Thinking in java-20 Java 初始化

Java 语言不厌其烦地试图保证变量在使用之前被合适地进行过初始化。

• 对于类中的变量，未被显式初始化的变量会被初始化为其默认值；
• 而对于方法中的变量，如果没有主动初始化，则编译器会以报错的方式保证变量初始化。

1.顺序初始化

在类内部，变量初始化的顺序是由其在类中的位置定义的。变量定义可能分散在类中各处，但是变量总是在任何方法被调用之前被初始化–甚至是构造函数。

package thinking.initialization;import static fqy.iss.utils.Print.print;public class OrderOfInitialization{    public static void main(String[] args)    {        House h = new House();        h.f();    }}class Window{    Window(int marker)    {        print("Window(" + marker + ")");    }}class House{    Window w1 = new Window(1);    House()    {        print("House()");        w3 = new Window(33);    }    Window w2 = new Window(2);    void f()    {        print("f()");    }    Window w3 = new Window(3);}//resultWindow(1)Window(2)Window(3)House()Window(33)f()

2.有静态数据情形

package fqy.iss.thinking.initialization;import static fqy.iss.utils.Print.print;class Bowl{    Bowl(int marker)    {        print("Bowl(" + marker + ")");    }    void f1(int marker)    {        print("f1(" + marker + ")");    }}class Table{    static Bowl bowl1 = new Bowl(1);    Table()    {        print("Table()");        bowl2.f1(1);    }    void f2(int marker)    {        print("f2(" + marker + ")");    }    static Bowl bowl2 = new Bowl(2);}class Cupboard{    Bowl bowl3 = new Bowl(3);    static Bowl bowl4 = new Bowl(4);    Cupboard()    {        print("Cupboard()");        bowl4.f1(2);    }    void f3(int marker)    {        print("f3(" + marker + ")");    }    static Bowl bowl5 = new Bowl(5);}public class StaticInitialization{    public static void main(String[] args)    {        print("Creating new Cupboard() in main");        new Cupboard();        print("Creating new Cupboard() in main");        new Cupboard();        table.f2(1);        cupboard.f3(1);    }    static Table table = new Table();    static Cupboard cupboard = new Cupboard(); }//resultBowl(1)Bowl(2)Table()f1(1)Bowl(4)Bowl(5)Bowl(3)Cupboard()f1(2)Creating new Cupboard() in mainBowl(3)Cupboard()f1(2)Creating new Cupboard() in mainBowl(3)Cupboard()f1(2)f2(1)f3(1)

1. 静态数据仅仅被加载一次，只有一处副本存在；
2. 类中数据初始化顺序是：静态数据按顺序初始化，然后是非静态数据顺序初始化，然后才执行构造函数。

2. 静态数据块初始化

package fqy.iss.thinking.initialization;import static fqy.iss.utils.Print.print;class Cup{    Cup(int marker)    {        print("Cup(" + marker + ")");    }    void f(int marker)    {        print("f(" + marker + ")");    }}class Cups{    static Cup cup1;    static Cup cup2;    static    {        cup1 = new Cup(1);        cup2 = new Cup(2);    }    Cups()    {        print("Cups()");    }}public class ExplicitStatic{    public static void main(String[] args)    {        print("Inside main()");        Cups.cup1.f(99); // (1)    }    // static Cups cups1 = new Cups(); // (2)    // static Cups cups2 = new Cups(); // (2)}//resultInside main()Cup(1)Cup(2)f(99)