java.lang · java.base · since Java 1.0

String

declaration

public final class String
        implements CharSequence, Comparable<String>, Serializable

Immutable UTF-16 character sequence. The most used class in Java: literals are pooled, methods return new strings, and comparison is always equals(), never ==.

Immutable — safe to share, cache, and key maps with
length()/charAt() count UTF-16 code units, not visible characters
Compact strings: Latin-1 content stored as bytes internally

Key methods

`boolean equals(Object) / equalsIgnoreCase(String)`	Content comparison — the correct way.
`String substring(int begin, int end)`	Half-open range copy.
`boolean contains/startsWith/endsWith`	Containment tests.
`String strip() / isBlank()`	Unicode-aware trim and whitespace test.
`String replace(CharSequence, CharSequence)`	Literal replacement of all occurrences.
`String[] split(String regex)`	Regex split — escape literal dots!
`static String join(CharSequence delim, ...)`	Join pieces with a delimiter.
`String formatted(Object... args)`	printf-style formatting (instance form of String.format).
`IntStream chars() / codePoints()`	Stream over code units / code points.
`String intern()`	Canonical pooled instance — rarely needed, measure before using.

Example

Java

String csv = String.join(", ", List.of("a", "b", "c"));
String padded = "%5d".formatted(42);
"Ciao 🇮🇹".length();        // 9 — code units, not characters!