第4章 初始化和清除（下）

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
"
                + "boolean " + t + "
"
                + "char " + (int)c + " 
"
                + "byte " + b + "
"
                + "short " + s + " 
"
                + "int " + i + "
"
                + "long " + l + " 
"
                + "float " + f + " 
"
                + "double " + d + "
");
    }
}

运行结果：

Data type Inital value
boolean false
char 0 
byte 0
short 0 
int 0
long 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 main
Bow(3)
Cupboard()
f(2)
Creating new Cupboard() in main
Bow(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