4.5 String Transforming and Splitting

String methods return new strings. They do not change the original string.

String raw = " Java ";
String clean = raw.trim();

System.out.println(raw);    // " Java "
System.out.println(clean);  // "Java"

trim() and strip()

trim() removes leading and trailing whitespace that is common in older ASCII-style text.

"  Java  ".trim();  // "Java"

strip() also removes leading and trailing whitespace, but it follows modern Unicode whitespace rules.

"  Java  ".strip();  // "Java"

For most beginner console programs, both appear to do the same thing. Prefer strip() when working with modern Java and international text.

toLowerCase() and toUpperCase()

toLowerCase() returns a lowercase version of the string. toUpperCase() returns an uppercase version.

"Java".toLowerCase();  // "java"
"Java".toUpperCase();  // "JAVA"

These methods are often used before comparison:

String command = input.nextLine().strip().toLowerCase();

if (command.equals("quit")) {
    System.out.println("Bye");
}

replace(old, new)

replace(old, new) returns a new string where every matching part is replaced.

"a-b-c".replace("-", "_");  // "a_b_c"
"banana".replace("na", "NA");  // "baNANA"

For literal text replacement, use replace. Java also has replaceAll, which uses regular expressions, but that belongs to a later topic.

split(regex)

split(regex) cuts a string into an array of strings.

String line = "java,arrays,strings";
String[] parts = line.split(",");

System.out.println(parts[0]);  // java
System.out.println(parts[1]);  // arrays
System.out.println(parts[2]);  // strings

The parameter is a regular expression. A comma is simple, but characters like . and | have special meanings in regex, so they require extra care.

String.join(separator, parts)

String.join(separator, parts) does the opposite: it joins many strings into one.

String[] words = {"java", "arrays", "strings"};
String label = String.join(" / ", words);

System.out.println(label);  // java / arrays / strings

split and join are useful for CSV-style lines, tags, search keywords, and small text-cleaning tasks.