Thinking in java Chapter5

Posted 2020-12-02 erinchen

5.6 成员初始化

• 方法的局部变量 未初始化 编译提示
• 类的数据成员 基本类型（boolean char byte short int long float double ）未初始化，有初值

Data type    Initial value
boolen       false
char         [] #char 值为0  控制字符 NUL 转义字符
byte         0
short        0
int          0
long         0
float        0.0
double       0.0
reference   null

5.6.1 指定初始化

类成员变量定义（注意float f = 3.14f）
对象初始化

5.7 构造器初始化

自动初始化，在构造器被调用之前发生

# i 首先置为0，然后是7  所有基本类型和对象引用，自动初始化
public class Counter {
    int i;
    Counter(){i=7;}

    public static void main(String[] args) {
        Counter counter = new Counter();
    }
}

5.7.1 初始化顺序

即便字段散步于方法定义之间，它们仍会在任何方法（包括构造器）被调用之前得到初始化。

package Chapter5Initial;

import static net.mindview.util.Print.*;

class Window {
    Window(int marker) {
        print("window(" + marker + ")");
    }
}

class House {
    Window w1 = new Window(1); //Before constructor

    House() {
        // Show that we're in the constructor
        print("House()");
        w3 = new Window(33); // reinitialize w3
    }

    Window w2 = new Window(2);// After constructor

    void f() {
        print("f()");
    }

    Window w3 = new Window(3);// At end
}

public class OrderOfInitialization {
    public static void main(String[] args) {
        House h = new House();
        h.f(); // Show that construction is done
    }

}

window(1)
window(2)
window(3)
House()
window(33)
f()

package Chapter5Initial;

public class InitialOrderTest {
    // 静态字段
    public static String staticField = "静态字段";

    // 静态代码块
    static {
        System.out.println(staticField);
        System.out.println("静态代码块");
    }

    // 字段
    public String field = "字段";

    // 初始化块
    {
        System.out.println(field);
        System.out.println("初始化块");
    }

    // 构造方法
    public InitialOrderTest() {
        System.out.println("构造方法");
    }

    public static void main(String[] args) {
        new InitialOrderTest();
        System.out.println("");
        new InitialOrderTest();
    }

}
静态字段
静态代码块
字段
初始化块
构造方法

字段
初始化块
构造方法

5.7.2 静态数据的初始化

初始化顺序：先静态，后 非静态
静态初始化 只在类对象首次加载 进行一次
没有显示使用static关键字，构造方法实际也是静态方法。

5.7.3 显式的静态初始化

static {
}

5.7.4 非静态实例初始化

import static net.mindview.util.Print.print;

class Mug {
    Mug(int marker){
        print("Mug(" + marker + ")");
    }
    void f(int marker){
        print("f()(" + marker + ")");
    }
}

public class Mugs {
    // 字段
    Mug mug1;
    Mug mug2;

    // 初始化块,在两个构造方法前执行
    {
        mug1 = new Mug(1);
        mug2 = new Mug(2);
        print("mug1 & mug2 initialized");
    }

    Mugs(){
        print("Mugs()");
    }
    Mugs(int i){
        print("Mugs(int)");
    }
    public static void main(String[] args) {
        print("Inside main()");
        new Mugs();
        print("new Mugs() completed");
        new Mugs(1);
        print("new Mugs(1) completed");
    }
}

5.8 数组初始化

int[] a1;(推荐)
int a1[];(c++)
赋值 复制引用

public class ArrayNew {
    public static void main(String[] args) {
        int[] a;
        Random rand = new Random();
        System.out.println(String.valueOf(rand.nextInt(20)));
        System.out.println(String.valueOf(rand.nextInt(20)));
        System.out.println(String.valueOf(rand.nextInt(20)));
        System.out.println(String.valueOf(rand.nextInt(20)));
        a = new int[rand.nextInt(20)];
        print("length of a = " + a.length);
        print(Arrays.toString(a));
    }
}

public class practice16 {
    public static void main(String[] args) {
        Random random = new Random();
        String[] strings = new String[random.nextInt(20)];
        for(int i = 0;i< strings.length;i++){
            strings[i] = String.valueOf(i);
        }
        for(int i = 0;i< strings.length;i++){
            System.out.println(strings[i]);
        }
        System.out.println(Arrays.toString(strings));
    }

}
public class Practice17 {
    Practice17(String arg){
        System.out.println(arg);
    }
    public static void main(String[] args) {
        Random random = new Random();
        Practice17[] practice17s = new Practice17[random.nextInt(20)];
        for (int i = 0; i<practice17s.length;i++){
            practice17s[i] = new Practice17(Integer.toString(i));
        }
    }
}

5.8.1 可变参数列表

class  A{}

public class NewVarArgs {

//    static void printArray(Object[] args){
//        for(Object obj:args)
//            System.out.print(obj +" ");
//        System.out.println();
//
//    }
//    public static void main(String[] args) {
//        printArray(new Object[]{new Integer(47),new Float(3.14),new Double(11.22)});
//        printArray(new Object[]{"one","two","three"});
//        printArray(new Object[]{new A(),new A(),new A()}); //Chapter5Initial.A@511d50c0 类名+对象地址
//    }
    static void printArray(Object...args){
        for(Object obj:args)
            System.out.print(obj +" ");
        System.out.println();

    }
    public static void main(String[] args) {
        printArray(new Integer(47),new Float(3.14),new Double(11.22));
        printArray(47,3.14F,11.11);
        printArray("one","two","three");
        printArray(new A(),new A(),new A());
        printArray((Object[])new Integer[]{1,2,3,4});//编译器填充数组
        printArray();

    }
}

可变参数

public class OptionalTrainlingArguments {
    static void f(int required,String... trailing){ //可变参数必须String
        System.out.println("required: " + required + "");
        for(String s: trailing)
            System.out.print(s+" ");
        System.out.println();
    }

    public static void main(String[] args) {
        f(1,"one");
        f(2,"two","three");
        f(0);
    }
}

public class VarargType {
    static void f(Character... args) {
        System.out.println(args.getClass()); //class [Ljava.lang.Character;
        System.out.println(" length " + args.length);
    }

    static void g(int... args) {
        System.out.println(args.getClass());
        System.out.println(" length " + args.length);
    }

    public static void main(String[] args) {
        f('a');
        f();
        g(1);
        g();
        System.out.println("int[] " + new int[0].getClass()); //int[] class [I

    }
}

5.9 枚举类型

public enum  Spiciness {
    NOT,MILD,MEDIUM,HOT,FLAMING //大写，若多单词，下划线隔开
}

public class SimpleEnumUse {
    public static void main(String[] args) {
        Spiciness howHot = Spiciness.MEDIUM; //创建枚举类型的引用，将其赋值给某个实例
        System.out.println(howHot);
    }
}

public class EnumOrder {
    public static void main(String[] args) {
        for (Spiciness s : Spiciness.values()) // static values()方法
            System.out.println(s + ".ordinal" + s.ordinal()); // 特定enum常量的声明顺序
    }
}

public class Burrito { //n. (墨西哥) 肉馅(或豆馅)玉米粉圆饼
    Spiciness degree;
    public Burrito(Spiciness degree){this.degree = degree;}
    public void  describe(){
        System.out.print("This burrito is ");
        switch (degree){
            case NOT:
                System.out.println("not spicy at all.");
                break;
            case MILD:
            case MEDIUM:
                System.out.println("a little hot.");
                break;
            case HOT:
            case FLAMING:
            default:
                System.out.println("maybe too hot.");
        }
    }
    public static void main(String[] args) {
        Burrito plain = new Burrito(Spiciness.NOT), // 朴素的，plain rice 白饭
                greenChile = new Burrito(Spiciness.MEDIUM), //青椒 绿色智利
                jalapeno = new Burrito(Spiciness.HOT); //墨西哥胡椒
        plain.describe();
        greenChile.describe();
        jalapeno.describe();
    }
}
This burrito is not spicy at all.
This burrito is a little hot.
This burrito is maybe too hot.


public class Practice21 {
    static void describe(Money money){ // 静态方法
        switch (money){
            case FEN:
                System.out.println(money + " is Fen.");
                break;
            case JIAO:
                System.out.println(money + " is JIAO.");
                break;
            case YUAN:
                System.out.println(money + " is YUAN.");
                break;
            case TENYUAN:
                System.out.println(money + " is TENYUAN.");
                break;
            case HUNDRED:
                System.out.println(money + " is HUNDRED.");
                break;
            case THOUSAND:
                System.out.println(money + " is THOUSAND.");
                break;
        }
    }

    public static void main(String[] args) {
        for(Money money: Money.values()) {
            System.out.println(money + ".value: " + money.ordinal());
            describe(money);
        }

    }
}