Java BasicsJava 基础知识

Java source does not run directly as machine codeJava 代码不是直接变成机器码

You write .java files. javac compiles them into .class bytecode. The JVM runs that bytecode on each platform.你写的是 .java 文件，javac 把它编译成 .class bytecode，然后 JVM 在不同系统上运行这些 bytecode。

• platform-independent: the same bytecode can run on JVMs for different platforms.：同一份 bytecode 可以交给不同平台的 JVM。
• memory management: objects are created with new and later collected by the garbage collector.：对象用 new 创建，没引用后由 garbage collector 回收。
• secure: Java limits many unsafe operations through type checks, access control, and runtime checks.：类型检查、访问控制和运行时检查比 C 的直接内存操作更受限制。

The three common parts inside a class一段 Java class 里最常见的三块

class Circle {
    private int x;
    private int y;
    private int r;

    double area() {
        return 3.14 * r * r;
    }
}

• class Circle: defines a type/template.：定义一个类型/模板。
• x, y, r: fields/attributes, the object's data.：field / attribute，也就是对象的数据。
• area(): a method, something the object can do.：method，也就是对象能做的事。

new creates the real objectnew 是真的创建对象

Circle c1 = new Circle();
Circle c2 = new Circle(2, 5, 10);

Circle is the class. new Circle(...) creates the object, also called an instance. A constructor initializes the object when it is born.Circle 是 class，new Circle(...) 创建出来的是 object，也叫 instance。constructor 是对象刚出生时负责初始化的特殊方法。

class Circle {
    private int x, y, r;

    Circle() {
        this(1, 1, 1);
    }

    Circle(int x, int y, int r) {
        this.x = x;
        this.y = y;
        this.r = r;
    }
}

Access control protects object state访问控制是为了保护对象状态

Instead of letting outside code write c.r = -100, hide data and control updates through methods.不建议让外面直接 c.r = -100。更好的做法是把数据藏起来，再通过 method 检查。

class Circle {
    private int r;

    public void setRadius(int r) {
        if (r <= 0) {
            throw new IllegalArgumentException("radius must be positive");
        }
        this.r = r;
    }
}