Chapter 24: String Methods in Java

This chapter is only about common String instance methods—the toolbox you use every day. String concepts were Chapter 23; StringBuilder is Chapter 25.

1. Topic title

Important String methods: inspection, search, slice, clean, split, replace, case

2. What it means

Each method below runs on a String value and usually returns a new String (or a boolean / int / char / String[]) without changing the original—because String is immutable.

3. Why it is used

Parsing CSV-ish lines, normalizing user input, extracting file extensions, comparing case-insensitive commands, splitting log lines—methods turn raw text into decisions.

4. Real-world example

A signup field returns " Ana ". You trim(), then check isBlank() (Java 11+) or length() == 0, then compare email domain with endsWith—small pipeline of method calls.

5. Java code example

public class StringMethodsDemo {
    public static void main(String[] args) {
        String s = "  Hello, Java world!  ";
 
        System.out.println("length: " + s.length());
        System.out.println("trim: '" + s.trim() + "'");
        System.out.println("charAt 7: " + s.trim().charAt(7));
 
        String core = "Hello, Java world!";
        System.out.println("substring(7, 11): '" + core.substring(7, 11) + "'");
 
        System.out.println("indexOf Java: " + core.indexOf("Java"));
        System.out.println("contains world: " + core.contains("world"));
 
        System.out.println("equals HELLO: " + core.equals("HELLO"));
        System.out.println("equalsIgnoreCase hello: " + core.equalsIgnoreCase("hello, java world!"));
 
        String csv = "one,two,three";
        String[] parts = csv.split(",");
        System.out.println("split parts: " + parts.length + " first=" + parts[0]);
 
        String messy = "foo_bar_foo";
        System.out.println("replace bar -> dash: " + messy.replace("_", "-"));
 
        System.out.println("toLower: " + "MixedCase".toLowerCase());
 
        String empty = "   ";
        System.out.println("isEmpty: " + empty.isEmpty());
        System.out.println("isBlank: " + empty.isBlank());
    }
}

Note: substring(begin, end) uses begin inclusive, end exclusive.

6. Explanation of code

Method	Role
`length()`	number of `char` code units
`charAt(i)`	single `char` at index
`substring(b, e)`	slice `[b, e)`
`indexOf(sub)`	first index or `-1`
`contains(sub)`	convenience search
`equals` / `equalsIgnoreCase`	value compare
`trim()`	strip leading/trailing whitespace (limited Unicode awareness—good enough for many tasks)
`split(regex)`	`String[]` (regex power = watch `.` and special chars)
`replace(old, new)`	literal char sequence replace, all occurrences
`toLowerCase` / `toUpperCase`	locale rules exist—use overload with `Locale` when language matters
`isEmpty`	length 0
`isBlank`	Java 11+, true if empty or only whitespace

7. Common mistakes

split(".") splitting on “any character” — . is regex special; use "\\." for a literal dot.

Thinking replace uses regex — replace is literal; replaceAll uses regex (different method).

Substring index confusion — end exclusive; easy to be off by one.

Calling charAt out of range — StringIndexOutOfBoundsException.

Locale surprises on toLowerCase()** for Turkish I issues—rare in homework, real in global apps.

8. Best practices

Chain thoughtfully — each call allocates; fine for small data.

Prefer isBlank over trim + isEmpty when on Java 11+.

Use strip (Java 11+) for Unicode-aware trimming when you need stricter whitespace handling than trim.

Read Javadoc once for startsWith / endsWith / regionMatches when protocols need them.

9. Small practice task

Given String raw = " SKU-12345;Widget;9.99 ":

trim the whole string.
split on ";" into three parts.
Print SKU, name, and price as separate lines (price stays String for now).

Handle wrong segment count with a simple if (parts.length < 3) message.

Beginner tip

When indexOf returns -1, do not pass that to substring blindly—check first.