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【零碎小知识点】（二）Java基础与常用API编码实践

news 2025/9/2 8:37:11

Java基础与常用API编码实践

下面我将根据你提到的Java基础知识点，提供相应的代码示例和练习，帮助你提高编码能力。

1. 核心语法与面向对象编程

基本数据类型与流程控制

public class BasicSyntax {public static void main(String[] args) {// 1. 数据类型与变量byte aByte = 127;                    // 8位short aShort = 32767;                // 16位int anInt = 2147483647;              // 32位long aLong = 9223372036854775807L;   // 64位float aFloat = 3.14f;                // 32位浮点double aDouble = 3.1415926535;       // 64位浮点char aChar = 'A';                    // 16位Unicodeboolean aBoolean = true;             // 真值// 2. 流程控制// if-elseif (anInt > 1000) {System.out.println("大于1000");} else if (anInt > 500) {System.out.println("大于500但小于等于1000");} else {System.out.println("小于等于500");}// switch (Java 12+更强大的switch表达式)int day = 3;String dayType = switch (day) {case 1, 2, 3, 4, 5 -> "工作日";case 6, 7 -> "周末";default -> "无效日期";};System.out.println("日期类型: " + dayType);// 循环// for循环for (int i = 0; i < 5; i++) {System.out.println("For循环: " + i);}// while循环int count = 0;while (count < 3) {System.out.println("While循环: " + count);count++;}// do-while循环int num = 5;do {System.out.println("Do-While循环: " + num);num--;} while (num > 0);}
}

面向对象编程实践

// 封装示例
class BankAccount {private String accountNumber;private double balance;// 构造方法public BankAccount(String accountNumber, double initialBalance) {this.accountNumber = accountNumber;this.balance = initialBalance;}// 封装的方法public void deposit(double amount) {if (amount > 0) {balance += amount;System.out.println("存款成功，当前余额: " + balance);}}public void withdraw(double amount) {if (amount > 0 && amount <= balance) {balance -= amount;System.out.println("取款成功，当前余额: " + balance);} else {System.out.println("取款失败，余额不足");}}// getter方法public double getBalance() {return balance;}public String getAccountNumber() {return accountNumber;}
}// 继承与多态示例
class Animal {private String name;public Animal(String name) {this.name = name;}public void makeSound() {System.out.println("动物发出声音");}public String getName() {return name;}
}class Dog extends Animal {public Dog(String name) {super(name); // 调用父类构造方法}@Overridepublic void makeSound() {System.out.println(getName() + "汪汪叫");}// super关键字使用示例public void superMakeSound() {super.makeSound(); // 调用父类方法}
}class Cat extends Animal {public Cat(String name) {super(name);}@Overridepublic void makeSound() {System.out.println(getName() + "喵喵叫");}
}// 抽象类与接口
abstract class Shape {protected String color;public Shape(String color) {this.color = color;}// 抽象方法public abstract double area();// 具体方法public String getColor() {return color;}
}interface Drawable {void draw(); // 接口方法默认publicdefault void display() { // 默认方法System.out.println("展示图形");}static void printInfo() { // 静态方法System.out.println("这是一个可绘制接口");}
}class Circle extends Shape implements Drawable {private double radius;public Circle(String color, double radius) {super(color);this.radius = radius;}@Overridepublic double area() {return Math.PI * radius * radius;}@Overridepublic void draw() {System.out.println("绘制一个" + color + "的圆形，半径: " + radius);}
}// 内部类示例
class OuterClass {private int outerField = 10;// 成员内部类class InnerClass {void display() {System.out.println("访问外部类字段: " + outerField);}}// 静态内部类static class StaticNestedClass {void display() {System.out.println("静态内部类");}}// 方法内部类void methodWithClass() {class LocalClass {void display() {System.out.println("方法内部类");}}LocalClass local = new LocalClass();local.display();}
}public class OOPPractice {public static void main(String[] args) {// 封装测试BankAccount account = new BankAccount("123456", 1000);account.deposit(500);account.withdraw(200);System.out.println("最终余额: " + account.getBalance());// 继承与多态测试Animal myDog = new Dog("Buddy");Animal myCat = new Cat("Whiskers");myDog.makeSound(); // 多态：运行时调用Dog的方法myCat.makeSound(); // 多态：运行时调用Cat的方法// 抽象类与接口测试Circle circle = new Circle("红色", 5.0);System.out.println("圆形面积: " + circle.area());circle.draw();circle.display();Drawable.printInfo(); // 调用接口静态方法// 内部类测试OuterClass outer = new OuterClass();OuterClass.InnerClass inner = outer.new InnerClass();inner.display();OuterClass.StaticNestedClass staticNested = new OuterClass.StaticNestedClass();staticNested.display();outer.methodWithClass();}
}

2. 常用API实践

String和Object类

import java.util.Arrays;public class CommonAPIPractice {public static void main(String[] args) {// String类实践String str1 = "Hello";String str2 = "Hello";String str3 = new String("Hello");String str4 = "World";// == 和 equals 区别System.out.println("str1 == str2: " + (str1 == str2)); // true，字符串常量池同一对象System.out.println("str1 == str3: " + (str1 == str3)); // false，不同对象System.out.println("str1.equals(str3): " + str1.equals(str3)); // true，内容相同// String不可变性演示String original = "Hello";String modified = original.concat(" World");System.out.println("Original: " + original); // 不变System.out.println("Modified: " + modified); // 新对象// StringBuffer vs StringBuilder// StringBuffer是线程安全的，StringBuilder不是但更快StringBuffer stringBuffer = new StringBuffer();stringBuffer.append("Hello").append(" ").append("World");System.out.println("StringBuffer: " + stringBuffer.toString());StringBuilder stringBuilder = new StringBuilder();stringBuilder.append("Hello").append(" ").append("World");System.out.println("StringBuilder: " + stringBuilder.toString());// Object类方法实践Person person1 = new Person("Alice", 25);Person person2 = new Person("Alice", 25);Person person3 = new Person("Bob", 30);System.out.println("person1.equals(person2): " + person1.equals(person2));System.out.println("person1.equals(person3): " + person1.equals(person3));System.out.println("person1 hashCode: " + person1.hashCode());System.out.println("person2 hashCode: " + person2.hashCode());System.out.println("person3 hashCode: " + person3.hashCode());System.out.println("person1 toString: " + person1.toString());System.out.println("person1 class: " + person1.getClass().getName());// Arrays类实践int[] numbers = {3, 1, 4, 1, 5, 9, 2, 6};System.out.println("Original array: " + Arrays.toString(numbers));// 排序Arrays.sort(numbers);System.out.println("Sorted array: " + Arrays.toString(numbers));// 二分查找int index = Arrays.binarySearch(numbers, 5);System.out.println("Index of 5: " + index);// 填充int[] filledArray = new int[5];Arrays.fill(filledArray, 7);System.out.println("Filled array: " + Arrays.toString(filledArray));// 比较int[] array1 = {1, 2, 3};int[] array2 = {1, 2, 3};int[] array3 = {1, 2, 4};System.out.println("array1 equals array2: " + Arrays.equals(array1, array2));System.out.println("array1 equals array3: " + Arrays.equals(array1, array3));// Math类实践System.out.println("Math.abs(-5): " + Math.abs(-5));System.out.println("Math.sqrt(25): " + Math.sqrt(25));System.out.println("Math.pow(2, 3): " + Math.pow(2, 3));System.out.println("Math.max(10, 20): " + Math.max(10, 20));System.out.println("Math.min(10, 20): " + Math.min(10, 20));System.out.println("Math.random(): " + Math.random()); // 0.0到1.0之间的随机数// 生成特定范围的随机数int min = 10, max = 20;int randomNum = min + (int)(Math.random() * ((max - min) + 1));System.out.println("Random between " + min + " and " + max + ": " + randomNum);// Date和Calendar实践// 旧版Date（已不推荐用于日期操作）java.util.Date oldDate = new java.util.Date();System.out.println("Old Date: " + oldDate);// Calendar类java.util.Calendar calendar = java.util.Calendar.getInstance();System.out.println("Current time: " + calendar.getTime());System.out.println("Year: " + calendar.get(java.util.Calendar.YEAR));System.out.println("Month: " + (calendar.get(java.util.Calendar.MONTH) + 1)); // 月份从0开始System.out.println("Day: " + calendar.get(java.util.Calendar.DAY_OF_MONTH));// 添加时间calendar.add(java.util.Calendar.DAY_OF_MONTH, 7);System.out.println("Date after 7 days: " + calendar.getTime());// 新版Java时间API (Java 8+)java.time.LocalDate localDate = java.time.LocalDate.now();System.out.println("LocalDate: " + localDate);java.time.LocalDateTime localDateTime = java.time.LocalDateTime.now();System.out.println("LocalDateTime: " + localDateTime);java.time.format.DateTimeFormatter formatter = java.time.format.DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");System.out.println("Formatted: " + localDateTime.format(formatter));}
}// 用于演示equals和hashCode的类
class Person {private String name;private int age;public Person(String name, int age) {this.name = name;this.age = age;}// 重写equals方法@Overridepublic boolean equals(Object obj) {if (this == obj) return true;if (obj == null || getClass() != obj.getClass()) return false;Person person = (Person) obj;return age == person.age && name.equals(person.name);}// 重写hashCode方法@Overridepublic int hashCode() {int result = name.hashCode();result = 31 * result + age;return result;}// 重写toString方法@Overridepublic String toString() {return "Person{name='" + name + "', age=" + age + "}";}
}

3. 练习题目

基础练习

编写一个程序，找出100以内的所有素数
实现一个简单的计算器，支持加减乘除运算
编写一个程序，反转一个字符串（不要使用内置的reverse方法）

面向对象练习

设计一个简单的图书馆管理系统，包含Book、Library和Member类
创建一个图形继承体系，包含Shape、Circle、Rectangle和Triangle类
实现一个简单的银行账户系统，支持存款、取款和转账功能

API使用练习

编写一个程序，统计一篇文章中每个单词出现的次数
实现一个日期工具类，提供日期格式化、计算日期差等功能
创建一个学生管理系统，使用Arrays或Collections对学生进行排序和搜索

4. 编码风格建议

命名规范：
- 类名使用大驼峰：MyClass
- 方法和变量使用小驼峰：myMethod, myVariable
- 常量全大写：MAX_VALUE
注释：
- 使用Javadoc注释类和方法
- 复杂逻辑添加行内注释
代码组织：
- 一个类一个文件
- 使用包组织相关类
- 保持方法简短，单一职责
异常处理：
- 使用try-catch处理可能出现的异常
- 不要捕获异常后什么都不做