String and String Methods

String is Java's standard string type from java.lang, so it needs no import. It manages its character count and storage automatically, making it the usual choice for text. Java strings are immutable: operations that appear to edit a string return a new string instead.

Create, read, and print

A string can be initialized from text and changed by assignment. scanner.next() reads one word; use scanner.nextLine() when spaces must be preserved.

String language = "Java";
String topic = scanner.nextLine();
System.out.println(language + ": " + topic);

Length and state

length() returns the number of characters as an int. isEmpty() reports whether the string contains no characters. Neither operation changes the string.

String text = "Leaflet";
System.out.println(text.length());  // 7
System.out.println(text.isEmpty()); // false

Access individual characters

charAt(index) accesses a character with a zero-based index. The first character is at index 0 and the final character is at text.length() - 1. It checks bounds and throws StringIndexOutOfBoundsException for an invalid index.

String text = "Leaflet";
System.out.println(text.charAt(0));                 // L
System.out.println(text.charAt(text.length() - 1)); // t

Append and insert

String objects cannot be modified in place. Use + or concat() to create a new string. When repeated append or insert operations are needed, use a mutable StringBuilder.

String text = "Leaf";
text = text.concat("let"); // Leaflet
text += " course";         // Leaflet course

StringBuilder builder = new StringBuilder(text);
builder.insert(7, " Java"); // Leaflet Java course
text = builder.toString();

Extract, find, and edit

substring(begin, end) returns a new string from begin up to, but not including, end. indexOf(value) returns the starting index of the first match, or -1 if no match exists. replace(old, replacement) also returns a new string.

String text = "Leaflet course";
String name = text.substring(0, 7); // Leaflet
int position = text.indexOf("course"); // 8

if (position != -1) {
    text = text.replace("course", "lab"); // Leaflet lab
}

text = text.substring(0, 7); // Leaflet

Compare strings

equals() and !equals() test content equality. compareTo() returns a negative value, zero, or a positive value when the current string sorts before, equals, or sorts after its argument. Do not use == to compare string contents.

String first = "apple";
String second = "banana";

if (!first.equals(second) && first.compareTo(second) < 0) {
    System.out.println(first + " comes first");
}