1.8 Strings

Learning Objectives

•Create and concatenate String values.
•Use core String methods: length(), charAt(), substring(), equals(), toUpperCase()/toLowerCase().
•Explain why String is immutable and why that matters for equality comparisons.

Introduction

`String` represents an immutable sequence of characters and is one of the most heavily used reference types in Java. "Immutable" means once a `String` object is created, its contents never change — every String method that appears to modify a string (like `toUpperCase()`) actually returns a brand-new `String`.

Key Concepts

String literals are pooled — identical literals may share the same object in memory.

Use `.equals()` to compare string *content*; `==` compares references and can give misleading results for Strings built at runtime.

Common methods: `length()`, `charAt(int)`, `substring(int, int)`, `indexOf(String)`, `trim()`, `split(String)`.

The `+` operator concatenates strings; concatenating with non-String values calls `toString()` implicitly.

Java Syntax

Code Examples

`length()` returns the character count. `toUpperCase()` returns a new uppercase String. `substring(0, 4)` extracts characters 0–3. `==` compares object references (false, since `new String(...)` forces a new object), while `.equals()` correctly compares content (true).

Common Mistakes

⚠Comparing Strings with `==` instead of `.equals()`, which can pass for literals (due to pooling) but silently fail for Strings built with `new String(...)` or concatenation at runtime.
⚠Forgetting that `substring(start, end)` excludes the character at index `end`.
⚠Assuming String methods mutate the original — they don't; the result must be captured or used directly.

Practice

Practice 1

Programming Exercisemedium

Write a program that stores your full name in a String and prints its length and its uppercase version.

Summary

Strings are immutable sequences of characters. Always compare content with `.equals()`, not `==`, and remember that every "modifying" method actually returns a new String.

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