Built-in Types and Variables
内置类型与变量
What you’ll learn: How Rust primitives, strings, mutability, and conversions differ from Java’s type model.
本章将学习： Rust 的基础类型、字符串、可变性和类型转换，分别和 Java 的类型模型有哪些差别。Difficulty: 🟢 Beginner
难度： 🟢 初级
Rust looks familiar at first because it has integers, booleans, strings, and variables. The differences start showing up once ownership, mutability, and explicit conversions enter the picture.
Rust 乍一看不难认，因为它也有整数、布尔、字符串和变量。真正的差异会在所有权、可变性和显式转换这些地方开始冒出来。
Primitive Types
基础类型
| Java | Rust | Notes |
|---|---|---|
int | i32 | explicit width 位宽显式 |
long | i64 | explicit width 位宽显式 |
double | f64 | default floating-point choice 默认浮点选择 |
boolean | bool | same general role 职责类似 |
char | char | Unicode scalar value, not UTF-16 code unit Unicode 标量值，不是 UTF-16 单元 |
byte | u8 or i8 | choose signedness explicitly 显式区分有无符号 |
Rust forces width and signedness into the spelling. That removes guesswork at API boundaries.
Rust 把位宽和有无符号直接写进类型名里，这样 API 边界上的歧义会少很多。
Variables and Mutability
变量与可变性
#![allow(unused)]
fn main() {
let name = "Ada";
let mut count = 0;
count += 1;
}Bindings are immutable by default. This is one of the earliest places where Rust asks for more explicit intent than Java.
绑定默认不可变，这是 Rust 最早要求“意图必须写清楚”的地方之一。
Shadowing
遮蔽
#![allow(unused)]
fn main() {
let port = "8080";
let port: u16 = port.parse().unwrap();
}Shadowing lets a name be rebound with a new type or refined value. It is often cleaner than introducing parsedPort-style names everywhere.
遮蔽允许同一个名字重新绑定成更精确的值或新类型。很多时候，它比到处起 parsedPort 这种名字更清爽。
String vs &str
String 与 &str
This is the first string distinction Java developers must really learn.
这是 Java 开发者必须认真吃透的第一个字符串差异。
| Rust type | Rough Java intuition | Meaning |
|---|---|---|
String | owned String拥有所有权的字符串 | heap-allocated, owned text 堆分配、拥有所有权 |
&str | read-only string view 只读字符串视图 | borrowed string slice 借用的字符串切片 |
If a function only needs to read text, prefer &str.
如果函数只是读取文本，优先用 &str。
Formatting and Printing
格式化与打印
#![allow(unused)]
fn main() {
let name = "Ada";
let score = 42;
println!("{name} scored {score}");
}Rust formatting uses macros rather than overloaded println methods.
Rust 的格式化基于宏，不是靠一堆重载打印方法。
Explicit Conversions
显式转换
Rust avoids many implicit numeric conversions:
Rust 会尽量避免隐式数值转换：
#![allow(unused)]
fn main() {
let x: i32 = 10;
let y: i64 = x as i64;
}That can feel verbose at first, but it reduces accidental widening and narrowing.
刚开始会觉得啰嗦，但它确实减少了很多无意间的扩大和截断。
Advice
建议
- Use immutable bindings unless mutation is genuinely needed.
除非真的要改，否则先用不可变绑定。 - Prefer
&strfor input parameters andStringfor owned returned text.
输入参数优先&str，需要返回拥有所有权的文本时再用String。 - Read the type annotations in compiler diagnostics carefully; they are often the fastest way to learn.
编译器诊断里的类型信息要认真看，那往往是学得最快的入口。
This chapter is where the surface syntax still feels easy. The harder conceptual shift begins when values start moving rather than merely being referenced.
这一章还只是表层语法，整体不算难。真正更硬的思维切换，会从“值开始 move，而不是只是被引用”那里开始。