Java · Lesson 7 of 15

Strings and StringBuilder

Immutable strings, common methods, comparison and building text efficiently.

Beginner
12 min read
3 objectives

Before this lessonLesson 6: Exception Handling

What you will learn

Use core String methods
Compare with equals
Build text with StringBuilder

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A String is a sequence of characters. In Java strings are immutable: every operation that seems to change one actually returns a new string, and the original stays untouched.

Everyday methods

public class Main {
    public static void main(String[] args) {
        String s = "  Hello, Java  ";
        System.out.println(s.trim());
        System.out.println(s.toUpperCase());
        System.out.println(s.length());
        System.out.println(s.trim().substring(0, 5));
        System.out.println(s.contains("Java"));
        System.out.println(s.replace("Java", "World").trim());
    }
}

Output

Hello, Java
  HELLO, JAVA  
15
Hello
true
Hello, World

Comparing strings

Never compare strings with ==. It checks whether two variables point at the same object, not whether the text matches. Use equals.

public class Main {
    public static void main(String[] args) {
        String a = new String("hi");
        String b = new String("hi");
        System.out.println(a == b);
        System.out.println(a.equals(b));
        System.out.println("Hi".equalsIgnoreCase("hI"));
    }
}

Output

false
true
true

Splitting, joining and formatting

public class Main {
    public static void main(String[] args) {
        String[] parts = "a,b,c".split(",");
        System.out.println(parts.length);
        System.out.println(String.join("-", parts));
        System.out.println(String.format("%s scored %d (%.1f%%)", "Ada", 42, 93.456));
    }
}

Output

3
a-b-c
Ada scored 42 (93.5%)

StringBuilder

Because strings are immutable, gluing many pieces in a loop with + creates many throwaway objects. StringBuilder is a mutable buffer built for this.

public class Main {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        for (int i = 1; i <= 5; i++) {
            sb.append(i).append(' ');
        }
        System.out.println(sb.toString().trim());
        System.out.println(sb.reverse().toString().trim());
    }
}

Output

1 2 3 4 5
5 4 3 2 1

Up next · Lesson 8ArraysFixed-size arrays, loops over them, 2D arrays and the Arrays utility class.