Strings · Java · CodeMIRAI

The basics

Java

public class Main {
    public static void main(String[] args) {
        String course = "Java Programming";
        System.out.println(course.length());           // 16
        System.out.println(course.charAt(0));          // J
        System.out.println(course.indexOf("Pro"));     // 5
        System.out.println(course.substring(5));       // Programming
        System.out.println(course.substring(0, 4));    // Java
        System.out.println(course.toUpperCase());      // JAVA PROGRAMMING
        System.out.println(course.contains("gram"));   // true
    }
}

Positions start at 0. substring(start, end) includes start and stops just before end. indexOf returns -1 when the text isn't found.

Strings never change

Java

public class Main {
    public static void main(String[] args) {
        String name = "asha";
        name.toUpperCase();              // makes a new String, which is thrown away
        System.out.println(name);        // asha
        name = name.toUpperCase();       // keep the new String
        System.out.println(name);        // ASHA
    }
}

Strings are immutable: methods like toUpperCase and replace return a new String and leave the original untouched.

Comparing: equals, not ==

Java

public class Main {
    public static void main(String[] args) {
        String a = "hello";
        String b = new String("hello");
        System.out.println(a == b);                                // false: two different objects
        System.out.println(a.equals(b));                           // true: the same text
        System.out.println("Hello".equalsIgnoreCase("HELLO"));     // true
        System.out.println("apple".compareTo("banana") < 0);       // true: apple sorts first
    }
}

== asks whether two variables refer to the same object. equals asks whether the text is the same, which is nearly always what you mean.

Everyday methods

Java

public class Main {
    public static void main(String[] args) {
        String raw = "  Learn Java Today  ";
        System.out.println("[" + raw.trim() + "]");                // [Learn Java Today]
        System.out.println(raw.trim().replace("Java", "Python"));  // Learn Python Today
        System.out.println("ha".repeat(3));                        // hahaha
        System.out.println("report.pdf".endsWith(".pdf"));         // true
        System.out.println("".isEmpty() + " " + "   ".isBlank());  // true true

        String csv = "Asha,Leo,Mei";
        String[] names = csv.split(",");
        System.out.println(names.length + " names, first is " + names[0]);
        System.out.println(String.join(" | ", names));             // Asha | Leo | Mei
    }
}

Building strings in a loop: StringBuilder

Java

public class Main {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        for (int i = 1; i <= 5; i++) {
            sb.append(i);
            if (i < 5) sb.append(", ");
        }
        System.out.println(sb.toString());   // 1, 2, 3, 4, 5
        System.out.println(sb.reverse());    // 5 ,4 ,3 ,2 ,1
    }
}

Because Strings never change, every + creates a brand-new String. In a loop that runs thousands of times, that's a lot of waste. A StringBuilder is a growable buffer: append as much as you like, then call toString() once.

Formatted output

Java

public class Main {
    public static void main(String[] args) {
        String name = "Mei";
        double score = 92.456;
        int rank = 3;
        System.out.println(String.format("%s scored %.1f (rank %d)", name, score, rank));
        System.out.printf("%-8s|%6.2f|%n", name, score);
        System.out.printf("%05d%n", 42);
    }
}

Specifier	Means
`%s`	a String (or anything, via `toString`)
`%d`	a whole number
`%.2f`	a decimal number with 2 places, rounded
`%n`	a line break
`%-8s`	left-aligned in a column 8 characters wide
`%05d`	padded with zeros to 5 digits

Text blocks

Since Java 15, three double quotes start a multi-line text block, with no \n or + needed:

Java

public class Main {
    public static void main(String[] args) {
        String json = """
            {
              "course": "Java",
              "lessons": 23
            }
            """;
        System.out.print(json);
    }
}

The indentation shared by every line is removed, so the block can be indented to match your code.