Java 字符串操作之contains 和 substring 方法最佳实践与常见问题

public boolean contains(CharSequence s)

// String 类中的实际实现
public boolean contains(CharSequence s) {
    return indexOf(s.toString()) >= 0;
}

String text = "Java programming is fun";
System.out.println(text.contains("pro"));  // true
System.out.println(text.contains("Python")); // false

public String substring(int beginIndex)
public String substring(int beginIndex, int endIndex)

String str = "HelloWorld";
// 从索引5开始到结尾
System.out.println(str.substring(5)); // "World"
// 从索引0开始到5（不包含5）
System.out.println(str.substring(0, 5)); // "Hello"

// 索引越界
str.substring(10); // StringIndexOutOfBoundsException
// 参数顺序错误
str.substring(5, 0); // StringIndexOutOfBoundsException

public static String findSubstringWithContext(String text, String subString) {
    if (text.contains(subString)) {
        int startIndex = text.indexOf(subString);
        int endIndex = startIndex + subString.length();
        return text.substring(0, endIndex);
    }
    return "子串未找到";
}

public static String getSubstringContext(String text, String sub) {
    if (!text.contains(sub)) {
        return "未找到子串";
    }
    int start = text.indexOf(sub);
    int end = start + sub.length();
    String before = (start > 0) ? text.substring(0, start) : "";
    String after = (end < text.length()) ? text.substring(end) : "";
    return "前文: \"" + before + "\", 子串: \"" + sub + "\", 后文: \"" + after + "\"";
}

String text = "这是一个测试字符串，用于测试子串出现的完整前后内容。";
String subString = "测试";
System.out.println(getSubstringContext(text, subString));
// 输出: 前文: "这是一个", 子串: "测试", 后文: "字符串，用于测试子串出现的完整前后内容。"

String csv = "张三,20,男";
String[] parts = csv.split(","); // ["张三", "20", "男"]

String input = "a1b2c3";
String replaced = input.replaceAll("\\d", ""); // "abc"

String userInput = "  Hello World  ";
String trimmed = userInput.trim(); // "Hello World"
boolean empty = trimmed.isEmpty(); // false

String filename = "document.txt";
boolean isTextFile = filename.endsWith(".txt"); // true

// 解决方案：统一转换为小写
if (text.toLowerCase().contains(sub.toLowerCase())) { ... }

// 正确计算结束索引
int endIndex = startIndex + subString.length();

String chinese = "你好世界";
// substring 正确处理多字节字符
System.out.println(chinese.substring(0, 2)); // "你好"

// 当需要多次检查包含关系时
int index = text.indexOf(sub);
if (index != -1) {
    // 直接使用index，避免二次查找
    String before = text.substring(0, index);
    // ...
}

public class StringOperations {
    public static void main(String[] args) {
        String text = "Java编程很有趣，Java是最好的编程语言之一";
        String sub = "Java";
        String context = getSubstringContext(text, sub);
        System.out.println(context);
    }
    public static String getSubstringContext(String text, String sub) {
        if (text == null || sub == null) {
            return "输入不能为null";
        }
        if (!text.contains(sub)) {
            return "未找到子串: \"" + sub + "\"";
        }
        int start = text.indexOf(sub);
        int end = start + sub.length();
        String before = (start > 0) ? text.substring(0, start) : "[开头]";
        String after = (end < text.length()) ? text.substring(end) : "[结尾]";
        return String.format("前文: \"%s\", 子串: \"%s\", 后文: \"%s\"", before, sub, after);
    }
}

前文: "", 子串: "Java", 后文: "编程很有趣，Java是最好的编程语言之一"