String.replace() / replaceAll() / replaceFirst()

// Replaces all occurrences of a literal string.
str.replace(CharSequence target, CharSequence replacement);

// Replaces all substrings that match a regular expression.
str.replaceAll(String regex, String replacement);

// Replaces only the first substring that matches a regular expression.
str.replaceFirst(String regex, String replacement);

// Use replace() to replace all occurrences of a literal string.
String str = "りんごとみかんとりんご";
System.out.println(str.replace("りんご", "ぶどう")); // Prints "ぶどうとみかんとぶどう".

// Remove specific characters by replacing them with an empty string.
String spaced = "H e l l o";
System.out.println(spaced.replace(" ", "")); // Prints "Hello".

// Use replaceAll() to replace matches using a regular expression.
String text = "電話: 090-1234-5678";
System.out.println(text.replaceAll("[0-9]", "*")); // Prints "電話: ***-****-****".

// Collapse multiple spaces into one.
String messy = "Hello   Java   World";
System.out.println(messy.replaceAll("\\s+", " ")); // Prints "Hello Java World".

// Use replaceFirst() to replace only the first match.
String log = "[ERROR] disk full [ERROR] retry";
System.out.println(log.replaceFirst("\\[ERROR\\]", "[WARN]"));
// Prints "[WARN] disk full [ERROR] retry".

// replace() treats the target as a literal, so . and * are not regex metacharacters.
String dotStr = "1.2.3";
System.out.println(dotStr.replace(".", "-")); // Prints "1-2-3".

replace() operates on literal strings and does not interpret regular expressions. This means characters like . and * are treated as plain characters.

replaceAll() and replaceFirst() support regex-based replacement. If the replacement string contains characters with special meaning in regex — such as backslashes or dollar signs — you must escape them using Matcher.quoteReplacement(). If you call these methods repeatedly in performance-sensitive code, consider using Pattern.compile() to reuse a compiled pattern.

To search within a string, see indexOf() / contains(). To split a string, see split().