计算与软件工程I12 数字和静态

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Wrapping a primitive
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Autoboxing
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Math methods
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Static methods
• Java is object-oriented, but there is a special case where there is no need to have an instance of the class.
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final 方法
• 使用final 方法的原因有两个。
– 第一个原因是把方法锁定，以预防任 何继承类修改它的意义。这是出于设 计的考虑：你想要确保在继承中方法 行为保持不变，并且不会被重载。
– 使用final方法的第二个原因是效率。 如果你将一个方法指明为final，就是 同意编译器将针对该方法的所有调用 都转为内嵌（inline）调用。
– Static variables in a class are initialized before any object of that class can be created.
– Static variables in a class are initialized before any static method of the class runs.
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final 参数
• class Gizmo { • public void spin() {} •} • public class FinalArguments { • void with(final Gizmo g) { • //! g = new Gizmo(); // Illegal -- g is final •} • void without(Gizmo g) { • g = new Gizmo(); // OK -- g not final • g.spin(); •} • // void f(final int i) { i++; } // Can't change • // You can only read from a final primitive: • int g(final int i) { return i + 1; } • public static void main(String[] args) { • FinalArguments bf = new FinalArguments(); • bf.without(null); • bf.with(null); •} • } ///:~
其他 public 类，就必须使用关键字 import 来使得 mypackage 中的名称可以被使用。另一个选择是给出完整 的名称：
• mypackage.MyClass m = new mypackage.MyClass(); • 关键字 import 可使之更加简洁： • import mypackage.*; • // . . . • MyClass m = new MyClass();
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A single copy for static variable
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Initializing a static variable
• The keyword static let a method run without any instance of the class.
• A static method means “behavior not dependent on an instance variable, so no instance/object is required. Just the class.”
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空白final
• class Poppet { • private int i; • Poppet(int ii) { i = ii; } •} • public class BlankFinal { • private final int i = 0; // Initialized final • private final int j; // Blank final • private final Poppet p; // Blank final reference • // Blank finals MUST be initialized in the constructor: • public BlankFinal() { • j = 1; // Initialize blank final • p = new Poppet(1); // Initialize blank final reference •} • public BlankFinal(int x) { • j = x; // Initialize blank final • p = new Poppet(x); // Initialize blank final reference •} • public static void main(String[] args) { • new BlankFinal(); • new BlankFinal(47); •} • } ///:~
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Static final variables are constants
• A variable marked final means that – once initialized - it can never change.
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Autoboxing works almost everywhere
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• Static variables are initialized when a class is loaded.
• A class is loaded because the JVM decides it's time to load it.
• And there are two guarantees about static initialization:
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Turing a primitive number into a String
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Date
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static import
• Saving typing • Harder to read • Naming conflicts
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包
• 定义：
– 包（package）是类的容器，用来保存 划分的类名空间
• 声明
– package pkg1[.pkg2[.pkg3]];
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• Marking the constructor private. • You are free to combine static and
non-static methods in a class.
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Wrappers have static utility mehtods
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包的使用
• 假设文件的名称是 MyClass.java，这就意味着在该文件中 有且只有一个 public类，该类的名称必须是 MyClass（注 意大小写）：
• package mypackage; • public class MyClass { • // . . . • 现在，如果有人想用 MyClass 或者是mypackage 中的任何
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Numbers and statics
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Math methods
• There’s no global thing in Java. • Math methods never use instance
variables.
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Static methods
• Often, a class with static methods is not meant to be instantiated.