Chapter 23: String in Java

This chapter is only about String as a concept: what kind of type it is, how assignment and comparison behave, and immutability. Individual methods get their own tour in Chapter 24; StringBuilder is Chapter 25.

1. Topic title

String in Java: immutable text, references, literals, and equals vs ==

2. What it means

String is a class in the Java standard library. A variable of type String holds a reference to an object that represents text (Unicode code units under the hood).

Immutable means: once a String object is created, its characters are not changed in place. Operations like toUpperCase() return a new String (or the same instance when optimized—do not rely on that for logic).

String s = "hi";
s = s + "!"; // new String object, old "hi" may be garbage collected

3. Why it is used

Text is everywhere: names, JSON, file paths, error messages. String is the default way Java represents read-only character sequences in APIs.

Immutability makes sharing safe: many parts of the program can hold references to the same "ERROR" literal without one caller mutating everyone else’s view.

4. Real-world example

A HTTP header value like "application/json" is fixed for a given response. You pass it around as String, compare with .equals, never worry another module trimmed your copy in place—because it cannot.

5. Java code example

public class StringConceptDemo {
    public static void main(String[] args) {
        String a = "java";
        String b = "java";
        String c = new String("java");
 
        System.out.println("a == b: " + (a == b));
        System.out.println("a == c: " + (a == c));
        System.out.println("a.equals(c): " + a.equals(c));
 
        String name = "  Alex  ";
        String trimmed = name.trim();
        System.out.println("same object after trim? " + (name == trimmed));
        System.out.println("trimmed value: '" + trimmed + "'");
 
        String path = "C:\\data";
        String longer = path + "\\file.txt";
        System.out.println(longer);
    }
}

Typical output: a == b is often true for identical literals (interning in the constant pool—implementation detail). a == c is false because new forces a fresh object. a.equals(c) is true for same characters.

6. Explanation of code

• == on objects compares references (same object in memory?).
• .equals compares logical content for String (case-sensitive by default).
• trim() returns a new string without leading/trailing spaces; original name unchanged.
• + concatenation builds new strings; fine for small demos, expensive in tight loops (Chapter 25).

7. Common mistakes

Using == for business equality of user input or database text—use .equals.

Calling .equals on a possibly-null variable first — user.equals("admin") throws if user is null. Prefer "admin".equals(user).

Trying to mutate a character like s[0] = 'X' — not valid Java syntax; String has no public index setter.

8. Best practices

Default to .equals for value comparison; reserve == for enums or intentional identity checks.

Use text blocks """ (Java 15+) for multiline SQL or JSON in modern codebases—optional sugar once basics stick.

Avoid heavy + chains in loops—use StringBuilder when profiling shows a hotspot.

9. Small practice task

Create two strings: String x = new String("ok"); and String y = new String("ok");. Print x == y and x.equals(y). Then assign String z = "ok"; and compare x == z and x.equals(z). Write one sentence in a comment explaining what surprised you.

Beginner tip

Think String = read-only snapshot of text. If you need a scratchpad that changes character by character, you are already hinting at StringBuilder.