4.4 成员初始化

局部变量在使用之前必须初始化。

package com.sunny.hello.c4;public class Test {    void f() {        int i;        //i++; 编译错误，因为i未初始化    }}

但是，若将基本类型设为一个类的数据成员，情况则会变得稍微有些不同。由于任何方法都可以初始化或使用那个数据，所以在正式使用前，若还是强迫程序员将其初始化为一个适当的值，就可能不是一种实际的做法。然而，若为其赋予一个垃圾值，同样

是非常不安全的。因此，一个类的所有基本类型数据成员都会保证获得一个初始值。

package com.sunny.hello.c4;public class InitialValues {    public static void main(String[] args) {        Measurement d = new Measurement();        d.print();    }}class Measurement {    boolean t; char c; byte b; short s; int i; long l; float f; double d; void print() { System.out.println("Data type Inital value\n" + "boolean " + t + "\n" + "char " + (int)c + " \n" + "byte " + b + "\n" + "short " + s + " \n" + "int " + i + "\n" + "long " + l + " \n" + "float " + f + " \n" + "double " + d + "\n"); } }

运行结果：

Data type Inital valueboolean falsechar 0 byte 0short 0 int 0long 0 float 0.0 double 0.0

 

　　4.4.1 规定初始化

class Measurement {    boolean b = true;    char c = 'x';    byte B = 47;    short s = 60;    int i = 999;    long l = 1;    float f = 3.14f;    double d = 3.14159;    Depth o = new Depth();        // ...}class Depth {    }

 

　　4.4.2 构建器初始化

1.初始化顺序

package com.sunny.hello.c4;public class OrderOfInitialization {    public static void main(String[] args) {        Card t = new Card();        t.f();    }}class Tag {    Tag(int marker) {        System.out.println("Tag(" + marker + ")");    }}class Card {    Tag t1 = new Tag(1);    Card() {        System.out.println("Card()");        t1 = new Tag(33);    }    Tag t2 = new Tag(2);    void f() {        System.out.println("f()");    }    Tag t3 = new Tag(3);}

运行结果：

Tag(1)Tag(2)Tag(3)Card()Tag(33)f()

在执行Card这个类的构造函数之前，会对Card类中所有的成员变量进行初始化。

package com.sunny.hello.c4;public class StaticInitialization {    public static void main(String[] args) {        System.out.println("Creating new Cupboard() in main");        new Cupboard();        System.out.println("Creating new Cupboard() in main");        new Cupboard();        t2.f2(1);        t3.f3(1);    }    static Table t2 = new Table();    static Cupboard t3 = new Cupboard();}class Bow {    Bow(int marker) {        System.out.println("Bow(" + marker + ")");    }    void f(int marker) {        System.out.println("f(" + marker + ")");    }}class Table {    static Bow b1 = new Bow(1);    Table() {        System.out.println("Table()");        b1.f(1);    }    void f2(int marker) {        System.out.println("f2(" + marker + ")");    }    static Bow b2 = new Bow(2);}class Cupboard {    Bow b3 = new Bow(3);    static Bow b4 = new Bow(4);    Cupboard() {        System.out.println("Cupboard()");        b4.f(2);    }    void f3(int marker) {        System.out.println("f3(" + marker + ")");    }    static Bow b5 = new Bow(5);}

运行结果：

Bow(1)Bow(2)Table()f(1)Bow(4)Bow(5)Bow(3)Cupboard()f(2)Creating new Cupboard() in mainBow(3)Cupboard()f(2)Creating new Cupboard() in mainBow(3)Cupboard()f(2)f2(1)f3(1)

 

 

4.5 数组初始化

4.5.1 多维数组

4.6 总结