Java String Manipulation: Replace, Split, and Join

1. Replacing Text

replace(): Simple Character/String Replacement

Replace all occurrences of a character or literal substring:

String text = "apple-banana-orange";  
String replaced = text.replace("-", ", ");  
// "apple, banana, orange"  

replaceAll() & replaceFirst(): Regex-Based Replacement

• replaceAll(String regex, String replacement): Replace all regex matches.

String data = "Error: 404; Error: 500";  
String cleaned = data.replaceAll("Error: \\d+", "OK");  
// "OK; OK"  

• replaceFirst(String regex, String replacement): Replace only the first match.

String log = "[WARN] Disk full. [WARN] Low memory.";  
log = log.replaceFirst("\\[WARN\\]", "[INFO]");  
// "[INFO] Disk full. [WARN] Low memory."  

Key Difference:

• replace() works with literals.
• replaceAll()/replaceFirst() use regex (slower for simple cases).

2. Splitting Strings

split(String regex): Divide a String into an array using regex.

String csv = "Java,Python,C++,JavaScript";  
String[] langs = csv.split(", ");  // ["Java", "Python", "C++", "JavaScript"]  

Handling Special Characters: Escape regex symbols like ., |, or $:

String path = "file.name.txt";  
String[] parts = path.split("\\.");  // ["file", "name", "txt"]  

Limiting Splits: Optional limit parameter restricts the number of splits:

String data = "one:two:three:four";  
String[] result = data.split(":", 2);  // ["one", "two:three:four"]  

3. Joining Strings

String.join() (Java 8+): Concatenate elements with a delimiter.

List<String> list = Arrays.asList("Java", "C#", "Python");  
String joined = String.join(" | ", list);  // "Java | C# | Python"  

Use Case: Building CSV lines or URLs:

String[] params = {"sort=asc", "page=2"};  
String url = "https://api.com/data?" + String.join("&", params);  

4. Converting Between Types

String ↔ char[]

// String to char[]  
char[] chars = "Hello".toCharArray();  

// char[] to String  
String s = new String(chars);  

String ↔ byte[] (with Encoding)

// String to UTF-8 bytes  
byte[] bytes = "text".getBytes(StandardCharsets.UTF_8);  

// byte[] to String  
String restored = new String(bytes, StandardCharsets.UTF_8);  

Other Conversions

// String to int  
int num = Integer.parseInt("42");  

// int to String  
String s = Integer.toString(42);  // or String.valueOf(42)  

Caution: Handle NumberFormatException for invalid parses.

Common Mistakes

❌ Forgetting Immutability:

String s = "hello";  
s.replace("h", "H");  // Original unchanged!  
s = s.replace("h", "H");  // ✅  

❌ Unescaped Regex in split():

"a|b|c".split("|");  // Incorrect! Splits every character.  
"a|b|c".split("\\|");  // ✅ ["a", "b", "c"]  

FAQ