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4、Java中的基础类

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4.1 字符串处理

字符串是Java中常用的数据类型,用于表示和操作字符序列。Java提供了丰富的字符串处理类。

4.1.1 创建字符串

String str1 = "Hello, World!"; String str2 = new String("Hello, World!");

4.1.2 字符串方法

获取字符串长度

String str = "Hello, World!"; int length = str.length(); System.out.println("Length: " + length); // 输出:Length: 13

字符串拼接

String firstName = "John"; String lastName = "Doe"; String fullName = firstName + " " + lastName; System.out.println("Full Name: " + fullName); // 输出:Full Name: John Doe

字符串比较

String str1 = "Hello"; String str2 = "hello"; boolean isEqual = str1.equals(str2); boolean isEqualIgnoreCase = str1.equalsIgnoreCase(str2); System.out.println("isEqual: " + isEqual); // 输出:isEqual: false System.out.println("isEqualIgnoreCase: " + isEqualIgnoreCase); // 输出:isEqualIgnoreCase: true

查找子字符串

String str = "Hello, World!"; int index = str.indexOf("World"); System.out.println("Index of 'World': " + index); // 输出:Index of 'World': 7

截取子字符串

String str = "Hello, World!"; String subStr = str.substring(7, 12); System.out.println("Sub String: " + subStr); // 输出:Sub String: World

转换大小写

String str = "Hello, World!"; String upperStr = str.toUpperCase(); String lowerStr = str.toLowerCase(); System.out.println("Upper Case: " + upperStr); // 输出:Upper Case: HELLO, WORLD! System.out.println("Lower Case: " + lowerStr); // 输出:Lower Case: hello, world!

4.2 集合框架:List、Set、Map

Java集合框架提供了一组接口和类,用于存储和操作一组数据。

4.2.1 List接口

List接口表示有序的元素集合,可以包含重复的元素。常用的实现类有ArrayListLinkedList

ArrayList示例

import java.util.ArrayList; import java.util.List; public class Main { public static void main(String[] args) { List<String> fruits = new ArrayList<>(); fruits.add("Apple"); fruits.add("Banana"); fruits.add("Orange"); System.out.println("Fruits: " + fruits); // 输出:Fruits: [Apple, Banana, Orange] fruits.remove("Banana"); System.out.println("After removal: " + fruits); // 输出:After removal: [Apple, Orange] String firstFruit = fruits.get(0); System.out.println("First Fruit: " + firstFruit); // 输出:First Fruit: Apple } }

4.2.2 Set接口

Set接口表示不包含重复元素的集合。常用的实现类有HashSetTreeSet

HashSet示例

import java.util.HashSet; import java.util.Set; public class Main { public static void main(String[] args) { Set<String> fruits = new HashSet<>(); fruits.add("Apple"); fruits.add("Banana"); fruits.add("Orange"); fruits.add("Apple"); // 重复添加不会生效 System.out.println("Fruits: " + fruits); // 输出:Fruits: [Apple, Banana, Orange] } }

4.2.3 Map接口

Map接口表示键值对映射。常用的实现类有HashMapTreeMap

HashMap示例

import java.util.HashMap; import java.util.Map; public class Main { public static void main(String[] args) { Map<String, Integer> fruitPrices = new HashMap<>(); fruitPrices.put("Apple", 3); fruitPrices.put("Banana", 2); fruitPrices.put("Orange", 5); System.out.println("Fruit Prices: " + fruitPrices); // 输出:Fruit Prices: {Apple=3, Banana=2, Orange=5} int applePrice = fruitPrices.get("Apple"); System.out.println("Price of Apple: " + applePrice); // 输出:Price of Apple: 3 fruitPrices.remove("Banana"); System.out.println("After removal: " + fruitPrices); // 输出:After removal: {Apple=3, Orange=5} } }

4.3 异常处理:失败的艺术

异常处理是指程序在运行过程中出现错误时的处理机制。Java通过try-catch块来处理异常。

4.3.1 基本语法

try-catch

public class Main { public static void main(String[] args) { try { int result = 10 / 0; // 可能抛出ArithmeticException System.out.println("Result: " + result); } catch (ArithmeticException e) { System.out.println("Error: Division by zero is not allowed."); // 输出:Error: Division by zero is not allowed. } } }

try-catch-finally

public class Main { public static void main(String[] args) { try { int result = 10 / 0; // 可能抛出ArithmeticException System.out.println("Result: " + result); } catch (ArithmeticException e) { System.out.println("Error: Division by zero is not allowed."); // 输出:Error: Division by zero is not allowed. } finally { System.out.println("This block is always executed."); // 输出:This block is always executed. } } }

4.3.2 自定义异常

可以通过继承Exception类来创建自定义异常。

class CustomException extends Exception { public CustomException(String message) { super(message); } } public class Main { public static void main(String[] args) { try { throw new CustomException("This is a custom exception."); } catch (CustomException e) { System.out.println("Caught custom exception: " + e.getMessage()); // 输出:Caught custom exception: This is a custom exception. } } }