Strings and text blocks

Text is everywhere — input, output, files, JSON — and String has a few rules that trip up every newcomer.

A String is an object holding a sequence of characters, and it is immutable: methods like toUpperCase() or replace() return a new string and leave the original unchanged. If you ignore the return value, nothing happens.

Compare string contents with a.equals(b) (or equalsIgnoreCase), never with ==. == checks whether two references point at the same object; it sometimes appears to work because string literals are shared, then fails for strings built at runtime.

Useful methods: length(), charAt(i), substring(start, end) (end is exclusive), indexOf, contains, startsWith, strip() (trims whitespace), isBlank(), split(","), repeat(n), and String.join(", ", list).

Concatenation with + is fine for a few pieces. In a loop, use a StringBuilder, which is mutable and avoids creating a new string each iteration: sb.append(x) then sb.toString().

For formatting, String.format("%s is %d years", name, age) or the instance form "%.2f".formatted(price) work like printf.

Since Java 15, text blocks make multi-line strings readable. They start with """ followed by a newline; the common indentation is stripped automatically:

String json = """
    {"name": "Ana", "age": 31}
    """;

Inside text blocks you don't need to escape double quotes, which makes them great for JSON, SQL and HTML snippets.

Example

public class Strings {
    public static void main(String[] args) {
        String name = "  Ada Lovelace ";
        String clean = name.strip();                  // "Ada Lovelace"
        name.toUpperCase();                           // result ignored: name unchanged

        String typed = new String("Ada Lovelace");    // a different object
        System.out.println(clean == typed);           // false
        System.out.println(clean.equals(typed));      // true

        System.out.println(clean.substring(0, 3));    // Ada
        System.out.println(String.join("|", clean.split(" "))); // Ada|Lovelace

        var sb = new StringBuilder();
        for (int i = 1; i <= 3; i++) sb.append(i).append(',');
        System.out.println(sb);                       // 1,2,3,

        String html = """
            <p>Hello, %s</p>
            """.formatted(clean);
        System.out.print(html);                       // <p>Hello, Ada Lovelace</p>
    }
}

Remember

Strings are immutable — always use the returned value
Compare contents with equals, never ==
Use StringBuilder in loops and text blocks for multi-line text

Watch out for

Writing s.trim(); on its own line and expecting s to change.
Comparing user input with == — it fails for strings created at runtime.

Check yourself

After `String s = "hi"; s.toUpperCase();` what is s?

Official docs: docs.oracle.com/javase/tutorial/java/data/strings.html

Example

Remember

Watch out for

Check yourself

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