); // 实际类型为D
ob = c;
System.out.println("ob now refers to c");
if (ob instanceof D) System.out.println("ob can be cast to D");
else System.out.println("ob cannot be cast to D"); // 实际类型为C
// 所有对象都是Object的实例
if (a instanceof Object) System.out.println("a may be cast to Object");
if (b instanceof Object) System.out.println("b may be cast to Object");
}
}
// 演示assert断言（需启用-ea参数）
class AssertDemo {
static int val = 3;
static int getnum() {
return val--;
}
public static void main(String args[]) {
int n;
for (int i = 0; i < 10; i++) {
n = getnum();
assert n > 0; // 断言n>0，当n=0时失败
System.out.println("n is " + n);
}
}
}
// 错误使用assert的示例（副作用）
class AssertDemo {
static int val = 3;
static int getnum() {
return val--;
}
public static void main(String args[]) {
int n = 0;
for (int i = 0; i < 10; i++) {
// 错误：assert包含赋值操作，禁用断言时n不会被赋值
assert (n = getnum()) > 0; // 不良实践
System.out.println("n is " + n);
}
}
}
// 计算直角三角形斜边（使用Math类静态方法）
class Hypot {
public static void main(String args[]) {
double side1 = 3.0;
double side2 = 4.0;
// 必须通过类名调用静态方法
double hypot = Math.sqrt(Math.pow(side1, 2) + Math.pow(side2, 2)); // sqrt和pow
System.out.println("Given sides of lengths " + side1 + " and " + side2 +
" the hypotenuse is " + hypot);
}
}
// 使用静态导入简化Math方法调用
import static java.lang.Math.sqrt;
import static java.lang.Math.pow;
class Hypot {
public static void main(String args[]) {
double side1 = 3.0;
double side2 = 4.0;
// 直接调用静态方法，无需类名
double hypot = sqrt(pow(side1, 2) + pow(side2, 2)); // 简化调用
System.out.println("Given sides of lengths " + side1 + " and " + side2 + " the hypotenuse is " + hypot);
}
}
// 构造函数重载（传统方式）
class MyClass {
int a;
int b;
// 分别初始化a和b MyClass(int i, int j) {
a = i;
b = j;
}
// 将a和b初始化为相同值
MyClass(int i) {
a = i;
b = i;
}
// 默认构造函数：a和b为0
MyClass() {
a = 0;
b = 0;
}
}
// 构造函数重载（使用this()调用其他构造函数）
class MyClass {
int a;
int b;
// 主构造函数 MyClass(int i, int j) {
a = i;
b = j;
}
// 调用主构造函数
MyClass(int i) {
this(i, i); // 调用MyClass(i, i)
}
// 调用单参数构造函数 MyClass() {
this(0); // 调用MyClass(0)
}
}
参考来源