目录
1.2.1UML：数据字段封装 Data Field Encapsulation
1.4基本数据类型变量 vs对象类型变量 Variables of Primitive Data Types vs Object Types
2.3静态变量与非静态变量 Static vs. Non-static variables
2.4静态与非静态方法Static vs. Non-static methods
2.8访问修饰符和访问器/修改器方法Visibility Modifiers and Accessor/Mutator Methods
1.对象 Object
1.1介绍
- 一个对象代表了现实世界中可以明确识别的实体，它与具有共同属性的对象类/模板不同。
An object represents an entity in the real world that can be distinctly identified from a class/templates of objects with common properties.
- 对象具有独特的状态和行为：An object has a unique state and behavior
·对象的状态由一组数据字段（属性）及其当前值组成。the state of an object consists of a set of data fields (properties) with their current values
·对象的行为由一组实例方法定义。the behavior of an object is defined by a set of instance methods
1.2统一建模语言（UML）
- 统一建模语言（UML）是软件工程领域的一种通用建模语言，旨在为面向对象系统的可视化设计提供一种标准方式。The Unified Modeling Language (UML) is a general-purpose modeling language in the field of software engineering that is intended to provide a standard way to visualize the design of a object-oriented system.
1.2.1UML：数据字段封装 Data Field Encapsulation
- 对象数组Array of Objects
·对象数组是引用变量的数组（类似于之前看到过的多维数组） An array of objects is an array of reference variables (like the multi-dimensional arrays seen before)
>Circle[] circleArray = new Circle[10];
>circleArray[0] = new Circle();
>circleArray[1] = new Circle(5);
1.3访问对象 Accessing Objects
- 引用对象的数据Referencing the object’s data：
objectRefVar.data
·eg：myCircle.radius
- 调用对象的方法Invoking the object’s method：
objectRefVar.methodName(arguments)
·eg：myCircle.getArea()
1.4基本数据类型变量 vs对象类型变量 Variables of Primitive Data Types vs Object Types
- 区别
- 复制
1.5垃圾回收Garbage Collection
- 之前由c1引用的对象不再被引用，它被称为垃圾。
The object previously referenced by c1 is no longer referenced, it is called garbage
- 垃圾会被JVM自动回收，这个过程叫做垃圾回收。
Garbage is automatically collected by the JVM, a process called garbage collection
- 在一些较早的语言中，如C和C++，人们必须显式地释放/删除不再使用的数据/对象。
In older languages, like C and C++, one had to explicitly deallocate/delete unused data/objects
2.类 Classes
2.1介绍
- 在Java中，类是定义相同类型对象的模板 In Java classes are templates that define objects of the same type
- Java类使用：
·非静态/实例变量来定义数据字段 non-static/instance variables to define data fields
·非静态/实例方法来定义行为 non-static/instance methods to define behaviors
- 一个类提供了一种特殊类型的方法，称为构造函数，它被用来根据类构造对象。A class provides a special type of methods called constructors which are invoked to construct objects from the class
- 示例：
2.2构造器Constructors
- 构造器的名称必须与类名相同。Constructors must have the same name as the class itself.
- 构造器没有返回类型——甚至连void也没有。Constructors do not have a return type—not even void.
- 构造器在创建对象时通过new运算符被调用——它们将对象初始化为引用变量：Constructors are invoked using the new operator when an object is created – they initialize objects to reference variables
ClassName o = new ClassName( );
- 一个类可以不声明构造器：如果类中没有显式声明构造器，编译器会隐式地为类声明一个无参的默认构造器，其方法体为空。A class may be declared without constructors: a no-arg default constructor with an empty body is implicitly declared in the class
2.3静态变量与非静态变量 Static vs. Non-static variables
- 静态变量和常量：Static variables and constants
·静态变量是整个类的全局变量，适用于该类的所有对象实例。global variables for the entire class: for all objects instances of this class
·静态变量被该类的所有实例共享 Static variables are shared by all the instances of the class:
·例如：
static int count = 0;
static final double PI = 3.141592;
- 非静态变量/实例变量是对象的数据字段：Non-static/instance variables are date fields of objects
·例如：
System.out.println(myCircle.radius);
System.out.println(yourCircle.radius);
2.4静态与非静态方法Static vs. Non-static methods
- 静态方法：Static methods
·被该类的所有实例共享，不与特定对象绑定。Shared by all the instances of the class - not tied to a specific object
·eg：double d = Math.pow(3, 2);
- 非静态方法/实例方法 Non-static/instance methods
·必须通过类的对象实例调用：must be invoked from an object instance of the class
·eg: double d1 = myCircle.getArea();
double d2 = yourCircle.getArea();
2.5.变量默认值：
- Java不会为方法内部的局部变量分配默认值。Java assigns no default value to a local variable inside a method.
- 数据字段有默认值（类中的全局变量）Data fields have default values
2.6 Date类
- 方法：
+Date()
构造一个表示当前时间的Date对象 Constructs a Date object for the current time
+Date(elapseTime: long)
构造一个表示自1970年1月1日（GMT）以来经过的毫秒数的Date对象 Constructs a Date object for a given time in milliseconds elapsed since January 1, 1970, GMT.
+toString(): String
返回表示日期和时间的字符串Returns a string representing the date and time.
+getTime(): long
返回自1970年1月1日（GMT）以来的毫秒数 Returns the number of milliseconds since January 1, 1970, GMT.
+setTime(elapseTime: long): void
设置对象中的新经过时间。1970年1月1日（GMT）被称为Unix时间或Unix纪元时间。Sets a new elapse time in the object. January 1, 1970, GMT is called the Unix time (or Unix epoch time)
- 声明方式：
java.util.Date date = new java.util.Date();
System.out.println(date.toString());
2.7 Random类
- 方法 ：
+Random()
构造一个以当前时间为种子的Random对象 Constructs a Random object with the current time as its seed
+Random(seed: long)
构造一个以指定值作为种子的Random对象。种子值用于确保生成的随机数序列是可重复的。Constructs a Random object with a specified seed.
+nextInt(): int
返回一个随机的int值。Returns a random int value.
+nextInt(n: int): int
返回一个介于0和n（不包括n）之间的随机int值 Returns a random int value between 0 and n (exclusive).
+nextLong(): long
返回一个随机的long值Returns a random long value.
+nextDouble(): double
返回一个介于0.0和1.0（不包括1.0）之间的随机double值 Returns a random double value between 0.0 and 1.0 (exclusive).
+nextFloat(): float
返回一个介于0.0F和1.0F（不包括1.0F）之间的随机float值 Returns a random float value between 0.0F and 1.0F (exclusive).
+nextBoolean(): boolean
返回一个随机的boolean值。Returns a random boolean value.
- 声明方式：
Random random1 = new Random(3);
for (int i = 0; i < 10; i++)
System.out.print(random1.nextInt(1000) + " ");
2.8访问修饰符和访问器/修改器方法Visibility Modifiers and Accessor/Mutator Methods
- 默认情况下，类、变量或方法可以被同一包内的任何类访问。By default, the class, variable, or method can be accessed by any class in the same package.
·public（UML中用+表示）
>类、数据或方法是对任何包内的任何类可见的。The class, data, or method is visible to any class in any package.
·private（UML中用-表示）
>数据或方法只能被声明它的类访问 - 用于保护数据！The data or methods can be accessed only by the declaring class - To protect data!
- getField（访问器）和setField（修改器）方法用于读取和修改私有属性。 getField (accessors) and setField (mutators) methods are used to read and modify private properties.
2.8.1修饰符 Pmodifiers
- private修饰符限制访问权限仅限于类内部The private modifier restricts access to within a class
- 默认修饰符（即不加任何修饰符）限制访问权限仅限于包内部。The default modifier restricts access to within a package
- public修饰符——无限制访问。public – unrestricted access