Mastering contains() in Java: How to Perform Efficient Substring Searches

1. Introduction: Why String Search Matters in Java

2. Basic Syntax and Characteristics of the contains() Method

Basic Syntax

boolean result = targetString.contains(searchString);

Sample Code

String message = "Java programming is fun!";
boolean hasKeyword = message.contains("programming");

System.out.println(hasKeyword); // Output: true

Characteristics of the Method

• Checks only for partial match: If you need an exact match, use equals() instead.
• Case-sensitive: For example, "Java" and "java" are treated as different (details explained later).
• Does not support regular expressions: Because it simply checks for the presence of a string, pattern matching requires matches() or the Pattern class.

Behavior When null Is Passed

String text = null;
System.out.println(text.contains("test")); // Exception occurs

3. Practical Usage Examples and Important Considerations

3-1. Basic Usage Example

String sentence = "今日はJavaの勉強をしています。";

if (sentence.contains("Java")) {
    System.out.println("Javaが含まれています。");
} else {
    System.out.println("Javaは含まれていません。");
}

3-2. Case Sensitivity

String text = "Welcome to Java";
System.out.println(text.contains("java")); // false

String text = "Welcome to Java";
System.out.println(text.toLowerCase().contains("java")); // true

3-3. Handling of null and Empty Strings

String text = null;
System.out.println(text.contains("test")); // Runtime error

if (text != null && text.contains("test")) {
    // Safe to process
}

String sample = "test";
System.out.println(sample.contains("")); // true

3-4. Does Not Support Multiple Keyword Searches

String target = "エラーコード123：アクセス拒否";
if (target.contains("エラー") || target.contains("拒否")) {
    System.out.println("問題のあるメッセージです。");
}

List<String> keywords = Arrays.asList("エラー", "障害", "失敗");
boolean found = keywords.stream().anyMatch(target::contains);

4. Methods Frequently Compared with contains()

4-1. Difference from equals(): Exact Match vs. Partial Match

String a = "Java";
String b = "Java";

System.out.println(a.equals(b));      // true: Exact match
System.out.println(a.contains("av")); // true: Partial match

4-2. Difference from indexOf(): Whether You Need the Position

String text = "Hello, Java World!";
System.out.println(text.indexOf("Java"));    // 7
System.out.println(text.indexOf("Python"));  // -1

if (text.indexOf("Java") >= 0) {
    System.out.println("It is contained.");
}

4-3. Difference from matches(): Support for Regular Expressions

String text = "abc123";
System.out.println(text.matches(".*123")); // true
System.out.println(text.contains(".*123")); // false (not regex)

4-4. Summary of Feature Comparison

5. Common Use Cases and Sample Code

5-1. User Input Validation (Detecting Prohibited Words)

String input = "このアプリは最悪だ";
String banned = "最悪";

if (input.contains(banned)) {
    System.out.println("不適切な言葉が含まれています。");
}

List<String> bannedWords = Arrays.asList("最悪", "バカ", "死ね");
for (String word : bannedWords) {
    if (input.contains(word)) {
        System.out.println("不適切な言葉が含まれています: " + word);
        break;
    }
}

5-2. Log File Analysis (Detecting Specific Messages)

List<String> logs = Arrays.asList(
    "[INFO] サーバーが起動しました",
    "[ERROR] データベース接続失敗",
    "[WARN] メモリ使用率が高い"
);

for (String log : logs) {
    if (log.contains("ERROR")) {
        System.out.println("エラー発生ログ: " + log);
    }
}

5-3. Filtering Strings in a List (Using Stream API)

List<String> users = Arrays.asList("tanaka@example.com", "sato@gmail.com", "yamada@yahoo.co.jp");

List<String> gmailUsers = users.stream()
    .filter(email -> email.contains("@gmail.com"))
    .collect(Collectors.toList());

System.out.println(gmailUsers); // [sato@gmail.com]

5-4. Parsing HTTP Request Headers or URLs

String userAgent = "Mozilla/5.0 (iPhone; CPU iPhone OS 16_0 like Mac OS X)";
if (userAgent.contains("iPhone")) {
    System.out.println("スマートフォンからのアクセスです。");
}

5-5. Checking File Paths or Extensions

String filePath = "/usr/local/data/sample.csv";
if (filePath.contains(".csv")) {
    System.out.println("CSVファイルです。");
}

Practical Considerations

• Apply normalization (e.g., toLowerCase(), trim()) when accuracy is required.
• For large-scale data, consider Stream API or regex.
• Remember that contains() is partial match—combine with other conditions for safer logic.

6. Performance Considerations

6-1. Internal Behavior and Time Complexity of contains()

for (String line : hugeTextList) {
    if (line.contains("error")) {
        // processing
    }
}

6-2. Techniques for Improving Performance During Frequent Searches

• Convert all strings to lowercase in advance

List<String> normalizedList = originalList.stream()
    .map(String::toLowerCase)
    .collect(Collectors.toList());

• Use Stream API with parallel() for parallel processing

List<String> result = hugeTextList.parallelStream()
    .filter(line -> line.contains("keyword"))
    .collect(Collectors.toList());

• Use regular expressions for complex search patterns

Pattern pattern = Pattern.compile("error|fail|fatal");
for (String log : logs) {
    if (pattern.matcher(log).find()) {
        // matched
    }
}

6-3. Memory Efficiency and Reusability Considerations

• Avoid creating unnecessary string instances.
• For large datasets, consider buffering or chunk processing.
• Caching repeated contains() results may improve performance in certain cases.

7. Comparison with Other Programming Languages

7-1. Python: Simple Partial Match with the in Operator

text = "Hello, Python!"
if "Python" in text:
    print("含まれています")

• in is a language operator, not a method.
• Python is also case-sensitive for string comparison.
• None produces an exception; null checks are required.

7-2. JavaScript: Partial Match with includes()

const text = "JavaScript is fun";
console.log(text.includes("fun")); // true

• Passing undefined does not throw an exception; it simply returns false.
• includes() also works on arrays, increasing its versatility.

7-3. C#: Contains() Similar to Java

string text = "Welcome to C#";
bool result = text.Contains("C#");

• C# Contains() is case-sensitive by default, but you can ignore case using StringComparison.OrdinalIgnoreCase.
• Passing null triggers ArgumentNullException.

7-4. Comparison Table Across Languages

Summary: Choose the Right Syntax for Your Use Case

8. Frequently Asked Questions (FAQ)

Q1. Is contains() case-sensitive?

String input = "Welcome to Java";
boolean result = input.toLowerCase().contains("java");

Q2. How can I check partial matches using regular expressions?

import java.util.regex.Pattern;
import java.util.regex.Matcher;

String text = "注文番号: A123456";
Pattern pattern = Pattern.compile("A\\d+");
Matcher matcher = pattern.matcher(text);

if (matcher.find()) {
    System.out.println("パターンに一致しました。");
}

Q3. What happens if I call contains() on null?

String target = null;
System.out.println(target.contains("test")); // Error

if (target != null && target.contains("test")) {
    System.out.println("含まれています。");
}

Q4. What happens if I pass an empty string (“”) to contains()?

String text = "Java";
System.out.println(text.contains("")); // true

Q5. Can contains() search multiple keywords at once?

String text = "本日はシステムエラーが発生しました";
if (text.contains("エラー") || text.contains("障害") || text.contains("失敗")) {
    System.out.println("問題が検出されました。");
}

List<String> keywords = Arrays.asList("エラー", "障害", "失敗");
boolean found = keywords.stream().anyMatch(text::contains);

Q6. When should I use contains() vs indexOf()?

• Use contains() when you simply want to know if the substring exists.
• Use indexOf() when you also need the position.

String text = "Error: Disk full";
if (text.contains("Error")) {
    int pos = text.indexOf("Error");
    System.out.println("Position: " + pos);
}

9. Conclusion

• Basic syntax and return values
• Case sensitivity and how to handle it
• Handling nulls and empty strings
• Differences from other string comparison methods
• Practical use cases: validation, log search, Stream processing
• Performance considerations and optimization techniques
• Comparison with Python, JavaScript, and C#
• Frequently asked questions and troubleshooting tips